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Title: The Economy of Workshop Mainipulation - A logical method of learning constructive mechanics
Author: Richards, J. T.
Language: English
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* * * * *
THE ECONOMY
OF
WORKSHOP MANIPULATION.
THE ECONOMY
OF
WORKSHOP MANIPULATION.
_A LOGICAL METHOD OF LEARNING CONSTRUCTIVE
MECHANICS._
ARRANGED WITH QUESTIONS
FOR THE USE OF
APPRENTICE ENGINEERS AND STUDENTS.
BY
J. RICHARDS,
AUTHOR OF "A TREATISE ON THE CONSTRUCTION AND OPERATION OF
WOOD-WORKING MACHINES," "THE OPERATOR'S HANDBOOK," "WOOD
CONVERSION BY MACHINERY," AND OTHER WRITINGS ON MECHANICAL
SUBJECTS.
LONDON:
E. & F. N. SPON, 48 CHARING CROSS.
NEW YORK: 446 BROOME STREET.
1876.
[_All rights reserved._]
_Entered, according to Act of Congress, in the year 1875, by
JOHN RICHARDS,
In the Office of the Librarian of Congress, at Washington._
PREFACE.
The contents of the present work, except the Introduction and the
chapter on Gauges, consist mainly in a revision of a series of
articles published in "Engineering" and the Journal of the Franklin
Institute, under the head of "The Principles of Shop Manipulation,"
during 1873 and 1874.
The articles alluded to were suggested by observations made in
actual practice, and by noting a "habit of thought" common among
learners, which did not seem to accord with the purely scientific
manner in which mechanical subjects are now so constantly treated.
The favourable reception which the articles on "Shop Manipulation"
met with during their serial publication, and various requests for
their reproduction in the form of a book, has led to the present
edition.
The addition of a few questions at the end of each chapter, some
of which are not answered in the text, it is thought will assist
the main object of the work, which is to promote a habit of logical
investigation on the part of learners.
It will be proper to mention here, what will be more fully pointed
out in the Introduction, that although workshop processes may
be scientifically explained and proved, they must nevertheless
be learned logically. This view, it is hoped, will not lead to
anything in the book being construed as a disparagement of the
importance of theoretical studies.
Success in Technical Training, as in other kinds of education,
must depend greatly upon how well the general mode of thought
among learners is understood and followed; and if the present work
directs some attention to this matter it will not fail to add
something to those influences which tend to build up our industrial
interests.
J. R.
10 JOHN STREET, ADELPHI,
LONDON, 1875.
CONTENTS.
CHAP. PAGE
INTRODUCTION, 1
I. PLANS OF STUDYING, 6
II. MECHANICAL ENGINEERING, 13
III. ENGINEERING AS A CALLING, 17
IV. THE CONDITIONS OF APPRENTICESHIP, 18
V. THE OBJECT OF MECHANICAL INDUSTRY, 25
VI. ON THE NATURE AND OBJECTS OF MACHINERY, 28
VII. MOTIVE MACHINERY, 29
VIII. WATER POWER, 35
IX. WIND POWER, 41
X. MACHINERY FOR TRANSMITTING AND DISTRIBUTING POWER, 42
XI. SHAFTS FOR TRANSMITTING POWER, 44
XII. BELTS FOR TRANSMITTING POWER, 48
XIII. GEARING AS A MEANS OF TRANSMITTING POWER, 51
XIV. HYDRAULIC APPARATUS FOR TRANSMITTING POWER, 53
XV. PNEUMATIC MACHINERY FOR TRANSMITTING POWER, 55
XVI. MACHINERY OF APPLICATION, 57
XVII. MACHINERY FOR MOVING AND HANDLING MATERIAL, 60
XVIII. MACHINE COMBINATION, 67
XIX. THE ARRANGEMENT OF ENGINEERING ESTABLISHMENTS, 71
XX. GENERALISATION OF SHOP PROCESSES, 74
XXI. MECHANICAL DRAWING, 78
XXII. PATTERN MAKING AND CASTING, 90
XXIII. FORGING, 100
XXIV. TRIP-HAMMERS, 106
XXV. CRANK-HAMMERS, 108
XXVI. STEAM-HAMMERS, 109
XXVII. COMPOUND HAMMERS, 112
XXVIII. TEMPERING STEEL, 114
XXIX. FITTING AND FINISHING, 118
XXX. TURNING LATHES, 121
XXXI. PLANING OR RECIPROCATING MACHINES, 128
XXXII. SLOTTING MACHINES, 134
XXXIII. SHAPING MACHINES, 135
XXXIV. BORING AND DRILLING, 136
XXXV. MILLING, 140
XXXVI. SCREW-CUTTING, 143
XXXVII. STANDARD MEASURES, 145
XXXVIII. GAUGING IMPLEMENTS, 147
XXXIX. DESIGNING MACHINES, 152
XL. INVENTION, 159
XLI. WORKSHOP EXPERIENCE, 165
THE ECONOMY
OF
WORKSHOP MANIPULATION.
_INTRODUCTION._
In adding another to the large number of books which treat upon
Mechanics, and especially of that class devoted to what is called
Mechanical Engineering, it will be proper to explain some of the
reasons for preparing the present work; and as these explanations
will constitute a part of the work itself, and be directed to a
subject of some interest to a learner, they are included in the
Introduction.
First I will notice that among our many books upon mechanical
subjects there are none that seem to be directed to the instruction
of apprentice engineers; at least, there are none directed to that
part of a mechanical education most difficult to acquire, a power
of analysing and deducing conclusions from commonplace matters.
Our text-books, such as are available for apprentices, consist
mainly of mathematical formulæ relating to forces, the properties
of material, examples of practice, and so on, but do not deal with
the operation of machines nor with constructive manipulation,
leaving out that most important part of a mechanical education,
which consists in special as distinguished from general knowledge.
The theorems, formulæ, constants, tables, and rules, which are
generally termed the principles of mechanics, are in a sense
only symbols of principles; and it is possible, as many facts
will prove, for a learner to master the theories and symbols of
mechanical principles, and yet not be able to turn such knowledge
to practical account.
A principle in mechanics may be known, and even familiar to a
learner, without being logically understood; it might even be
said that both theory and practice may be learned without the
power to connect and apply the two things. A person may, for
example, understand the geometry of tooth gearing and how to lay
out teeth of the proper form for various kinds of wheels, how to
proportion and arrange the spokes, rims, hubs, and so on; he may
also understand the practical application of wheels as a means of
varying or transmitting motion, but between this knowledge and a
complete wheel lies a long train of intricate processes, such as
pattern-making, moulding, casting, boring, and fitting. Farther
on comes other conditions connected with the operation of wheels,
such as adaptation, wear, noise, accidental strains, with many
other things equally as important, as epicycloidal curves or other
geometrical problems relating to wheels.
Text-books, such as relate to construction, consist generally of
examples, drawings, and explanations of machines, gearing, tools,
and so on; such examples are of use to a learner, no doubt, but in
most cases he can examine the machines themselves, and on entering
a shop is brought at once in contact not only with the machines
but also with their operation. Examples and drawings relate to
_how_ machines are constructed, but when a learner comes to the
actual operation of machines, a new and more interesting problem is
reached in the reasons _why_ they are so constructed.
The difference between _how_ machinery is constructed and _why_
it is so constructed, is a wide one. This difference the reader
should keep in mind, because it is to the second query that the
present work will be mainly addressed. There will be an attempt--an
imperfect one, no doubt, in some cases--to deduce from practice
the causes which have led to certain forms of machines, and to
the ordinary processes of workshop manipulation. In the mind of a
learner, whether apprentice or student, the strongest tendency is
to investigate why certain proportions and arrangement are right
and others wrong--why the operations of a workshop are conducted
in one manner instead of another? This is the natural habit of
thought, and the natural course of inquiry and investigation is
deductive.
Nothing can be more unreasonable than to expect an apprentice
engineer to begin by an inductive course in learning and reasoning
about mechanics. Even if the mind were capable of such a course,
which can not be assumed in so intricate and extensive a subject
as mechanics, there would be a want of interest and an absence
of apparent purpose which would hinder or prevent progress. Any
rational view of the matter, together with as many facts as can be
cited, will all point to the conclusion that apprentices must learn
deductively, and that some practice should accompany or precede
theoretical studies. How dull and objectless it seems to a young
man when he toils through "the sum of the squares of the base and
perpendicular of a right-angle triangle," without knowing a purpose
to which this problem is to be applied; he generally wonders why
such puzzling theorems were ever invented, and what they can have
to do with the practical affairs of life. But if the same learner
were to happen upon a builder squaring a foundation by means of the
rule "six, eight, and ten," and should in this operation detect the
application of that tiresome problem of "the sum of the squares,"
he would at once awake to a new interest in the matter; what was
before tedious and without object, would now appear useful and
interesting. The subject would become fascinating, and the learner
would go on with a new zeal to trace out the connection between
practice and other problems of the kind. Nothing inspires a learner
so much as contact with practice; the natural tendency, as before
said, is to proceed deductively.
A few years ago, or even at the present time, many school-books
in use which treat of mechanics in connection with natural
philosophy are so arranged as to hinder a learner from grasping a
true conception of force, power, and motion; these elements were
confounded with various agents of transmission, such as wheels,
wedges, levers, screws, and so on. A learner was taught to call
these things "mechanical powers," whatever that may mean, and
to compute their power as mechanical elements. In this manner
was fixed in the mind, as many can bear witness, an erroneous
conception of the relations between power and the means for its
transmission; the two things were confounded together, so that
years, and often a lifetime, has not served to get rid of the
idea of power and mechanism being the same. To such teaching can
be traced nearly all the crude ideas of mechanics so often met
with among those well informed in other matters. In the great
change from empirical rules to proved constants, from special
and experimental knowledge to the application of science in the
mechanic arts, we may, however, go too far. The incentives to
substitute general for special knowledge are so many, that it may
lead us to forget or underrate that part which cannot come within
general rules.
The labour, dirt, and self-denial inseparable from the acquirement
of special knowledge in the mechanic arts are strong reasons
for augmenting the importance and completeness of theoretical
knowledge, and while it should be, as it is, the constant object to
bring everything, even manipulative processes, so far as possible,
within general rules, it must not be forgotten that there is a
limit in this direction.
In England and America the evils which arise from a false or over
estimate of mere theoretical knowledge have thus far been avoided.
Our workshops are yet, and must long remain, our technological
schools. The money value of bare theoretical training is so fast
declining that we may be said to have passed the point of reaction,
and that the importance of sound practical knowledge is beginning
to be more felt than it was some years ago. It is only in those
countries where actual manufactures and other practical tests are
wanting, that any serious mistake can be made as to what should
constitute an education in mechanics. Our workshops, if other means
fail, will fix such a standard; and it is encouraging to find
here and there among the outcry for technical training, a note of
warning as to the means to be employed.
During the meeting of the British Association in Belfast (1874),
the committee appointed to investigate the means of teaching
Physical Science, reported that "the most serious obstacle
discovered was an absence from the minds of the pupils of a firm
and clear grasp of the concrete facts forming a base of the
reasoning processes they are called upon to study; and that the
use of text-books should be made subordinate to an attendance upon
lectures and demonstrations."
Here, in reference to teaching science, and by an authority which
should command our highest confidence, we have a clear exposition
of the conditions which surround mechanical training, with,
however, this difference, that in the latter "demonstration" has
its greatest importance.
Professor John Sweet of Cornell University, in America, while
delivering an address to the mechanical engineering classes,
during the same year, made use of the following words: "It is not
what you 'know' that you will be paid for; it is what you can
'perform,' that must measure the value of what you learn here."
These few words contain a truth which deserves to be earnestly
considered by every student engineer or apprentice; as a maxim
it will come forth and apply to nearly everything in subsequent
practice.
I now come to speak directly of the present work and its objects.
It may be claimed that a book can go no further in treating of
mechanical manipulation than principles or rules will reach, and
that books must of necessity be confined to what may be called
generalities. This is in a sense true, and it is, indeed, a most
difficult matter to treat of machine operations and shop processes;
but the reason is that machine operations and shop processes have
not been reduced to principles or treated in the same way as
strains, proportions, the properties of material, and so on. I do
not claim that manipulative processes can be so generalised--this
would be impossible; yet much can be done, and many things regarded
as matters of special knowledge can be presented in a way to
come within principles, and thus rendered capable of logical
investigation.
Writers on mechanical subjects, as a rule, have only theoretical
knowledge, and consequently seldom deal with workshop processes.
Practical engineers who have passed through a successful experience
and gained that knowledge which is most difficult for apprentices
to acquire, have generally neither inclination nor incentives to
write books. The changes in manipulation are so frequent, and the
operations so diversified, that practical men have a dread of the
criticisms which such changes and the differences of opinion may
bring forth; to this may be added, that to become a practical
mechanical engineer consumes too great a share of one's life to
leave time for other qualifications required in preparing books.
For these reasons "manipulation" has been neglected, and for the
same reasons must be imperfectly treated here. The purpose is not
so much to instruct in shop processes as to point out how they can
be best learned, the reader for the most part exercising his own
judgment and reasoning powers. It will be attempted to point out
how each simple operation is governed by some general principle,
and how from such operations, by tracing out the principle which
lies at the bottom, it is possible to deduce logical conclusions
as to what is right or wrong, expedient or inexpedient. In this
way, it is thought, can be established a closer connection between
theory and practice, and a learner be brought to realise that he
has only his reasoning powers to rely on; that formulæ, rules,
tables, and even books, are only aids to this reasoning power,
which alone can master and combine the symbol and the substance.
No computations, drawings, or demonstrations of any kind will
be employed to relieve the mind of the reader from the care of
remembering and a dependence on his own exertions. Drawings,
constants, formulæ, tables, rules, with all that pertains to
computation in mechanics, are already furnished in many excellent
books, which leave nothing to be added, and such books can be
studied at the same time with what is presented here.
The book has been prepared with a full knowledge of the fact, that
what an apprentice may learn, as well as the time that is consumed
in learning, are both measured by the personal interest felt in
the subject studied, and that such a personal interest on the part
of an apprentice is essential to permanent success as an engineer.
A general dryness and want of interest must in this, as in all
cases, be a characteristic of any writing devoted to mechanical
subjects: some of the sections will be open to this charge, no
doubt, especially in the first part of the book; but it is trusted
that the good sense of the reader will prevent him from passing
hurriedly over the first part, to see what is said, at the end, of
casting, forging, and fitting, and will cause him to read it as it
comes, which will in the end be best for the reader, and certainly
but fair to the writer.
CHAPTER I.
_PLANS OF STUDYING._
By examining the subject of applied mechanics and shop
manipulation, a learner may see that the knowledge to be acquired
by apprentices can be divided into two departments, that may be
called general and special. General knowledge relating to tools,
processes and operations, so far as their construction and action
may be understood from general principles, and without special
or experimental instruction. Special knowledge is that which is
based upon experiment, and can only be acquired by special, as
distinguished from general sources.
To make this plainer, the laws of forces, the proportion of parts,
strength of material, and so on, are subjects of general knowledge
that may be acquired from books, and understood without the aid
of an acquaintance with the technical conditions of either the
mode of constructing or the manner of operating machines; but
how to construct proper patterns for castings, or how the parts
of machinery should be moulded, forged, or fitted, is special
knowledge, and must have reference to particular cases. The
proportions of pulleys, bearings, screws, or other regular details
of machinery, may be learned from general rules and principles,
but the hand skill that enters into the manufacture of these
articles cannot be learned except by observation and experience.
The general design, or the disposition of metal in machine-framing,
can be to a great extent founded upon rules and constants that have
general application; but, as in the case of wheels, the plans of
moulding such machine frames are not governed by constant rules or
performed in a uniform manner. Patterns of different kinds may be
employed; moulds may be made in various ways, and at a greater and
less expense; the metal can be mixed to produce a hard or a soft
casting, a strong or a weak one; the conditions under which the
metal is poured may govern the soundness or shrinkage,--things that
are determined by special instead of general conditions.
The importance of a beginner learning to divide what he has to
learn into these two departments of special and general, has the
advantage of giving system to his plans, and pointing out that
part of his education which must be acquired in the workshop and
by practical experience. The time and opportunities which might
be devoted to learning the technical manipulations of a foundry,
for instance, would be improperly spent if devoted to metallurgic
chemistry, because the latter may be studied apart from practical
foundry manipulation, and without the opportunity of observing
casting operations.
It may also be remarked that the special knowledge involved in
applied mechanics is mainly to be gathered and retained by personal
observation and memory, and that this part is the greater one;
all the formulæ relating to machine construction may be learned
in a shorter time than is required to master and understand the
operations which may be performed on an engine lathe. Hence first
lessons, learned when the mind is interested and active, should
as far as possible include whatever is special; in short, no
opportunity of learning special manipulation should be lost. If a
wheel pattern come under notice, examine the manure in which it is
framed together, the amount of draught, and how it is moulded, as
well as to determine whether the teeth have true cycloidal curves.
Once, nearly all mechanical knowledge was of the class termed
special, and shop manipulations were governed by empirical rules
and the arbitrary opinions of the skilled; an apprentice entered a
shop to learn a number of mysterious operations, which could not be
defined upon principles, and only understood by special practice
and experiment. The arrangement and proportions of mechanism were
also determined by the opinions of the skilled, and like the
manipulation of the shop, were often hid from the apprentice, and
what he carried in his memory at the end of an apprenticeship was
all that he had gained. The tendency of this was to elevate those
who were the fortunate possessors of a strong natural capacity,
and to depress the position of those less fortunate in the matter
of mechanical "genius," as it was called. The ability to prepare
proper designs, and to succeed in original plans, was attributed
to a kind of intuitive faculty of the mind; in short, the mechanic
arts were fifty years ago surrounded by a superstition of a
different nature, but in its influences the same as superstition in
other branches of knowledge.
But now all is changed: natural phenomena have been explained
as being but the operation of regular laws; so has mechanical
manipulation been explained as consisting in the application of
general principles, not yet fully understood, but far enough,
so that the apprentice may with a substantial education, good
reasoning powers, and determined effort, force his way where once
it had to be begged. The amount of special knowledge in mechanical
manipulation, that which is irregular and modified by special
conditions, is continually growing less as generalisation and
improvement go on.
Another matter to be considered is that the engineering apprentice,
in estimating what he will have to learn, must not lose sight of
the fact that what qualifies an engineer of to-day will fall far
short of the standard that another generation will fix, and of that
period in which his practice will fall. This I mention because it
will have much to do with the conceptions that a learner will form
of what he sees around him. To anticipate improvement and change is
not only the highest power to which a mechanical engineer can hope
to attain, but is the key to his success.
By examining the history of great achievements in the mechanic
arts, it will be seen that success has been mainly dependent upon
predicting future wants, as well as upon an ability to supply such
wants, and that the commercial value of mechanical improvements
is often measured by conditions that the improvements themselves
anticipate. The invention of machine-made drills, for example,
was but a small matter; but the demand that has grown up since,
and because of their existence, has rendered this improvement one
of great value. Moulded bearings for shafts were also a trifling
improvement when first made, but it has since influenced machine
construction in America in a way that has given great importance to
the invention.
It is generally useless and injudicious to either expect or to
search after radical changes or sweeping improvements in machine
manufacture or machine application, but it is important in learning
how to construct and apply machinery, that the means of foreseeing
what is to come in future should at the same time be considered.
The attention of a learner can, for example, be directed to the
division of labour, improvements in shop system, how and where
commercial interests are influenced by machinery, what countries
are likely to develop manufactures, the influence of steam-hammers
on forging, the more extended use of steel when cheapened by
improved processes for producing it, the division of mechanical
industry into special branches, what kind of machinery may become
staple, such as shafts, pulleys, wheels, and so on. These things
are mentioned at random, to indicate what is meant by looking into
the future as well as at the present.
Following this subject of future improvement farther, it may be
assumed that an engineer who understands the application and
operation of some special machine, the principles that govern its
movements, the endurance of the wearing surfaces, the direction and
measure of the strains, and who also understands the principles of
the distribution of material, arrangement, and proportions,--that
such an engineer will be able to construct machines, the plans of
which will not be materially departed from so long as the nature
of the operations to which the machines are applied remain the same.
A proof of this proposition is furnished in the case of standard
machine tools for metal-cutting, a class of machinery that for many
years past has received the most thorough attention at the hands of
our best mechanical engineers.
Standard tools for turning, drilling, planing, boring, and so
on, have been changed but little during twenty years past, and
are likely to remain quite the same in future. A lathe or a
planing-machine made by a first-class establishment twenty years
ago has, in many cases, the same capacity, and is worth nearly
as much in value at the present time as machine tools of modern
construction--a test that more than any other determines their
comparative efficiency and the true value of the improvements
that have been made. The plans of the framing for machine tools
have been altered, and many improvements in details have been
added; yet, upon the whole, it is safe to assume, as before said,
that standard tools for metal-cutting have reached a state of
improvement that precludes any radical changes in future, so long
as the operations in metal-cutting remain the same.
This state of improvement which has been reached in machine-tool
manufacture, is not only the result of the skill expended on such
tools, but because as a notable exception they are the agents of
their own production; that is, machine tools produce machine tools,
and a maker should certainly become skilled in the construction of
implements which he employs continually in his own business. This
peculiarity of machine-tool manufactures is often overlooked by
engineers, and unfair comparisons made between machines of this
class and those directed to wood conversion and other manufacturing
processes, which machinists, as a rule, do not understand.
Noting the causes and conditions which have led to this perfection
in machine-tool manufacture, and how far they apply in the case of
other classes of machinery, will in a measure indicate the probable
improvements and changes that the future will produce.
The functions and adaptations of machinery constitute, as already
explained, the science of mechanical engineering. The functions
of a machine are a foundation on which its plans are based; hence
machine functions and machine effect are matters to which the
attention of an apprentice should first be directed.
In the class of mechanical knowledge that has been defined as
general, construction comes in the third place: first, machine
functions; next, plans or adaptation of machines; and third, the
manner of constructing machines. This should be the order of study
pursued in learning mechanical manipulation. Instead of studying
how drilling-machines, planing-machines or lathes are arranged,
and next plans of constructing them, and then the principles of
their operation, which is the usual course, the learner should
reverse the order, studying, first, drilling, planing, and turning
as operations; next, the adaptation of tools for the purposes; and
third, plans of constructing such tools.
Applied to steam-engines, the same rule holds good. Steam, as a
motive agent, should first be studied, then the operation of steam
machinery, and finally the construction of steam-engines. This is a
rule that may not apply in all cases, but the exceptions are few.
To follow the same chain of reasoning still farther, and to show
what may be gained by method and system in learning mechanics, it
may be assumed that machine functions consist in the application of
power, and therefore power should be first studied; of this there
can be but one opinion. The learner who sets out to master even the
elementary principles of mechanics without first having formed a
true conception of power as an element, is in a measure wasting his
time and squandering his efforts.
Any truth in mechanics, even the action of the "mechanical powers"
before alluded to, is received with an air of mystery, unless the
nature of power is first understood. Practical demonstration a
hundred times repeated does not create a conviction of truth in
mechanical propositions, unless the principles of operation are
understood.
An apprentice may learn that power is not increased or diminished
by being transmitted through a train of wheels which change both
speed and force, and he may believe the proposition without having
a "conviction" of its truth. He must first learn to regard power
as a constant and indestructible element--something that may be
weighed, measured, and transmitted, but not created or destroyed by
mechanism; then the nature of the mechanism may be understood, but
not before.
To obtain a true understanding of the nature of power is by no
means the difficulty for a beginner that is generally supposed;
and when once reached, the truth will break upon the mind like a
sudden discovery, and ever afterwards be associated with mechanism
and motion whenever seen. The learner will afterwards find himself
analysing the flow of water, the traffic in the streets, the
movement of ships and trains; even the act of walking will become
a manifestation of power, all clear and intelligible, without that
air of mystery that is otherwise inseparable from the phenomena of
motion. If the learner will go on farther, and study the connection
between heat and force, the mechanical equivalent of heat when
developed into force and motion, and the reconversion of power into
heat, he will have commenced at the base of what must constitute
a thorough knowledge of mechanics, without which he will have to
continually proceed under difficulties.
I am well aware of the popular opinion that such subjects
are too abstruse to be understood by practical mechanics--an
assumption that is founded mainly in the fact that the subject
of heat and motion are not generally studied, and have been too
recently demonstrated in a scientific way to command confidence
and attention; but the subject is really no more difficult to
understand in an elementary sense than that of the relation between
movement and force illustrated in the "mechanical powers" of
school-books, which no apprentice ever did or ever will understand,
except by first studying the principles of force and motion,
independent of mechanical agents, such as screws, levers, wedges,
and so on.
It is to be regretted that there have not been books especially
prepared to instruct mechanical students in the relations between
heat, force, motion, and practical mechanism. The subject is,
of course, treated at great length in modern scientific works,
but is not connected with the operations of machinery in a way
to be easily understood by beginners. A treatise on the subject,
called "The Correlation and Conservation of Forces," published
by D. Appleton & Co. of New York, is perhaps as good a book on
the subject as can at this time be referred to. The work contains
papers contributed by Professors Carpenter, Grove, Helmholtz,
Faraday, and others, and has the advantage of arrangement in short
sections, that compass the subject without making it tedious.
In respect to books and reading, the apprentice should supply
himself with references. A single book, and the best one that can
be obtained on each of the different branches of engineering,
is enough to begin with. A pocket-book for reference, such as
Molesworth's or Nystrom's, is of use, and should always be at
hand. For general reading, nothing compares with the scientific
and technical journals, which are now so replete with all kinds
of information. Beside noting the present progress of engineering
industry in all parts of the world, they contain nearly all besides
that a learner will require.
It will be found that information of improvements and mechanical
progress that a learner may gather from serial publications can
always be exchanged for special knowledge in his intercourse with
skilled workmen, who have not the opportunity or means of reading
for themselves; and what an apprentice may read and learn in an
hour can often be "exchanged" for experimental knowledge that has
cost years to acquire.
(1.) Into what two divisions can a knowledge of constructive
mechanics be divided?--(2.) Give an example of your own to
distinguish between special and general knowledge.--(3.) In
what manner is special knowledge mostly acquired?--(4.) What
has been the effect of scientific investigations upon special
knowledge?--(5.) What is meant by the division of labour?--(6.)
Why have engineering tools been less changed than most other
kinds of machinery during twenty years past?--(7.) What is meant
by machine functions; adaptation; construction?--(8.) Why has the
name "mechanical powers" been applied to screws, levers, wedges,
and so on?--(9.) Can power be conceived of as an element or
principle, independent of mechanism?
CHAPTER II.
_MECHANICAL ENGINEERING._
This work, as already explained, is to be devoted to mechanical
engineering, and in view of the difference of opinion that exists
as to what mechanical engineering comprehends, and the different
sense in which the term is applied, it will be proper to explain
what is meant by it here.
I am not aware that any one has defined what constitutes civil
engineering, or mechanical engineering, as distinguished one from
the other, nor is it assumed to fix any standard here farther than
to serve the purpose of explaining the sense in which the terms
will be used; yet there seems to be a clear line of distinction,
which, if it does not agree with popular use of the terms, at least
seems to be furnished by the nature of the business itself. It will
therefore be assumed that mechanical engineering relates to dynamic
forces and works that _involve machine motion_, and comprehends
the conditions of machine action, such as torsional, centrifugal,
intermittent, and irregular strains in machinery, arising out
of motion; the endurance of wearing surfaces, the constructive
processes of machine-making and machine effect in the conversion
of material--in short, agents for converting, transmitting, and
applying power.
Civil engineering, when spoken of, will be assumed as referring to
works that do not _involve machine motion_, nor the use of power,
but deal with static forces, the strength, nature, and disposition
of material under constant strains, or under measured strains,
the durability and resistance of material, the construction of
bridges, factories, roads, docks, canals, dams, and so on; also,
levelling and surveying. This corresponds to the most common use
of the term civil engineering in America, but differs greatly from
its application in Europe, where civil engineering is understood as
including machine construction, and where the term engineering is
applied to ordinary manufacturing processes.
Civil engineering, in the meaning assumed for the term, has
become almost a pure mathematical science. Constants are proved
and established for nearly every computation; the strength and
durability of materials, from long and repeated tests, has come
to be well understood; and as in the case of machine tools, the
uniformity of practice among civil engineers, and the perfection
of their works, attest how far civil engineering has become a
true science, and proves that the principles involved in the
construction of permanent works are well understood.
To estimate how much is yet to be learned in mechanical
engineering, we have only to apply the same test, and when we
contrast the great variance between the designs of machines and
the diversity of their operation, even when applied to similar
purposes, their imperfection is at once apparent. It must, however,
be considered that if the rules of construction were uniform, and
the principles of machine operation as well understood as the
strength and arrangement of material in permanent structures,
still there would remain the difficulty of adaptation to new
processes, which are continually being developed.
If the steam-engine, for instance, had forty years ago been brought
to such a state of improvement as to be constructed with standard
proportions and arrangement for stationary purposes, all the rules,
constants, and data of whatever kind that had been collected and
proved, would have been but of little use in adapting steam-engines
to railways and the purposes of navigation.
Mechanical engineering has by the force of circumstances been
divided up into branches relating to engineering tools, railway
machinery, marine engines, and so on; either branch of which
constitutes a profession within itself. Most thorough study will be
required to master general principles, and then a further effort to
acquire proficiency in some special branch, without which there is
but little chance of success at the present day.
To master the various details of machine manufacture, including
draughting, founding, forging, and fitting, is of itself a work
equal to most professional pursuits, to say nothing of manual
skill; and when we come to add machine functions and their
application, generating and transmitting power, with other things
that will necessarily be included in practice, the task assumes
proportions that makes it appear a hopeless one. Besides, the work
of keeping progress with the mechanic arts calls for a continual
accretion of knowledge; and it is no small labour to keep informed
of the continual changes and improvements that are going on in all
parts of the world, which may at any time modify and change both
machines and processes. But few men, even under the most favourable
conditions, have been able to qualify themselves as competent
mechanical engineers sooner than at forty years of age.
One of the earliest cares of an apprentice should be to divest his
mind of what I will call the romance of mechanical engineering,
almost inseparable from such views as are often acquired in
technological schools. He must remember that it is not a science he
is studying, and that mathematics deal only with one branch of what
is to be learned. Special knowledge, or what does not come within
the scope of general principles, must be gained in a most practical
way, at the expense of hard work, bruised fingers, and a disregard
of much that the world calls gentility.
Looking ahead into the future, the apprentice can see a field
for the mechanical engineer widening on every side. As the
construction of permanent works becomes more settled and
uniform, the application of power becomes more diversified,
and develops problems of greater intricacy. No sooner has some
great improvement, like railway and steam navigation, settled
into system and regularity than new enterprises begin. To offset
the undertaking of so great a work as the study of mechanical
engineering, there is the very important advantage of the
exclusiveness of the calling--a condition that arises out of its
difficulties. If there is a great deal to learn, there is also
much to be gained in learning it. It is seldom, indeed, that an
efficient mechanical engineer fails to command a place of trust and
honour, or to accumulate a competency by means of his calling.
If a civil engineer is wanted to survey railways, construct docks,
bridges, buildings, or permanent works of any kind, there are
scores of men ready for the place, and qualified to discharge
the duties; but if an engineer is wanted to design and construct
machinery, such a person is not easy to be found, and if found,
there remains that important question of competency; for the work
is not like that of constructing permanent works, where several
men may and will perform the undertaking very much in the same
manner, and perhaps equally well. In the construction of machinery
it is different; the success will be directly as the capacity of
the engineer, who will have but few precedents, and still fewer
principles, to guide him, and generally has to set out by relying
mainly upon his special knowledge of the operation and application
of such machines as he has to construct.
(1.) How may mechanical be distinguished from civil
engineering?--(2.) What test can be applied to determine the
progress made in any branch of engineering?--(3.) What are some
of the conditions which prevent the use of constants in machine
construction?--(4.) Is mechanical engineering likely to become
more exact and scientific?--(5.) Name some of the principal
branches of mechanical engineering.--(6.) Which is the most
extensive and important?
CHAPTER III.
_ENGINEERING AS A CALLING._
It may in the abstract be claimed that the dignity of any pursuit
is or should be as the amount of good it confers, and the influence
it exerts for the improvement of mankind. The social rank of
those engaged in the various avocations of life has, in different
countries and in different ages, been defined by various standards.
Physical strength and courage, hereditary privilege, and other
things that once recommended men for preferment, have in most
countries passed away or are regarded as matters of but little
importance, and the whole civilised world have agreed upon one
common standard, that knowledge and its proper use shall be the
highest and most honourable attainment to which people may aspire.
It may be useless or even wrong to institute invidious comparisons
between different callings which are all useful and necessary, and
the matter is not introduced here with any view of exalting the
engineering profession; it is for some reasons regretted that the
subject is alluded to at all, but there is too much to be gained by
an apprentice having a pride and love for his calling to pass over
the matter of its dignity as a pursuit without calling attention
to it. The gauntlet has been thrown down and comparison provoked
by the unfair and unreasonable place that the politician, the
metaphysician, and the moral philosopher have in the past assigned
to the sciences and constructive arts. Poetry, metaphysics,
mythology, war, and superstition have in their time engrossed the
literature of the world, and formed the subject of what was alone
considered education.
In a half century past all has changed; the application of the
sciences, the utilisation of natural forces, manufacturing, the
transportation of material, the preparation and diffusion of
printed matter, and other great matters of human interest, have
come to shape our laws, control commerce, establish new relations
between people and countries--in short, has revolutionised the
world. So rapid has been this change that it has outrun the powers
of conception, and people waken as from a dream to find themselves
governed by a new master.
Considering material progress as consisting primarily in the
demonstration of scientific truths, and secondly, in their
application to useful purposes, we can see the position of the
engineer as an agent in this great work of reconstruction now going
on around us. The position is a proud one, but not to be attained
except at the expense of great effort, and a denial of everything
that may interfere with the acquirement of knowledge during
apprenticeship and the study which must follow.
The mechanical engineer deals mainly with the natural forces, and
their application to the conversion of material and transport. His
calling involves arduous duties; he is brought in contact with
what is rough and repulsive, as well as what is scientific and
refined. He must include grease, dirt, manual labour, undesirable
associations, and danger with apprenticeship, or else be content to
remain without thoroughly understanding his profession.
(1.) What should determine the social rank of industrial
callings?--(2.) Why have the physical sciences and mechanic arts
achieved so honourable a position?--(3.) How may the general
object of the engineering arts be described?--(4.) What is the
difference between science and art as the terms are generally
employed in connection with practical industry?
CHAPTER IV.
_THE CONDITIONS OF APPRENTICESHIP._
Were it not that moral influences in learning mechanics, as in all
other kinds of education, lie at the bottom of the whole matter,
the subject of this chapter would not have been introduced. But
it is the purpose, so far as possible, to notice everything that
concerns an apprentice and learner, and especially what he has
to deal with at the outset; hence some remarks upon the nature
of apprentice engagements will not be out of place. To acquire
information or knowledge of any kind successfully and permanently,
it must be a work of free volition, as well as from a sense of duty
or expediency; and whatever tends to create love and respect for a
pursuit or calling, becomes one of the strongest incentives for
its acquirement, and the interest taken by an apprentice in his
business is for this reason greatly influenced by the opinions that
he may hold concerning the nature of his engagement.
The subject of apprentice engagements seems in the abstract to
be only a commercial one, partaking of the nature of ordinary
contracts, and, no doubt, can be so construed so far as being an
exchange of "considerations," but no farther. Its intricacy is
established by the fact that all countries where skilled labour
exists have attempted legislation to regulate apprenticeship, and
to define the terms and conditions between master and apprentice;
but, aside from preventing the abuse of powers delegated to
masters, and in some cases forcing a nominal fulfilment of
conditions defined in contracts, such legislation, like that
intended to control commerce and trade, or the opinions of men, has
failed to attain the objects for which it was intended.
This failure of laws to regulate apprenticeship, which facts
fully warrant us in assuming, is due in a large degree to the
impossibility of applying general rules to special cases; it may be
attributed to the same reasons which make it useless to fix values
or the conditions of exchange by legislation. What is required is
that the master, the apprentice, and the public should understand
the true relations between them--the value of what is given and
what is received on both sides. When this is understood, the whole
matter will regulate itself without any interference on the part of
the law.
The subject is an intricate one, and has been so much affected
by the influence of machine improvement, and a corresponding
decrease in what may be called special knowledge, that rules and
propositions which would fifty years ago apply to the conditions
of apprenticeship, will at the present day be wrong and unjust.
Viewed in a commercial sense, as an exchange of considerations or
values, apprenticeship can be regarded like other engagements;
yet, what an apprentice gives as well as what he receives are
alike too conditional and indefinite to be estimated by ordinary
standards. An apprentice exchanges unskilled or inferior labour for
technical knowledge, or for the privilege and means of acquiring
such knowledge. The master is presumed to impart a kind of special
knowledge, collected by him at great expense and pains, in return
for the gain derived from the unskilled labour of the learner.
This special knowledge given by the master may be imparted in a
longer or shorter time; it may be thorough and valuable, or not
thorough, and almost useless. The privileges of a shop may be such
as to offset a large amount of valuable labour on the part of the
apprentice, or these privileges may be of such a character as to be
of but little value, and teach inferior plans of performing work.
On the other hand, the amount that an apprentice may earn by his
labour is governed by his natural capacity, and by the interest
he may feel in advancing; also from the view he may take of the
equity of his engagement, and the estimate that he places upon
the privileges and instruction that he receives. In many branches
of business, where the nature of the operations carried on are
measurably uniform, and have not for a long time been much affected
by changes and improvements, the conditions of apprenticeship are
more easy to define; but mechanical engineering is the reverse
of this, it lacks uniformity both as to practice and what is
produced. To estimate the actual value of apprentice labour in
an engineering-work is not only a very difficult matter, but to
some extent impracticable even by those of long experience and
skilled in such investigations; and it is not to be expected that
a beginner will under such circumstances be able to understand the
value of such labour: he is generally led to the conclusion that he
is unfairly treated, that his services are not sufficiently paid
for, and that he is not advanced rapidly enough.
With these conclusions in his mind, but little progress will be
made, and hence the reason for introducing the subject here.
The commercial value of professional or technical knowledge is
generally as the amount of time, effort, and unpaid labour that has
been devoted to its acquirement. This value is sometimes modified
by the exclusiveness of some branch that has been made the object
of special study. Exclusiveness is, however, becoming exceptional,
as the secrets of manufacture and special knowledge are supplanted
by the application of general principles; it is a kind of
artificial protection thrown around certain branches of industry,
and must soon disappear, as unjust to the public and unnecessary to
success.
In business arrangements, technical knowledge and professional
experience become capital, and offset money or property, not under
any general rule, nor even as a consideration of which the law can
define the value or prescribe conditions for. The estimate placed
upon technical knowledge when rated as capital in the organisation
of business firms, and wherever it becomes necessary to give such
knowledge a commercial value, furnishes the best and almost the
only source from which an apprentice can form an opinion of the
money value of what he is to acquire during his apprenticeship.
An apprentice at first generally forms an exaggerated estimate of
what he has to learn; it presents to his mind not only a great
undertaking, but a kind of mystery, which he fears that he may
not be able to master. The next stage is when he has made some
progress, and begins to underrate the task before him, and imagine
that the main difficulties are past, that he has already mastered
all the leading principles of mechanics, which is, after all,
but a "small matter." In a third stage an apprentice experiences
a return of his first impressions as to the difficulties of his
undertaking; he begins to see his calling as one that must involve
endless detail, comprehending things which can only be studied in
connection with personal experience; he sees "the horizon widen as
it recedes," that he has hardly begun the task, instead of having
completed it--even despairs of its final accomplishment.
In the workshop, mechanical knowledge of some kind is continually
and often insensibly acquired by a learner, who observes the
operations that are going on around him; he is continually availing
himself of the experience of those more advanced, and learns by
association the rules and customs of the shop, of the business,
and of discipline and management. He gathers the technical terms
of the fitting-shop, the forge and foundry; notes the operations
of planing, turning, drilling, and boring, with the names and
application of the machines directed to these operations. He sees
the various plans of lifting and moving material, the arrangement
and relation of the several departments to facilitate the course
of the work in process; he also learns where the product of the
works is sold, discusses the merits and adaptation of what is
constructed, which leads to considering the wants that create a
demand for this product, and the extent and nature of the market in
which it is sold.
All these things constitute technical knowledge, and the privilege
of their acquirement is an element of value. The common view taken
of the matter, however, is that it costs nothing for a master to
afford these privileges--the work must at any rate be carried on,
and is not retarded by being watched and learned by apprentices.
Viewed from any point, the privileges of engineering establishments
have to be considered as an element of value, to be bought at a
price, just as a ton of iron or a certain amount of labour is; and
in a commercial sense, as an exchangeable equivalent for labour,
material, or money. In return a master receives the unskilled
labour or service of the learner; this service is presumed to be
given at a reduced rate, or sometimes without compensation, for the
privileges of the works and the instruction received.
In forming an estimate of the value of his services, an apprentice
sees what his hands have performed, compares it with what a skilled
man will do, and estimates accordingly, assuming that his earnings
are in proportion to what has been done; but this is a mistake, and
a very different standard must be assumed to arrive at the true
value of such unskilled labour.
Apprentice labour, as distinguished from skilled labour, has to
be charged with the extra attention in management, the loss that
is always occasioned by a forced classification of the work, the
influence in lowering both the quality and the amount of work
performed by skilled men, the risk of detention by failure or
accident, and loss of material; besides, apprentices must be
charged with the same, if not a greater expense than skilled
workmen, for light, room, oil, tools, and office service.
Attempts have been made in some of the best-regulated engineering
establishments to fix some constant estimate upon apprentice
labour, but, so far as known, without definite results in any case.
If not combined with skilled labour, it would be comparatively easy
to determine the value of apprentice labour; but when it comes up
as an item in the aggregate of labour charged to a machine or some
special work constructed, it is difficult, if not impossible, to
separate skilled from unskilled service.
Another condition of apprenticeship that is equally as difficult to
define as the commercial value of mechanical knowledge, or that of
apprentice labour, is the extent and nature of the facilities that
different establishments afford for learners.
In speaking of the mechanical knowledge to be gained, and of the
privileges afforded for learners in engineering-works in a general
way, it must, of course, be assumed that such works afford full
facilities for learning some branch of work by the best practice
and in the most thorough manner. Such establishments are, however,
graded from the highest class, on the best branches of work, where
a premium would be equitable, down to the lowest class, performing
only inferior branches of work, where there can be little if any
advantage gained by serving an apprenticeship.
Besides this want or difference of facilities which establishments
may afford, there is the farther distinction to be made between
an engineering establishment and one that is directed to the
manufacture of staple articles. This distinction between
engineering-works and manufacturing is quite plain to engineers
themselves, but in many cases is not so to those who are to enter
as apprentices, nor to their friends who advise them. In every case
where engagements are made there should be the fullest possible
investigation as to the character of the works, not only to protect
the learner, but to guard regular engineering establishments in
the advantages to be gained by apprentice labour. A machinist or
a manufacturer who employs only the muscular strength and the
ordinary faculties of workmen in his operations, can afford to pay
an apprentice from the beginning a fair share of his earnings;
but an engineering-work that projects original plans, generates
designs, and assumes risks based upon skill and special knowledge,
is very different from a manufactory. To manufacture is to carry
on regular processes for converting material; such processes being
constantly the same, or approximately so, and such as do not demand
much mechanical knowledge on the part of workmen.
The name of having been an apprentice to a famous firm may sometimes
have an influence in enabling an engineer to form advantageous
commercial connections, but generally an apprenticeship is of value
only as it has furnished substantial knowledge and skill; for every
one must sooner or later come down to the solid basis of their
actual abilities and acquirements. The engineering interest is by
far too practical to recognise a shadow instead of true substance,
and there is but little chance of deception in a calling which deals
mainly with facts, figures, and positive demonstration.
It is best, when an apprentice thinks of entering an engineering
establishment, to inquire of its character from disinterested
persons who are qualified to judge of the facilities it affords.
As a rule, every machine-shop proprietor imagines his own
establishment to combine all the elements of an engineering
business--and the fewer the facilities for learners, usually the
more extravagant this estimate; so that opinions in the matter, to
be relied upon, should come from disinterested sources.
In regard to premiums, it is a matter to be determined by the
facilities that a work may afford for teaching apprentices. To
include experience in all the departments of an engineering
establishment, within a reasonable term, none but those of unusual
ability can make their services of sufficient value to offset what
they receive; and there is no doubt but that premium engagements,
when the amount of the premium is based upon the facilities
afforded for learning, are fair and equitable.
There is, however, this to be remembered, that the considerations
which more especially balance premiums--such as a term at
draughting, designing, and office service--may be mainly acquired
by self-effort, while the practical knowledge of moulding, forging,
and fitting cannot; and an apprentice who has good natural
capacity, may, if industrious, by the aid of books and such
opportunities as usually exist, qualify himself very well without
including the premium departments in his course.
Finally, it must constantly be borne in mind that what will be
learned is no less a question of faculties than effort, and that
the means of succeeding are closed to none who at the beginning
form proper plans, and follow them persistently.
(1.) Why cannot the conditions of apprentice engagements be
determined by law?--(2.) In what manner does machine improvements
affect the conditions of apprenticeship?--(3.) What are the
considerations which pass from a master to an apprentice?--(4.)
What from an apprentice to a master?--(5.) Why is a particular
service of less value when performed by an apprentice than by a
skilled workman?--(6.) In what manner can technical knowledge
be made to balance or become capital?--(7.) Name two of the
principal distinctions between technical knowledge and property
as constituting capital.--(8.) What is the difference between
what is called engineering and regular manufactures?
CHAPTER V.
_THE OBJECT OF MECHANICAL INDUSTRY._
Mechanical engineering, like every other business pursuit, is
directed to the accumulation of wealth; and as the attainment
of any purpose is more surely achieved by keeping that purpose
continually in view, there will be no harm, and perhaps
considerable gain derived by an apprentice considering at the
beginning the main object to which his efforts will be directed
after learning his profession or trade. So far as an abstract
principle of motives, the subject is of course unfit to consider
in connection with engineering operations, or shop manipulation;
but business objects have a practical application to be followed
throughout the whole system of industrial pursuits, and are as
proper to be considered in connection with machine-manufacturing as
mechanical principles, or the functions and operation of machines.
The cost of production is an element that continually modifies
or improves manufacturing processes, determines the success
of every establishment, and must be considered continually in
making drawings, patterns, forgings, and castings. Machines are
constructed because of _the difference between what they cost and
what they sell for_--between their manufacturing cost and market
value when they are completed.
It seems hard to deprive engineering pursuits of the romance that
is often attached to the business, and bring it down to a matter
of commercial gain; but it is best to deal with facts, especially
when such facts have an immediate bearing upon the general object
in view. There is no intention in these remarks of disparaging the
works of many noble men, who have given their means, their time,
and sometimes their lives, to the advancement of the industrial
arts, without hope or desire of any other reward than the
satisfaction of having performed a duty; but we are dealing with
facts, and no false colouring should prevent a learner from forming
practical estimates of practical matters.
The following propositions will place this subject of aims and
objects before the reader in the sense intended:--
_First._ The main object of mechanical engineering is commercial
gain--the profits derived from planning and constructing machinery.
_Second._ The amount of gain so derived is as the difference
between the cost of constructing machinery, and the market value of
the machinery when completed.
_Third._ The difference between what it costs to plan and construct
machinery and what it will sell for, is generally as the amount of
engineering knowledge and skill brought to bear in the processes of
production.
This last sentence brings the matter into a tangible form, and
indicates what the subject of gain should have to do with what
an apprentice learns of machine construction. Success in an
engineering enterprise may be temporarily achieved by illegitimate
means--such as misrepresentation of the capacity and quality of
what is produced, the use of cheap or improper material, or by
copying the plans of others to avoid the expense of engineering
service--but in the end the permanent success of an engineering
business must rest upon the knowledge and skill that is connected
with it.
By examining into the facts, an apprentice will find that all truly
successful establishments have been founded and built upon the
mechanical abilities of some person or persons whose skill formed
a base upon which the business was reared, and that true skill is
the element which must in the end lead to permanent success. The
material and the labour which make up the first cost of machines
are, taking an average of various classes, nearly equally divided;
labour being in excess for the finer class of machinery, and the
material in excess for the coarser kinds of work. The material is
presumed to be purchased at the same rates by those of inferior
skill as by those that are well skilled, so that the difference in
the first, or manufacturing cost of machinery, is determined mainly
by skill.
Skill, in the sense employed here, consists not only in preparing
plans and in various processes for converting and shaping
material, but also in the general conduct of an establishment,
including estimates, records, system, and so on, which will be
noticed in their regular order. The amount of labour involved, and
consequently the first cost of machinery, is in a large degree as
the number of mechanical processes required, and the time consumed
in each operation; to reduce the number of these processes or
operations, shorten the time in which they may be performed,
and improve the quality of what is produced, is the business of
the mechanical engineer. A careful study of shop operations or
processes, including designing, draughting, moulding, forging, and
fitting, is the secret of success in engineering practice, or in
the management of manufactures. The advantages of an economical
design, and the most carefully-prepared drawings, are easily
neutralised and lost by careless or improper manipulation in the
workshop; an incompetent manager may waste ten pounds in shop
processes, while the commercial department of a work saves one
pound by careful buying and selling.
This importance of shop processes in machine construction is
generally realised by proprietors, but not thoroughly understood in
all of its bearings; an apprentice may notice the continual effort
that is made to augment the production of engineering-works, which
is the same thing as shortening the processes.
A machine may be mechanically correct, arranged with symmetry, true
proportions, and proper movements; but if such a machine has not
commercial value, and is not applicable to a useful purpose, it
is as much a failure as though it were mechanically inoperative.
In fact, this consideration of cost and commercial value must
be continually present; and a mechanical education that has not
furnished a true understanding of the relations between commercial
cost and mechanical excellence will fall short of achieving the
objects for which such an education is undertaken. By reasoning
from such premises as have been laid down, an apprentice may form
true standards by which to judge of plans and processes that he
is brought in contact with, and the objects for which they are
conducted.
(1.) To what general object are all pursuits directed?--(2.) What
besides wealth may be objects in the practice of engineering
pursuits?--(3.) Name some of the most common among the causes
which reduce the cost of production.--(4.) Name five of the
main elements which go to make up the cost of engineering
products.--(5.) Why is commercial success generally a true test
of the skill connected with engineering-works?
CHAPTER VI.
_ON THE NATURE AND OBJECTS OF MACHINERY._
Machines do not create or consume, but only transmit and apply
power; and it is only by conceiving of power as a constant element,
independent of every kind of machinery, that the learner can reach
a true understanding of the nature of machines. When once there is
in the mind a fixed conception of power, dissociated from every
kind of mechanism, there is laid, so to speak, a solid foundation
on which an understanding of machines may be built up.
To believe a fact is not to learn it, in the sense that these terms
may be applied to mechanical knowledge; to believe a proposition
is not to have a conviction of its truth; and what is meant by
learning mechanical principles is, as remarked in a previous place,
to have them so fixed in the mind that they will involuntarily
arise to qualify everything met with that involves mechanical
movement. For this reason it has been urged that learners should
begin by first acquiring a clear and fixed conception of power, and
next of the nature and classification of machines, for without the
first he cannot reach the second.
Machines may be defined in general terms as agents for converting,
transmitting, and applying power, or motion and force, which
constitute power. By machinery the natural forces are utilised,
and directed to the performance of operations where human strength
is insufficient, when natural force is cheaper, and when the rate
of movement exceeds what the hands can perform. The term "agent"
applied to machines conveys a true idea of their nature and
functions.
Machinery can be divided into four classes, each constituting a
division that is very clearly defined by functions performed, as
follows:--
_First._ Motive machinery for utilising or converting the natural
forces.
_Second._ Machinery for transmitting and distributing power.
_Third._ Machinery for applying power.
_Fourth._ Machinery of transportation.
Or, more briefly stated--
Motive machinery.
Machinery of transmission.
Machinery of application.
Machinery of transportation.
These divisions of machinery will next be treated of separately,
with a view of making the classification more clear, and to explain
the principles of operation in each division. This dissertation
will form a kind of base upon which the practical part of the
treatise will in a measure rest. It is trusted that the reader will
carefully consider each proposition that is laid down, and on his
own behalf pursue the subjects farther than the limits here permit.
(1.) To what three general objects are machines directed?--(2.)
How are machines distinguished from other works or
structures?--(3.) Into what four classes can machinery be
divided?--(4.) Name one principal type in each of these four
divisions.
CHAPTER VII.
_MOTIVE MACHINERY._
In this class belong--
Steam-engines.
Caloric or air engines.
Water-wheels or water-engines.
Wind-wheels or pneumatic engines.
These four types comprehend the motive-power in general use at the
present day. In considering different engines for motive-power in
a way to best comprehend their nature, the first view to be taken
is that they are all directed to the same end, and all deal with
the same power; and in this way avoid, if possible, the impression
of there being different kinds of power, as the terms water-power,
steam-power, and so on, seem to imply. We speak of steam-power,
water-power, or wind-power; but power is the same from whatever
source derived, and these distinctions merely indicate different
natural sources from which power is derived, or the different means
employed to utilise and apply it.
Primarily, power is a product of heat; and wherever force and
motion exist, they can be traced to heat as the generating
element: whether the medium through which the power is obtained be
by the expansion of water or gases, the gravity of water, or the
force of wind, heat will always be found as the prime source. So
also will the phenomenon of expansion be found a constant principle
of developing power, as will again be pointed out. As steam-engines
constitute a large share of the machinery commonly met with, and as
a class of machinery naturally engrosses attention in proportion,
the study of mechanics generally begins with steam-engines, or
steam machinery, as it may be called.
The subject of steam-power, aside from its mechanical
consideration, is one that may afford many useful lessons, by
tracing its history and influence, not only upon mechanical
industry, but upon human interests generally. This subject is often
treated of, and both its interest and importance conceded; but no
one has, so far as I know, from statistical and other sources,
ventured to estimate in a methodical way the changes that can be
traced directly and indirectly to steam-power.
The steam-engine is the most important, and in England and America
best known among motive agents. The importance of steam contrasted
with other sources of motive-power is due not so much to a
diminished cost of power obtained in this way, but for the reason
that the amount of power produced can be determined at will, and
in most cases without reference to local conditions; the machinery
can with fuel and water be transported from place to place, as in
the case of locomotives which not only supply power for their own
transit, but move besides vast loads of merchandise, or travel.
For manufacturing processes, one importance of steam-power rests
in the fact that such power can be taken to the material; and
beside other advantages gained thereby, is the difference in the
expense of transporting manufactured products and the raw material.
In the case of iron manufacture, for example, it would cost ten
times as much to transport the ore and the fuel used in smelting
as it does to transport the manufactured iron; steam-power saves
this difference, and without such power our present iron traffic
would be impossible. In a great many manufacturing processes steam
is required for heating, bleaching, boiling, and so on; besides,
steam is now to a large extent employed for warming buildings, so
that even when water or other power is employed, in most cases
steam-generating apparatus has to be set up in addition. In many
cases waste steam or waste heat from a steam-engine can be employed
for the purposes named, saving most of the expense that must be
incurred if special apparatus is employed.
Other reasons for the extended and general use of steam as a power,
besides those already named, are to be found in the fact that no
other available element or substance can be expanded to a given
degree at so small a cost as water; and that its temperature will
not rise to a point injurious to machinery, and, further, in the
very important property of lubrication which steam possesses,
protecting the frictional surfaces of pistons and valves, which it
is impossible to keep oiled because of their inaccessibility or
temperature.
The steam-engine, in the sense in which the term is employed, means
not only steam-using machinery, but steam-generating machinery or
plant; it includes the engine proper, with the boiler, mechanism
for feeding water to the boiler, machinery for governing speed,
indicators, and other details.
An apprentice must guard against the too common impression that
the engine, cylinder, piston, valves, and so on, are the main
parts of steam machinery, and that the boiler and furnace are
only auxiliaries. The boiler is, in fact, the base of the whole,
that part where the power is generated, the engine being merely
an agent for transmitting power from the boiler to work that is
performed. This proposition would, of course, be reached by any one
in reasoning about the matter and following it to a conclusion, but
the fact should be fixed in the mind at the beginning.
When we look at a steam-engine there are certain impressions
conveyed to the mind, and by these impressions we are governed in
a train of reflection that follows. We may conceive of a cylinder
and its details as a complete machine with independent functions,
or we can conceive of it as a mechanical device for transmitting
the force generated by a boiler, and this conception might be
independent of, or even contrary to, specific knowledge that we at
the same time possessed; hence the importance of starting with a
correct idea of the boiler being, as we may say, the base of steam
machinery.
As reading books of fiction sometimes expands the mind and enables
it to grasp great practical truths, so may a study of abstract
principles often enable us to comprehend the simplest forms
of mechanism. Even Humboldt and Agassiz, it is said, resorted
sometimes to imaginative speculations as a means of enabling them
to grasp new truths.
In no other branch of machinery has so much research and experiment
been made during eighty years past as in steam machinery, and,
strange to say, the greater part of this research has been directed
to the details of engines; yet there has been no improvement made
during the time which has effected any considerable saving of
heat or expense. The steam-engines of fifty years ago, considered
as steam-using machines, utilised nearly the same proportion of
the energy or power developed by the boiler as the most improved
engines of modern construction--a fact that in itself indicates
that an engine is not the vital part of steam machinery. There is
not the least doubt that if the efforts to improve steam-engines
had been mainly directed to economising heat and increasing
the evaporative power of boilers, much more would have been
accomplished with the same amount of research. This remark,
however, does not apply to the present day, when the principles of
steam-power are so well understood, and when heat is recognised
as the proper element to deal with in attempts to diminish the
expense of power. There is, of course, various degrees of economy
in steam-using as well as in steam-generating machinery; but so
long as the best steam machinery does not utilise but one-tenth
or one-fifteenth part of the heat represented in the fuel burned,
there need be no question as to the point where improvements in
such machinery should be mainly directed.
The principle upon which steam-engines operate may be briefly
explained as follows:--
A cubic inch of water, by taking up a given amount of heat,
is expanded to more than five hundred cubic inches of steam,
at a pressure of forty-five pounds to the square inch. This
extraordinary expansion, if performed in a close vessel, would
exert a power five hundred times as great as would be required
to force the same quantity of water into the vessel against this
expansive pressure; in other words, the volume of the water when
put into the vessel would be but one five-hundredth part of its
volume when it is allowed to escape, and this expansion, when
confined in a steam-boiler, exerts the force that is called
steam-power. This force or power is, through the means of the
engine and its details, communicated and applied to different kinds
of work where force and movement are required. The water employed
to generate steam, like the engine and the boiler, is merely an
agent through which the energy of heat is applied.
This, again, reaches the proposition that power is heat, and heat
is power, the two being convertible, and, according to modern
science, indestructible; so that power, when used, must give off
its mechanical equivalent of heat, or heat, when utilised, develop
its equivalent in power. If the whole amount of heat represented
in the fuel used by a steam-engine could be applied, the effect
would be, as before stated, from ten to fifteen times as great as
it is in actual practice, from which it must be inferred that a
steam-engine is a very imperfect machine for utilising heat. This
great loss arises from various causes, among which is that the
heat cannot be directly nor fully communicated to the water. To
store up and retain the water after it is expanded into steam, a
strong vessel, called a boiler, is required, and all the heat that
is imparted to the water has to pass through the plates of this
boiler, which stand as a wall between the heat and its work.
To summarise, we have the following propositions relating to steam
machinery:--
1. The steam-engine is an agent for utilising the power of heat and
applying it to useful purposes.
2. The power of a steam-engine is derived by expanding water in a
confining vessel, and employing the force exerted by pressure thus
obtained.
3. The power developed is as the difference of volume between the
feed-water forced into the boiler, and the volume of the steam that
is drawn from the boiler, or as the amount of heat taken up by the
water.
4. The heat that may be utilised is what will pass through the
plates of the boiler, and be taken up by the water, and is but a
small share of what the fuel produces.
5. The boiler is the main part, where power is generated, and the
engine is but an agent for transmitting this power to the work
performed.
6. The loss of power in a steam-engine arises from the heat carried
off in the exhaust steam, loss by radiation, and the friction of
the moving parts.
7. By condensing the steam before it leaves the engine, so that the
steam is returned to the air in the form of water, and of the same
volume as when it entered the boiler, there is a gain effected by
avoiding atmospheric pressure, varying according to the perfection
of the arrangements employed.
Engines operated by means of hot air, called caloric engines,
and engines operated by gas, or explosive substances, all act
substantially upon the same general principles as steam-engines;
the greatest distinction being between those engines wherein the
generation of heat is by the combustion of fuel, and those wherein
heat and expansion are produced by chemical action. With the
exception of a limited number of caloric or air engines, steam
machinery comprises nearly all expansive engines that are employed
at this day for motive-power; and it may be safely assumed that a
person who has mastered the general principles of steam-engines
will find no trouble in analysing and understanding any machinery
acting from expansion due to heat, whether air, gas, or explosive
agents be employed.
This method of treating the subject of motive-engines will no
doubt be presenting it in a new way, but it is merely beginning at
an unusual place. A learner who commences with first principles,
instead of pistons, valves, connections, and bearings, will find
in the end that he has not only adopted the best course, but the
shortest one to understand steam and other expansive engines.
(1.) What is principal among the details of steam
machinery?--(2.) What has been the most important improvement
recently made in steam machinery?--(3.) What has been the result
of expansive engines generally stated?--(4.) Why has water proved
the most successful among various expansive substances employed
to develop power?--(5.) Why does a condensing engine develop
more power than a non-condensing one?--(6.) How far back from
its development into power can heat be traced as an element in
nature?--(7.) Has the property of combustion a common source in
all substances?
CHAPTER VIII.
_WATER-POWER._
Water-wheels, next to steam-engines, are the most common motive
agents. For centuries water-wheels remained without much
improvement or change down to the period of turbine wheels, when
it was discovered that instead of being a very simple matter, the
science of hydraulics and water-wheels involved some very intricate
conditions, giving rise to many problems of scientific interest,
that in the end have produced the class known as turbine wheels.
A modern turbine water-wheel, one of the best construction,
operating under favourable conditions, gives a percentage of the
power of the water which, after deducting the friction of the
wheel, almost reaches the theoretical coefficient or equals the
gravity of the water; it may therefore be assumed that there will
in the future be but little improvement made in such water-wheels
except in the way of simplifying and cheapening their construction.
There is, in fact, no other class of machines which seem to
have reached the same state of improvement as water-wheels, nor
any other class of machinery that is constructed with as much
uniformity of design and arrangement, in different countries, and
by different makers.
Water-wheels, or water-power, as a mechanical subject, is
apparently quite disconnected with shop manipulation, but will
serve as an example for conveying general ideas of force and
motion, and, on these grounds, will warrant a more extended notice
than the seeming connection with the general subject calls for.
In the remarks upon steam-engines it was explained that power is
derived from heat, and that the water and the engine were both to
be regarded as agents through which power was applied, and further,
that power is always a product of heat. There is, perhaps, no
problem in the whole range of mechanics more interesting than to
trace the application of this principle in machinery; one that is
not only interesting but instructive, and may suggest to the mind
of an apprentice a course of investigation that will apply to many
other matters connected with power and mechanics.
Power derived from water by means of wheels is due to the gravity
of the water in descending from a higher to a lower level; but
the question arises, What has heat to do with this? If heat is
the source of power, and power a product of heat, there must be a
connection somewhere between heat and the descent of the water.
Water, in descending from one level to another, can give out no
more power than was consumed in raising it to the higher level, and
this power employed to raise the water is found to be heat. Water
is evaporated by heat of the sun, expanded until it is lighter
than the atmosphere, rises through the air, and by condensation
falls in the form of rain over the earth's surface; then drains
into the ocean through streams and rivers, to again resume its
round by another course of evaporation, giving out in its descent
power that we turn to useful account by means of water-wheels. This
principle of evaporation is continually going on; the fall of rain
is likewise quite constant, so that streams are maintained within a
sufficient regularity to be available for operating machinery.
The analogy between steam-power and water-power is therefore quite
complete. Water is in both cases the medium through which power is
obtained; evaporation is also the leading principle in both, the
main difference being that in the case of steam-power the force
employed is directly from the expansion of water by heat, and in
water-power the force is an indirect result of expansion of water
by heat.
Every one remembers the classification of water-wheels met with in
the older school-books on natural philosophy, where we are informed
that there are three kinds of wheels, as there were "three kinds
of levers"--namely, overshot, undershot, and breast wheels--with
a brief notice of Barker's mill, which ran apparently without any
sufficient cause for doing so. Without finding fault with the plan
of describing water-power commonly adopted in elementary books,
farther than to say that some explanation of the principles by
which power is derived from the water would have been more useful,
I will venture upon a different classification of water-wheels,
more in accord with modern practice, but without reference to the
special mechanism of the different wheels, except when unavoidable.
Water-wheels can be divided into four general types.
_First._ Gravity wheels, acting directly from the weight of the
water which is loaded upon a wheel revolving in a vertical plane,
the weight resting upon the descending side until the water has
reached the lowest point, where it is discharged.
_Second._ Impact wheels, driven by the force of spouting water
that expends its percussive force or momentum against the vanes
tangental to the course of rotation, and at a right angle to the
face of the vanes or floats.
_Third._ Reaction wheels, that are "enclosed," as it is termed,
and filled with water, which is allowed to escape under pressure
through tangental orifices, the propelling force being derived from
the unbalanced pressure within the wheel, or from the reaction due
to the weight and force of the water thrown off from the periphery.
_Fourth._ Pressure wheels, acting in every respect upon the
principle of a rotary steam-engine, except in the differences that
arise from operating with an elastic and a non-elastic fluid; the
pressure of the water resting continually against the vanes and
"abutment," without means of escape except by the rotation of the
wheel.
To this classification may be added combination wheels, acting
partly by the gravity and partly by the percussion force of
the water, by impact combined with reaction, or by impact and
maintained pressure.
Gravity, or "overshot" wheels, as they are called, for some reasons
will seem to be the most effective, and capable of utilising the
whole effect due to the gravity of the water; but in practice this
is not the case, and it is only under peculiar conditions that
wheels of this class are preferable to turbine wheels, and in no
case will they give out a greater per cent. of power than turbine
wheels of the best class. The reasons for this will be apparent by
examining the conditions of their operation.
A gravity wheel must have a diameter equal to the fall of water,
or, to use the technical name, the height of the head. The speed
at the periphery of the wheel cannot well exceed sixteen feet
per second without losing a part of the effect by the wheel
anticipating or overrunning the water. This, from the large
diameter of the wheels, produces a very slow axial speed, and a
train of multiplying gearing becomes necessary in order to reach
the speed required in most operations where power is applied.
This train of gearing, besides being liable to wear and accident,
and costing usually a large amount as an investment, consumes a
considerable part of the power by frictional resistance, especially
when such gearing consists of tooth wheels. Gravity wheels, from
their large size and their necessarily exposed situation, are
subject to be frozen up in cold climates; and as the parts are
liable to be first wet and then dry, or warm and cold by exposure
to the air and the water alternately, the tendency to corrosion
if constructed of iron, or to decay if of wood, is much greater
than in submerged wheels. Gravity wheels, to realise the highest
measure of effect from the water, require a diameter so great that
they must drag in the water at the bottom or delivering side, and
are for this reason especially affected by back-water, to which
all wheels are more or less liable from the reflux of tides or by
freshets. These disadvantages are among the most notable pertaining
to gravity wheels, and have, with other reasons--such as the
inconvenience of construction, greater cost, and so on--driven such
wheels out of use by the force of circumstances, rather than by
actual tests or theoretical deductions.
Impact wheels, or those driven by the percussive force of water,
including the class termed turbine water-wheels, are at this time
generally employed for heads of all heights.
The general theory of their action may be explained in the
following propositions:--
1. The spouting force of water is theoretically equal to its
gravity.
2. The percussive force of spouting water can be fully utilised if
its motion is altogether arrested by the vanes of a wheel.
3. The force of the water is greatest by its striking against
planes at right angles to its course.
4. Any force resulting from water rebounding from the vanes
parallel to their face, or at any angle not reverse to the motion
of the wheel, is lost.
5. This rebounding action becomes less as the columns of water
projected upon the wheel are increased in number and diminished in
size.
6. To meet the conditions of rotation in the wheel, and to
facilitate the escape of the water without dragging, after it has
expended its force upon the vanes, the reversed curves of the
turbine is the best-known arrangement.
It is, of course, very difficult to deal with so complex a subject
as the present one with words alone, and the reader is recommended
to examine drawings, or, what is better, water-wheels themselves,
keeping the above propositions in view.
Modern turbine wheels have been the subject of the most careful
investigation by able engineers, and there is no lack of
mathematical data to be referred to and studied after the general
principles are understood. The subject, as said, is one of great
complicity if followed to detail, and perhaps less useful to a
mechanical engineer who does not intend to confine his practice to
water-wheels, than other subjects that may be studied with greater
advantage. The subject of water-wheels may, indeed, be called an
exhausted one that can promise but little return for labour spent
upon it--with a view to improvements, at least. The efforts of the
ablest hydraulic engineers have not added much to the percentage of
useful effect realised by turbine wheels during many years past.
Reaction wheels are employed to a limited extent only, and will
soon, no doubt, be extinct as a class of water-wheels. In speaking
of reaction wheels, I will select what is called Barker's mill for
an example, because of the familiarity with which it is known,
although its construction is greatly at variance with modern
reaction wheels.
There is a problem as to the principle of action in a Barker wheel,
which although it may be very clear in a scientific sense, remains
a puzzle to the minds of many who are well versed in mechanics,
some contending that the power is directly from pressure, others
that it is from the dynamic effect due to reaction. It is one of
the problems so difficult to determine by ordinary standards, that
it serves as a matter of endless debate between those who hold
different views; and considering the advantage usually derived from
such controversies, perhaps the best manner of disposing of the
problem here is to state the two sides as clearly as possible, and
leave the reader to determine for himself which he thinks right.
Presuming the vertical shaft and the horizontal arms of a Barker
wheel to be filled with water under a head of sixteen feet, there
would be a pressure of about seven pounds upon each superficial
inch of surface within the cross arm, exerting an equal force in
every direction. By opening an orifice at the sides of these arms
equal to one inch of area, the pressure would at that point be
relieved by the escape of the water, and the internal pressure be
unbalanced to that extent. In other words, opposite this orifice,
and on the other side of the arm, there would be a force of seven
pounds, which being unbalanced, acts as a propelling power to drive
the wheel.
This is one theory of the principle upon which the Barker wheel
operates, which has been laid down in Vogdes' "Mensuration," and
perhaps elsewhere. The other theory alluded to is that, direct
action and reaction being equal, ponderable matter discharged
tangentally from the periphery of a wheel must create a reactive
force equal to the direct force with which the weight is thrown
off. To state it more plainly, the spouting water that issues from
the arm of a Barker wheel must react in the opposite course in
proportion to its weight.
The two propositions may be consistent with each other or even
identical, but there still remains an apparent difference.
The latter seems a plausible theory, and perhaps a correct one; but
there are two facts in connection with the operation of reaction
water-wheels which seem to controvert the latter and favour the
first theory, namely, that reaction wheels in actual practice
seldom utilise more than forty per cent. of useful effect from the
water, and that their speed may _exceed the initial velocity of
the water_. With this the subject is left as one for argument or
investigation on the part of the reader.
Pressure wheels, like gravity wheels, should, from theoretical
inference, be expected to give a high per cent. of power. The
water resting with the whole of its weight against the vanes or
abutments, and without chance of escape except by turning the
wheel, seems to meet the conditions of realising the whole effect
due to the gravity of the water, and such wheels would no doubt
be economical if they had not to contend with certain mechanical
difficulties that render them impracticable in most cases.
A pressure wheel, like a steam-engine, must include running contact
between water-tight surfaces, and like a rotary steam-engine, this
contact is between surfaces which move at different rates of speed
in the same joint, so that the wear is unequal, and increases as
the speed or the distance from the axis. When it is considered that
the most careful workmanship has never produced rotary engines that
would surmount these difficulties in working steam, it can hardly
be expected they can be overcome in using water, which is not only
liable to be filled with grit and sediment, but lacks the peculiar
lubricating properties of steam. A rotary steam-engine is in effect
the same as a pressure water-wheel, and the apprentice in studying
one will fully understand the principles of the other.
(1.) What analogy may be found between steam and water
power?--(2.) What is the derivation of the name turbine?--(3.)
To what class of water-wheels is this name applicable?--(4.)
How may water-wheels be classified?--(5.) Upon what principle
does a reaction water-wheel operate?--(6.) Can ponderable weight
and pressure be independently considered in the case?--(7.) Why
cannot radial running joints be maintained in machines?--(8.)
Describe the mechanism in common use for sustaining the weight of
turbine wheels, and the thrust of propeller shafts.
CHAPTER IX.
_WIND-POWER._
Wind-power, aside from the objections of uncertainty and
irregularity, is the cheapest kind of motive-power. Steam
machinery, besides costing a large sum as an investment, is
continually deteriorating in value, consumes fuel, and requires
continual skilled attention. Water-power also requires a large
investment, greater in many cases than steam-power, and in
many places the plant is in danger of destruction by freshets.
Wind-power is less expensive in every way, but is unreliable
for constancy except in certain localities, and these, as it
happens, are for the most part distant from other elements of
manufacturing industry. The operation of wind-wheels is so simple
and so generally understood that no reference to mechanism need
be made here. The force of the wind, moving in right lines, is
easily applied to producing rotary motion, the difference from
water-power being mainly in the comparative weakness of wind
currents and the greater area required in the vanes upon which
the wind acts. Turbine wind-wheels have been constructed on
very much the same plan as turbine water-wheels. In speaking of
wind-power, the propositions about heat must not be forgotten.
It has been explained how heat is almost directly utilised by
the steam-engine, and how the effect of heat is utilised by
water-wheels in a less direct manner, and the same connection will
be found between heat and wind-wheels or wind-power. Currents of
air are due to changes of temperature, and the connection between
the heat that produces such air currents and their application as
power is no more intricate than in the case of water-power.
(1.) What is the difference in general between wind and water
wheels?--(2.) Can the course of wind, like that of water, be
diverted and applied at pleasure?--(3.) On what principle does
wind act against the vanes of a wheel?--(4.) How may an analogy
between wind-power and heat be traced?
CHAPTER X.
_MACHINERY FOR TRANSMITTING AND DISTRIBUTING POWER._
To construe the term "transmission of power" in its full sense,
it will, when applied to machinery, include nearly all that has
motion; for with the exception of the last movers, or where power
passes off and is expended upon work that is performed, all
machinery of whatever kind may be called machinery of transmission.
Custom has, however, confined the use of the term to such devices
as are employed to convey power from one place to another, without
including organised machines through which power is directly
applied to the performance of work. Power is transmitted by means
of shafts, belts, friction wheels, gearing, and in some cases by
water or air, as various conditions of the work to be performed may
require. Sometimes such machinery is employed as the conditions do
not require, because there is, perhaps, nothing of equal importance
connected with mechanical engineering of which there exists a
greater diversity of opinion, or in which there is a greater
diversity of practice, than in devices for transmitting power.
I do not refer to questions of mechanical construction, although
the remark might be true if applied in this sense, but to the kind
of devices that may be best employed in certain cases.
It is not proposed at this time to treat of the construction
of machinery for transmitting power, but to examine into the
conditions that should determine which of the several plans of
transmitting is best in certain cases--whether belts, gearing, or
shafts should be employed, and to note the principles upon which
they operate. Existing examples do not furnish data as to the
advantages of the different plans for transmitting power, because
a given duty may be successfully performed by belts, gearing,
or shafts--even by water, air, or steam--and the comparative
advantages of different means of transmission is not always an easy
matter to determine.
Machinery of transmission being generally a part of the fixed
plant of an establishment, experiments cannot be made to institute
comparisons, as in the case of machines; besides, there are special
or local considerations--such as noise, danger, freezing, and
distance--to be taken into account, which prevent any rules of
general application. Yet in every case it may be assumed that some
particular plan of transmitting power is better than any other, and
that plan can best be determined by studying, first, the principles
of different kinds of mechanism and its adaptation to the special
conditions that exist; and secondly, precedents or examples.
A leading principle in machinery of transmission that more than
any other furnishes data for strength and proper proportions
is, that the stress upon the machinery, whatever it may be, is
inverse as the speed at which it moves. For example, a belt two
inches wide, moving one thousand feet a minute, will theoretically
perform the same work that one ten inches wide will do, moving at a
speed of two hundred feet a minute; or a shaft making two hundred
revolutions a minute will transmit four times as much power as a
shaft making but fifty revolutions in the same time, the torsional
strain being the same in both cases.
This proposition argues the expediency of reducing the proportions
of mill gearing and increasing its speed, a change which has
gradually been going on for fifty years past; but there are
opposing conditions which make a limit in this direction, such as
the speed at which bearing surfaces may run, centrifugal strain,
jar, and vibration. The object is to fix upon a point between what
high speed, light weight, cheapness of cost suggest, and what the
conditions of practical use and endurance demand.
(1.) What does the term "machinery of transmission" include,
as applied in common use?--(2.) Why cannot direct comparisons
be made between shafts, belts, and gearing?--(3.) Define
the relation between speed and strain in machinery of
transmission.--(4.) What are the principal conditions which limit
the speed of shafts?
CHAPTER XI.
_SHAFTS FOR TRANSMITTING POWER._
There is no use in entering upon detailed explanations of what
a learner has before him. Shafts are seen wherever there is
machinery; it is easy to see the extent to which they are employed
to transmit power, and the usual manner of arranging them. Various
text-books afford data for determining the amount of torsional
strain that shafts of a given diameter will bear; explain that
their capacity to resist torsional strain is as the cube of the
diameter, and that the deflection from transverse strains is so
many degrees; with many other matters that are highly useful and
proper to know. I will therefore not devote any space to these
things here, but notice some of the more obscure conditions that
pertain to shafts, such as are demonstrated by practical experience
rather than deduced from mathematical data. What is said will
apply especially to what is called line-shafting for conveying
and distributing power in machine-shops and other manufacturing
establishments. The following propositions in reference to shafts
will assist in understanding what is to follow:--
1. The strength of shafts is governed by their size and the
arrangement of their supports.
2. The capacity of shafts is governed by their strength and the
speed at which they run taken together.
3. The strains to which shafts are subjected are the torsional
strain of transmission, transverse strain from belts and wheels,
and strains from accidents, such as the winding of belts.
4. The speed at which shafts should run is governed by their size,
the nature of the machinery to be driven, and the kind of bearings
in which they are supported.
5. As the strength of shafts is determined by their size, and
their size fixed by the strains to which they are subjected,
strains are first to be considered.
There were three kinds of strain mentioned--torsional, deflective,
and accidental. To meet these several strains the same means have
to be provided, which is a sufficient size and strength to resist
them; hence it is useless to consider each of these different
strains separately. If we know which of the three is greatest, and
provide for that, the rest, of course, may be disregarded. This,
in practice, is found to be accidental strains to which shafts are
in ordinary use subjected, and they are usually made, in point
of strength, far in excess of any standard that would be fixed
by either torsional or transverse strain due to the regular duty
performed.
This brings us back to the old proposition, that for structures
which do not involve motion, mathematical data will furnish
dimensions; but the same rule will not apply in machinery. To
follow the proportions for shafts that would be furnished by
pure mathematical data would in nearly all cases lead to error.
Experience has demonstrated that for ordinary cases, where power is
transmitted and applied with tolerable regularity, a shaft three
inches in diameter, making one hundred and fifty revolutions a
minute, its bearings three to four diameters in length, and placed
ten feet apart, will safely transmit fifty horse-power.
By assuming this or any other well-proved example, and estimating
larger or smaller shafts by keeping their diameters as the cube
root of the power to be transmitted, the distance between bearings
as the diameter, and the speed inverse as the diameter, the reader
will find his calculations to agree approximately with the modern
practice of our best engineers. This is not mentioned to give
proportions for shafts, so much as to call attention to accidental
strains, such as winding belts, and to call attention to a marked
discrepancy between actual practice and such proportions as would
be given by what has been called the measured or determinable
strains to which shafts are subjected.
As a means for transmitting power, shafts afford the very
important advantage that power can be easily taken off at any
point throughout their length, by means of pulleys or gearing,
also in forming a positive connection between the motive-power
and machines, or between the different parts of machines. The
capacity of shafts in resisting torsional strain is as the cube of
their diameter, and the amount of torsional deflection in shafts
is as their length. The torsional capacity being based upon the
diameter, often leads to the construction of what may be termed
diminishing shafts, lines in which the diameter of the several
sections are diminished as the distance from the driving power
increases, and as the duty to be performed becomes less. This plan
of arranging line shafting has been and is yet quite common, but
certainly was never arrived at by careful observation. Almost every
plan of construction has both advantages and disadvantages, and
the best means of determining the excess of either, in any case,
is to first arrive at all the conditions as near as possible,
then form a "trial balance," putting the advantages on one side
and the disadvantages on the other, and footing up the sums for
comparison. Dealing with this matter of shafts of uniform diameter
and shafts of varying diameter in this way, there may be found in
favour of the latter plan a little saving of material and a slight
reduction of friction as advantages. The saving of material relates
only to first cost, because the expense of fitting is greater in
constructing shafts when the diameters of the different pieces
vary; the friction, considering that the same velocity throughout
must be assumed, is scarcely worth estimating.
For disadvantages there is, on the other hand, a want of uniformity
in fittings that prevents their interchange from one part of a line
shaft to the other--a matter of great importance, as such exchanges
are frequently required. A line shaft, when constructed with
pieces of varying diameter, is special machinery, adapted to some
particular place or duty, and not a standard product that can be
regularly manufactured as a staple article by machinists, and thus
afforded at a low price. Pulleys, wheels, bearings, and couplings
have all to be specially prepared; and in case of a change, or the
extension of lines of shafting, cause annoyance, and frequently
no little expense, which may all be avoided by having shafts of
uniform diameter. The bearings, besides being of varied strength
and proportions, are generally in such cases placed at irregular
intervals, and the lengths of the different sections of the shaft
are sometimes varied to suit their diameter. With line shafts of
uniform diameter, everything pertaining to the shaft--such as
hangers, couplings, pulleys, and bearings--is interchangeable; the
pulleys, wheels, bearings, or hangers can be placed at pleasure,
or changed from one part of the shaft to another, or from one part
of the works to another, as occasion may require. The first cost
of a line of shafting of uniform diameter, strong enough for a
particular duty, is generally less than that of a shaft consisting
of sections varying in size. This may at first seem strange, but a
computation of the number of supports required, with the expense of
special fitting, will in nearly all cases show a saving.
Attention has been called to this case as one wherein the
conditions of operation obviously furnish true data to govern the
arrangement of machinery, instead of the determinable strains
to which the parts are subjected, and as a good example of the
importance of studying mechanical conditions from a practical and
experimental point of view. If the general diameter of a shaft is
based upon the exact amount of power to be transmitted, or if the
diameter of a shaft at various parts is based upon the torsional
stress that would be sustained at these points, such a shaft would
not only fail to meet the conditions of practical use, but would
cost more by attempting such an adaptation. The regular working
strain to which shafts are subjected is inversely as the speed at
which they run. This becomes a strong reason in favour of arranging
shafts to run at a maximum speed, provided there was nothing more
than first cost to consider; but there are other and more important
conditions to be taken into account, principal among which are the
required rate of movement where power is taken off to machines, and
the endurance of bearings.
In the case of line shafting for manufactories, if the speed varies
so much from that of the first movers on machines as to require
one or more intermediate or countershafts, the expense would be
very great; on the contrary, if countershafts can be avoided, there
is a great saving of belts, bearings, machinery, and obstruction.
The practical limit of speed for line shafts is in a great measure
dependent upon the nature of the bearings, a subject that will be
treated of in another place.
(1.) What kind of strains are shafts subjected to?--(2.) What
determines the strength of shafts in resisting transverse
strain?--(3.) Why are shafts often more convenient than belts
for transmitting power?--(4.) What is the difference between the
strains to which shafts and belts are subjected?--(5.) What is
gained by constructing a line shaft of sections diminishing in
size from the first mover?--(6.) What is gained by constructing
line shafts of uniform diameter?
CHAPTER XII.
_BELTS FOR TRANSMITTING POWER._
The traction of belts upon pulleys, like that of locomotive wheels
upon railways, being incapable of demonstration except by actual
experience, for a long time hindered the introduction of belts as a
means of transmitting motion and power except in cases when gearing
or shafts could not be employed. Motion is named separately,
because with many kinds of machinery that are driven at high
speed--such as wood machines--the transmission of rapid movement
must be considered as well as power, and in ordinary practice it is
only by means of belts that such high speeds may be communicated
from one shaft to another.
The first principle to be pointed out in regard to belts, to
distinguish them from shafts as a means of transmitting power,
is that power is communicated by means of tensile instead of
torsional strain, the power during transmission being represented
in the difference of tension between the driving and the slack
side of belts. In the case of shafts, their length, or the
distance to which they may be extended in transmitting power,
is limited by torsional resistance; and as belts are not liable
to this condition, we may conclude that unless there are other
difficulties to be contended with, belts are more suitable than
shafts for transmitting power throughout long distances. Belts
suffer resistance from the air and from friction in the bearings of
supporting pulleys, which are necessary in long horizontal belts;
with these exceptions they are capable of moving at a very high
rate of speed, and transmitting power without appreciable loss.
Following this proposition into modern engineering examples, we
find how practice has gradually conformed to what these properties
in belts suggest. Wire and other ropes of small diameter, to avoid
air friction, and allowed to droop in low curves to avoid too many
supporting pulleys, are now in many cases employed for transmitting
power through long distances, as at Schaffhausen, in Germany.
This system has been very successfully applied in some cases for
distributing power in large manufacturing establishments. Belts,
among which are included all flexible bands, do not afford the
same facilities for taking off power at different points as shafts,
but have advantages in transmitting power to portable machinery,
when power is to be taken off at movable points, as in the case of
portable travelling cranes, machines, and so on.
An interesting example in the use of belts for communicating
power to movable machinery is furnished by the travelling cranes
of Mr Ramsbottom, in the shops of the L. & N. W. Railway, at
Crewe, England, where powerful travelling cranes receive both the
lifting and traversing power by means of a cotton rope not more
than three-fourths of an inch in diameter, which moves at a high
velocity, the motion being reduced by means of tangent wheels
and gearing to attain the force required in lifting heavy loads.
Observing the operation of this machinery, a person not familiar
with the relations between force and motion will be astonished at
the effect produced by the small rope which communicates power to
the machinery.
Considered as means for transmitting power, the contrast as to
advantages and disadvantages lies especially between belts and
gearing instead of between belts and shafts. It is true in extreme
cases, such as that cited at Crewe, or in conveying water-power
from inaccessible places, through long distances, the comparison
lies between belts and shafts; but in ordinary practice, especially
for first movers, the problem as to mechanism for conveying power
lies between belts and gear wheels. If experience in the use of
belts was thorough, as it is in the case of gearing, and if the
quality of belts did not form so important a part in the estimates,
there would be but little difficulty in determining where belts
should be employed and where gearing would be preferable. Belts are
continually taking the place of gearing even in cases where, until
quite recently, their use has been considered impracticable; one
of the largest rolling mills in Pittsburg, Pennsylvania, except
a single pair of spur wheels as the last movers at each train of
rolls, is driven by belts throughout.
Leaving out the matter of a positive relative movement between
shafts, which belts as a means of transmitting power cannot insure,
there are the following conditions that must be considered in
determining whether belts or other means should be employed in
transmitting power from one machine to another or between the parts
of machines.
1. The distance to which power is to be transmitted.
2. The speed at which the transmitting machinery must move.
3. The course or direction of transmission, whether in straight
lines or at angles.
4. The cost of construction and durability.
5. The loss of power during transmission.
6. Danger, noise, vibration, and jar.
In every case where there can be a question as to whether gearing
shafts or belts will be the best means of transmitting power,
the several conditions named will furnish a solution if they are
properly investigated and understood. Speed, noise, or angles may
become determinative conditions, and are such in a large number
of cases; first cost and loss of power are generally secondary
conditions. Applying these tests to cases where belts, shafts,
or wheels may be employed, a learner will soon find himself in
possession of knowledge to guide him in his own schemes, and enable
him to judge of the correctness of examples that come under his
notice.
It is never enough to know that any piece of work is commonly
constructed in some particular manner, or that a proposition is
generally accepted as being correct; a reason should be sought
for. Nothing is learned, in the true sense, until the reasons for
it are understood, and it is by no means sufficient to know from
observation alone that belts are best for high speeds, that gearing
is the best means of forming angles in transmitting power, or
that gearing consumes more power, and that belts produce less jar
and noise; the principles which lie at the bottom must be reached
before it can be assumed that the matter is fairly understood.
(1.) Why have belts been found better than shafts for
transmitting power through long distances?--(2.) What are the
conditions which limit the speed of belts?--(3.) Why cannot
belts be employed to communicate positive movement?--(4.) Would
a common belt transmit motion positively, if there were no slip
on the pulleys?--(5.) Name some of the circumstances to be
considered in comparing belts with gearing or shafts as a means
of transmitting power.
CHAPTER XIII.
_GEARING AS A MEANS OF TRANSMITTING POWER._
The term gearing, which was once applied to wheels, shafts, and the
general mechanism of mills and factories, has now in common use
become restricted to tooth wheels, and is in this sense employed
here. Gearing as a means of transmitting motion is employed when
the movement of machines, or the parts of machines, must remain
relatively the same, as in the case of the traversing screw of an
engine lathe--when a heavy force is transmitted between shafts
that are near to each other, or when shafts to be connected are
arranged at angles with each other. This rule is of course not
constant, except as to cases where positive relative motion has
to be maintained. Noise, and the liability to sudden obstruction,
may be reasons for not employing tooth wheels in many cases when
the distance between and the position of shafts would render
such a connection the most durable and cheap. Gearing under
ordinary strain, within limited speed, and when other conditions
admit of its use, is the cheapest and most durable mechanism
for transmitting power; but the amount of gearing employed in
machinery, especially in Europe, is no doubt far greater than it
will be in future, when belts are better understood.
No subject connected with mechanics has been more thoroughly
investigated than that of gearing. Text-books are replete with
every kind of information pertaining to wheels, at least so far
as the subject can be made a mathematical one; and to judge from
the amount of matter, formulæ, and diagrams, relating to the teeth
of wheels that an apprentice will meet with, he will no doubt be
led to believe that the main object of modern engineering is to
generate wheels. It must be admitted that the teeth of wheels
and the proportions of wheels is a very important matter to
understand, and should be studied with the greatest care; but it is
equally important to know how to produce the teeth in metal after
their configuration has been defined on paper; to understand the
endurance of teeth under abrasive wear when made of wrought or cast
iron, brass or steel; how patterns can be constructed from which
correct wheels may be cast, and how the teeth of wheels can be cut
by machinery, and so on.
A learner should, in fact, consider the application and operative
conditions of gearing as one of the main parts of the subject, and
the geometry or even the construction of wheels as subsidiary; in
this way attention will be directed to that which is most difficult
to learn, and a part for which facilities are frequently wanting.
Gearing may be classed into five modifications--spur wheels, bevel
wheels, tangent wheels, spiral wheels, and chain wheels; the last
I include among gearing because the nature of their operation is
analogous to tooth wheels, although at first thought chains seem
to correspond more to belts than gearing. The motion imparted
by chains meshing over the teeth of wheels is positive, and not
frictional as with belts; the speed at which such chains may run,
with other conditions, correspond to gearing.
Different kinds of gearing can be seen in almost every engineering
establishment, and in view of the amount of scientific information
available, it will only be necessary to point out some of the
conditions that govern the use and operation of the different kinds
of wheels. The durability of gearing, aside from breaking, is
dependent upon pressure and the amount of rubbing action that takes
place between the teeth when in contact. Spur wheels, or bevel
wheels, when the pitch is accurate and the teeth of the proper
form, if kept clean and lubricated, wear but little, because the
contact between the teeth is that of rolling instead of sliding.
In many cases, one wheel of a pair is filled with wooden cogs;
in this arrangement there are four objects, to avoid noise, to
attain a degree of elasticity in the teeth, to retain lubricants
by absorption in the wood, and to secure by wear a better
configuration of the teeth than is usually attained in casting, or
even in cutting teeth.
Tangent wheels and spiral gearing have only what is termed line
contact between the bearing surfaces, and as the action between
these surfaces is a sliding one, such wheels are subject to
rapid wear, and are incapable of sustaining much pressure, or
transmitting a great amount of power, except the surfaces be hard
and lubrication constant. In machinery the use of tangent wheels
is mainly to secure a rapid change of speed, usually to diminish
motion and increase force.
By placing the axes of tangent gearing so that the threads or
teeth of the pinions are parallel to the face of the driven teeth,
as in the planing machines of Messrs Wm. Sellers & Co., the
conditions of operation are changed, and an interesting problem
arises. The progressive or forward movement of the pinion teeth
may be equal to the sliding movement between the surfaces; and an
equally novel result is, that the sliding action is distributed
over the whole breadth of the driven teeth.
In spiral gearing the line of force is at an angle of forty-five
degrees with the bearing faces of the teeth, and the sliding
movement equal to the speed of the wheels at their periphery;
the bearing on the teeth, as before said, is one of line contact
only. Such wheels cannot be employed except in cases where an
inconsiderable force is to be transmitted. Spiral wheels are
employed to connect shafts that cross each other at right angles
but in different planes, and when the wheels can be of the same
size.
It may be mentioned in regard to rack gearing for communicating
movement to the carriages of planing machines or other purposes of
a similar nature: the rack can be drawn to the wheel, and a lifting
action avoided, by shortening the pitch of the rack, so that it
will vary a little from the driving wheel. The rising or entering
teeth in this case do not come in contact with those on the rack
until they have attained a position normal to the line of the
carriage movement.
(1.) Into what classes can gearing be divided?--(2.) What
determines the wearing capacity of gearing?--(3.) What is the
advantage gained by employing wooden cogs for gear wheels?--(4.)
Why are tangent or worm wheels not durable?
CHAPTER XIV.
_HYDRAULIC APPARATUS FOR TRANSMITTING POWER._
Although a system but recently developed, the employment of
hydraulic machinery for transmitting and applying power has reached
an extended application to a variety of purposes, and gives promise
of a still more extensive use in future. Considered as a means of
transmitting regularly a constant amount of power, water apparatus
is more expensive and inferior in many respects to belts or shafts,
and its use must be traced to some special principle involved which
adapts hydraulic apparatus to the performance of certain duties.
This principle will be found to consist in storing up power in
such a manner that it may be used with great force at intervals;
and secondly, in the facilities afforded for multiplying force
by such simple mechanism as pumps. An engine of ten-horse-power,
connected with machinery by hydraulic apparatus, may provide for a
force equal to one hundred horse-power for one-tenth part of the
time, the power being stored up by accumulators in the interval;
or in other words, the motive power acting continuously can be
accumulated and applied at intervals as it may be required for
raising weights, operating punches, compressive forging, or other
work of an intermittent character. Hydraulic machinery employed
for such purposes is more simple and inexpensive than gearing and
shafts, especially in the application of a great force acting for
a considerable distance, and where a cylinder and piston represent
a degree of strength which could not be attained with twice the
amount of detail, if gearing, screws, levers, or other devices were
employed instead.
Motion or power may be varied to almost any degree by the ratio
between the pistons of pumps and the pistons which give off the
power, the same general arrangement of machinery answering in all
cases; whereas, with gearing the quantity of machinery has to be
increased as the motive power and the applied power may vary in
time and force. This as said recommends hydraulic apparatus where a
great force is required at intervals, and it is in such cases that
it was first employed, and is yet for the most part used.
In the use of hydraulic apparatus for transmitting and applying
power, there is, however, this difficulty to be contended with:
water is inelastic, and for the performance of irregular duty,
there is a loss of power equal to the difference between the duty
that a piston may perform and what it does perform; that is, the
amount of water, and consequently the amount of power given off,
is as the movement and volume of the water, instead of as the work
done. The application of hydraulic machinery to the lifting and
handling of weights will be further noticed in another place.
(1.) Under what conditions is hydraulic apparatus a suitable
means for transmitting power?--(2.) To what class of operations
is hydraulic apparatus mostly applied?--(3.) Why is not water
as suitable a medium as air or steam in transmitting power for
general purposes?
CHAPTER XV.
_PNEUMATIC MACHINERY FOR TRANSMITTING POWER._
Pneumatic machinery, aside from results due to the elasticity of
air, is analogous in operation to hydraulic machinery.
Water may be considered as a rigid medium for transmitting power,
corresponding to shafts and gear wheels; air as a flexible or
yielding one, corresponding to belts. There is at this time but
a limited use of pneumatic apparatus for transmitting power, but
its application is rapidly extending, especially in transporting
material by means of air currents, and in conveying power to mining
machinery.
The successful application of the pneumatic system at the Mont
Cenis Tunnel in Italy, and at the Hoosac Tunnel in America, has
demonstrated the value of the system where the air not only served
to transmit power to operate the machinery but to ventilate the
mines at the same time. Air brakes for railway trains are another
example illustrating the advantages of pneumatic transmission;
the force being multiplied at the points where it is applied,
so that the connecting pipes are reduced to a small size, the
velocity of the air making up for a great force that formerly had
to be communicated through rods, chains, or shafts. The principal
object attained by the use of air to operate railway brakes is,
however, to maintain a connection throughout a train by means of
flexible pipes that accommodate themselves to the varying distance
between the carriages. Presuming that the flow of air in pipes is
not materially impeded by friction or angles, and that there will
be no difficulty in maintaining lubrication for pistons or other
inaccessible parts of machinery when driven by air, there seems to
be many reasons in favour of its use as a means of distributing
power in manufacturing districts. The diminished cost of motive
power when it is generated on a large scale, and the expense and
danger of maintaining an independent steam power for each separate
establishment where power is employed, especially in cities, are
strong reasons in favour of generating and distributing power by
compressed air, through pipes, as gas and water are now supplied.
Air seems to be the most natural and available medium for
transmitting and distributing power upon any general system like
water or gas, and there is every probability of such a system
existing at some future time. The power given out by the expansion
of air is not equal to the power consumed in compressing it, but
the loss is but insignificant compared with the advantages that
may be gained in other ways. There is no subject more interesting,
and perhaps few more important for an engineering student to study
at this time, than the transmission of power and the transport of
material by pneumatic apparatus.
In considering pneumatic machinery there are the following points
to which attention is directed:--
1. The value of pneumatic apparatus in reaching places where steam
furnaces cannot be employed.
2. The use that may be made of air after it has been applied as a
motive agent.
3. The saving from condensation, to which steam is exposed,
avoidance of heat, and the consequent contraction and expansion of
long conducting pipes.
4. The loss of power by friction and angles in conducting air
through pipes.
5. The lubrication of surfaces working under air pressure, such as
the pistons and valves of engines.
6. The diminished cost of generating power on a large scale,
compared with a number of separate steam engines distributed over
manufacturing districts.
7. The effect of pneumatic machinery in reducing insurance rates
and danger of fire.
8. The expense of the appliances of distribution and their
maintenance.
In passing thus rapidly over so important a subject, and one that
admits of so extended a consideration as machinery of transmission,
the reader can see that the purpose has been to touch only upon
such points as will lead to thought and investigation, and
especially to meet such queries as are most likely to arise in
the mind of a learner. In arranging and erecting machinery of
transmission, obviously the first problem must be, what kind of
machinery should be employed, and what are the conditions which
should determine the selection and arrangement? What has been
written has, so far as possible, been directed to suggesting proper
means of solving these questions.
(1.) In what respect are air and water like belts and gearing,
as means to transmit power?--(2.) What are some of the
principal advantages gained by employing air to operate railway
brakes?--(3.) Name some of the advantages of centralising motive
power.--(4.) Are the conditions of working an engine the same
whether air or steam is employed?
CHAPTER XVI.
_MACHINERY OF APPLICATION._
The term application has been selected as a proper one to
distinguish machines that expend and apply power, from those
that are employed in generating or transmitting power. Machines
of application employed in manufacturing, and which expend their
action on material, are directed to certain operations which are
usually spoken of as processes, such as cutting, compressing,
grinding, separating, and disintegrating.
By classifying these processes, it will be seen that there is in
all but a few functions to be performed by machines, and that they
all act upon a few general principles. Engineering tools employed
in fitting are, for example, all directed to the process of
cutting. Planing machines, lathes, drilling machines, and shaping
machines are all cutting machines, acting upon the same general
plan--that of a cleaving wedge propelled in straight or curved
lines.
Cutting, as a process in converting material, includes the force
to propel cutting edges, means to guide and control their action,
and mechanism to sustain and adjust the material acted upon. In
cutting with hand tools, the operator performs the two functions
of propelling and guiding the tools with his hands; but in what
is called power operations, machines are made to perform these
functions. In nearly all processes machines have supplanted hand
labour, and it may be noticed in the history and development of
machine tools that much has been lost in too closely imitating hand
operations when machines were first applied. To be profitable,
machines must either employ more force, guide tools with more
accuracy, or move them at greater speed, than is attainable by
hand. Increased speed may, although more seldom, be an object in
the employment of machinery, as well as the guidance of implements
or increased force in propelling them. The hands of workmen are not
only limited as to the power that may be exerted, and unable to
guide tools with accuracy, but are also limited to a slow rate of
movement, so that machines can be employed with great advantage in
many operations where neither the force nor guidance of tools are
wanting.
There is nothing more interesting, or at the same time more useful,
in the study of mechanics, than to analyse the action of cutting
machines or other machinery of application, and to ascertain in
examples that come under notice whether the main object of a
machine is increased force, more accurate guidance, or greater
speed than is attainable by hand operations. Cutting machines as
explained may be directed to either of these objects singly, or to
all of them together, or these objects may vary in their relative
importance in different operations; but in all cases where machines
are profitably employed, their action can be traced to one or more
of the functions named.
To follow this matter further. It will be found in such machines as
are directed mainly to augmenting force or increasing the amount of
power that may be applied in any operation, such as sawing wood or
stone, the effect produced when compared to hand labour is nearly
as the difference in the amount of power applied; and the saving
that such machines effect is generally in the same proportion. A
machine that can expend ten horse-power in performing a certain
kind of work, will save ten times as much as a machine directed
to the same purpose expending but one horse-power; this of course
applies to machines for the performance of the coarser kinds of
work, and employed to supplant mere physical effort. In other
machines of application, such as are directed mainly to guidance,
or speed of action, such as sewing machines, dove-tailing machines,
gear-cutting machines, and so on, there is no relation whatever
between the increased effect that may be produced and the amount of
power expended.
The difference between hand and machine operations, and the
labour-saving effect of machines, will be farther spoken of in
another place; the subject is alluded to here, only to enable the
reader to more fully distinguish between machinery of transmission
and machinery of application. Machinery of application, directed to
what has been termed compression processes, such as steam hammers,
drops, presses, rolling mills, and so on, act upon material that is
naturally soft and ductile, or when it is softened by heat, as in
the case of forging.
In compression processes no material is cut away as in cutting
or grinding, the mass being forced into shape by dies or forms
that give the required configuration. The action of compressing
machines may be either intermittent, as in the case of rolling
mills; percussive, as in steam hammers, where a great force acts
throughout a limited distance; or gradual and sustained, as in
press forging. Machines of application, for abrading or grinding,
are constantly coming more into use; their main purpose being to
cut or shape material too hard to be acted upon by compression or
by cutting processes. It follows that the necessity for machines
of this kind is in proportion to the amount of hard material which
enters into manufactures; in metal work the employment of hardened
steel and iron is rapidly increasing, and as a result, grinding
machines have now a place among the standard machine tools of a
fitting shop.
Grinding, no doubt, if traced to the principles that lie at the
bottom, is nothing more than a cutting process, in which the
edges employed are harder than any material that can be made into
cutters, the edges firmly supported by being imbedded into a mass
as the particles of sand are in grindstones, or the particles of
emery in emery wheels.
Separating machines, such as bolts and screens, which may be
called a class, require no explanation. The employment of magnetic
machines to separate iron and brass filings or shop waste, may be
noted as a recent improvement of some importance.
Disintegrating machines, such as are employed for pulverising
various substances, grinding grain or pulp, separating fibrous
material, and so on, are, with some exceptions, simple enough
to be readily understood. One of these exceptions is the rotary
"disintegrators," recently introduced, about the action of which
some diversity of opinion exists. The effect produced is certainly
abrasive wear, the result of the pieces or particles striking one
against another, or against the revolving beaters and casing. The
novelty of the process is in the augmented effect produced by a
high velocity, or, in other words, the rapidity of the blows.
(1.) Name five machines as types of those employed in the general
processes of converting material.--(2.) Name some machines, the
object of which is to augment force--One to attain speed--One
directed to the guidance of tools.--(3.) What is the difference
between the hot and cold treatment of iron as to processes--As to
dimensions?--(4.)
CHAPTER XVII.
_MACHINERY FOR MOVING AND HANDLING MATERIAL._
Steam and other machinery applied to the transport of material and
travel, in navigation and by railways, comprises the greater share
of what may be called engineering products; and when we consider
that this vast interest of steam transport is less than a century
old, and estimate its present and possible future influence on
human affairs, we may realise the relation that mechanical science
bears to modern civilisation.
To follow out the application of power to the propulsion of vessels
and trains, with the many abstruse problems that would of necessity
be involved, would be to carry this work far beyond the limits
within which it is most likely to be useful to the apprentice
engineer; besides, it would be going beyond what can properly be
termed manipulation.
Marine and railway engineering have engrossed the best talent in
the world; investigation and research has been expended upon these
subjects in a degree commensurate with their importance, and it
would be hard to suggest a single want in the many able text-books
that have been prepared upon the subjects. Marine and railway
engineering are sciences that may, in a sense, be separated from
the ordinary constructive arts, and studied at the end of a course
in mechanical engineering, but are hardly proper subjects for an
apprentice to take up at the beginning.
In treating of machinery for transport, as a class, the subject,
as far as noticed here, will be confined to moving and handling
material as one of the processes of manufacturing, and especially
in connection with machine construction. If the amount of time,
expense, labour, and machinery devoted to handling material in
machine shops is estimated, it becomes a matter of astonishment to
as many as have not previously investigated the subject; as an item
of expense the handling, often exceeds the fitting on large pieces,
and in the heavier class of work demands the most careful attention
to secure economical manipulation.
It will be well for an apprentice to begin at once, as soon as he
commences a shop course, to note the manner of handling material,
watching the operation of cranes, hoists, trucks, tackle, rollers;
in short, everything that has to do with moving and handling. The
machinery and appliances in ordinary use are simple enough in a
mechanical sense, but the principles of handling material are by
no means as plain or easy to understand. The diversity of practice
seen in various plans of handling and lifting weights fully attests
the last proposition, and it is questionable whether there is any
other branch of mechanical engineering that is treated less in a
scientific way than machinery of this class. I do not allude to
the mechanism of cranes and other devices, which are usually well
proportioned and generally well arranged, but to the adaptation
of such machinery with reference to special or local conditions.
There are certain inherent difficulties that have to be encountered
in the construction and operation of machinery, for lifting
and handling, that are peculiar to it as a class; among these
difficulties is the transmission of power to movable mechanism, the
intermittent and irregular application of power, severe strains,
also the liability to accidents and breakage from such machinery
being controlled by the judgment of attendants.
Ordinary machinery, on the reverse, is stationary, generally
consumes a regular amount of power, is not subjected to such
uncertain strains, and as a rule acts without its operation being
controlled by the will of attendants.
The functions required in machinery for handling material in a
machine shop correspond very nearly to those of the human hands.
Nature in this, as in all other things, where a comparison is
possible, has exceeded man in adaptation; in fact, we cannot
conceive of anything more perfect than the human hands for handling
material--a duty that forms a great share of all that we term
labour.
Considered mechanically as a means of handling material, the human
hands are capable of exerting force in any direction, vertically,
horizontally, or at any angle, moving at various rates of speed,
as the conditions may require, and with varying force within the
limits of human strength. These functions enable us to pick up or
lay down a weight slowly and carefully, to transport it at a rapid
rate to save time, to move it in any direction, and without the
least waste of power, except in the case of carrying small loads,
when the whole body has to be moved, as in ascending or descending
stairs. The power travelling cranes, that are usually employed in
machine-fitting establishments, are perhaps the nearest approach
that has been made to the human frame in the way of handling
mechanism; they, however, lack that very important feature of a
movement, the speed of which is graduated at will. It is evident
that in machinery of any kind for handling and lifting that moves
at a uniform rate of speed, and this rate of speed adapted, as it
must be, to the conditions of starting or depositing a load, much
time must be lost in the transit, especially when the load is moved
for a considerable distance. This uniform speed is perhaps the
greatest defect in the lifting machinery in common use, at least in
such as is driven by power.
In handling a weight with the hands it is carefully raised, and
laid down with care, but moved as rapidly as possible throughout
the intervening distance; this lesson of nature has not been
disregarded. We find that the attention of engineers has been
directed to this principle of variable speed to be controlled at
will. The hydraulic cranes of Sir William Armstrong, for example,
employ this principle in the most effective manner, not only
securing rapid transit of loads when lifted, but depositing or
adjusting them with a care and precision unknown to mechanism
positively geared or even operated by friction brakes.
The principles of all mechanism for handling loads should be such
as to place the power, the rate of movement, and the direction of
the force, within the control of an operator, which, as has been
pointed out, is the same thing in effect as the action of the human
hands.
The safety, simplicity, and reliable action of hydraulic machinery
has already led to its extensive employment for moving and lifting
weights, and it is fair to assume that the importance and success
of this invention fully entitle it to be classed as one of the
most important that has been made in mechanical engineering during
fifty years past. The application of hydraulic force in operating
the machinery used in the processes for steel Bessemer manufacture,
is one of the best examples to illustrate the advantages and
principles of the hydraulic system. Published drawings and
descriptions of Bessemer steel plant explain this hydraulic
machinery.
There is, however, a principle in hydraulic machinery that must
be taken into account, in comparing it with positively geared
mechanism, which often leads to loss of power that in many cases
will overbalance any gain derived from the peculiar action of
hydraulic apparatus. I allude to the loss of power incident to
dealing with an inelastic medium, where the amount of force
expended is constant, regardless of the resistance offered. A
hydraulic crane, for instance, consumes power in proportion to
its movements, and not as the amount of duty performed; it takes
the same quantity of water to fill the cylinders of such cranes,
whether the water exert much or little force in moving the pistons.
The difference between employing elastic mediums like air and
steam, and an inelastic medium like water, for transmitting force
in performing irregular duty, has been already alluded to, and
forms a very interesting study for a student in mechanics, leading,
as it does, to the solution of many problems concerning the use and
effect of power.
The steam cranes of Mr Morrison, which resemble hydraulic cranes,
except that steam instead of water is employed as a medium for
transmitting force, combine all the advantages of hydraulic
apparatus, except positive movement, and evade the loss of power
that occurs in the use of water. The elasticity of the steam is
found in practice to offer no obstacle to steady and accurate
movement of a load, provided the mechanism is well constructed,
while the loss of heat by radiation is but trifling.
To return to shop processes in manufacturing. Material operated
upon has to be often, sometimes continually, moved from one place
to another to receive successive operations, and this movement
may be either vertically or horizontally as determined, first,
by the relative facility with which the material may be raised
vertically, or moved horizontally, and secondly, by the value
of the ground and the amount of room that may be available, and
thirdly by local conditions of arrangement. In large cities, where
a great share of manufacturing is carried on, the value of ground
is so great that its cost becomes a valid reason for constructing
high buildings of several storeys, and moving material vertically
by hoists, thus gaining surface by floors, instead of spreading
the work over the ground; nor is there any disadvantage in high
buildings for most kinds of manufacture, including machine fitting
even, a proposition that will hardly be accepted in Europe, where
fitting operations, except for small pieces, are rarely performed
on upper floors.
Vertical handling, although it consumes more power, as a rule costs
less, is more convenient, and requires less room than horizontal
handling, which is sure to interfere more or less with the
constructive operations of a workshop. In machine fitting there is
generally a wrong estimate placed upon the value of ground floors,
which are no doubt indispensable for the heaviest class of work,
and for the heaviest tools; but with an ordinary class of work,
where the pieces do not exceed two tons in weight, upper floors if
strong are quite as convenient, if there is proper machinery for
handling material; in fact, the records of any establishment, where
cost accounts are carefully made up, will show that the expense
of fitting on upper floors is less than on ground floors. This is
to be accounted for by better light, and a removal of the fitting
from the influences and interference of other operations that must
necessarily be carried on upon ground floors.
For loading and unloading carts and waggons, the convenience of
the old outside sling is well known; it is also a well-attested
fact that accidents rarely happen with sling hoists, although
they appear to be less safe than running platforms or lifts. As
a general rule, the most dangerous machinery for handling or
raising material is that which pretends to dispense with the care
and vigilance of attendants, and the safest machinery that which
enforces such attention. The condition which leads to danger
in hoisting machinery is, that the power employed is opposed
to the force of gravity, and as the force of gravity is acting
continually, it is always ready to take advantage of the least
cessation in the opposing force employed, and thus drag away the
weight for which the two forces are contending; as a weight when
under the influence of gravity is moved at an accelerated velocity,
if gravity becomes the master, the result is generally a serious
accident. Lifting may be considered a case wherein the contrivances
of man are brought to bear in overcoming or opposing a natural
force; the imperfect force of the machinery is liable to accident
or interruption, but gravity never fails to act. Acting on every
piece of matter in proportion to its weight must be some force
opposing and equal to that of gravity; for example, a piece of iron
lying on a bench is opposed by the bench and held in resistance to
gravity, and to move this piece of iron we have to substitute some
opposing force, like that of the hands or lifting mechanism, to
overcome gravity.
As molecular adhesion keeps the particles of matter together so
as to form solids, so the force of gravity keeps objects in their
place; and to attain a proper conception of forces, especially in
handling and moving material, it is necessary to familiarise the
mind with this thought.
The force of gravity acts only in one direction--vertically, so
that the main force of hoisting and handling machinery which
opposes gravity must also act vertically, while the horizontal
movement of objects may be accomplished by simply overcoming the
friction between them and the surfaces on which they move. This is
seen in practice. A force of a hundred pounds may move a loaded
truck, which it would require tons to lift; hence the horizontal
movements of material may be easily accomplished by hand with
the aid of trucks and rollers, so long as it is moved on level
planes; but if a weight has to be raised even a single inch by
reason of irregularity in floors, the difference between overcoming
frictional contact and opposing gravity is at once apparent.
One of the problems connected with the handling of material is to
determine where hand-power should stop and motive-power begin--what
conditions will justify the erection of cranes, hoists, or
tramways, and what conditions will not. Frequent mistakes are made
in the application of power when it is not required, especially
for handling material; the too common tendency of the present day
being to apply power to every purpose where it is possible, without
estimating the actual saving that, may be effected. A common
impression is that motive power, wherever applied to supplant hand
labour in handling material, produces a gain; but in many cases
the fallacy of this will be apparent, when all the conditions are
taken into account.
Considered upon grounds of commercial expediency as a question of
cost alone, it is generally cheaper to move material by hand when
it can be easily lifted or moved by workmen, when the movement
is mainly in a horizontal direction, and when the labour can be
constantly employed; or, to assume a general rule which in practice
amounts to much the same thing, vertical lifting should be done by
motive power, and horizontal movement for short distances performed
by hand. There is nothing more unnatural than for men to carry
loads up stairs or ladders; the effort expended in such cases is
one-half or more devoted to raising the weight of the body, which
is not utilised in the descent, and it is always better to employ
winding or other mechanism for raising weights, even when it is to
be operated by manual labour. Speaking of this matter of carrying
loads upward, I am reminded of the fact that builders in England
and America, especially in the latter country, often have material
carried up ladders, while in some of the older European countries,
where there is but little pretension to scientific manipulation,
bricks are usually tossed from one man to another standing on
ladders at a distance of ten to fifteen feet apart.
To conclude. The reader will understand that the difficulties
and diversity of practice, in any branch of engineering, create
similar or equal difficulties in explaining or reasoning about the
operations; and the most that can be done in the limited space
allotted here to the subject of moving material, is to point out
some of the principles that should govern the construction and
adaptation of handling machinery, from which the reader can take up
the subject upon his own account, and follow it through the various
examples that may come under notice.
To sum up--We have the following propositions in regard to moving
and handling material:
1. The most economical and effectual mechanism for handling is that
which places the amount of force and rate of movement continually
under the control of an operator.
2. The necessity for, and consequent saving effected by,
power-machinery for handling is mainly in vertical lifting,
horizontal movement being easily performed by hand.
3. The vertical movement of material, although it consumes more
power, is more economical than horizontal handling, because less
floor room and ground surface is required.
4. The value of handling machinery, or the saving it effects, is as
the constancy with which it operates; such machinery may shorten
the time of handling without cheapening the expense.
5. Hydraulic machinery comes nearest to filling the required
conditions in handling material, and should be employed in cases
where the work is tolerably uniform, and the amount of handling
will justify the outlay required.
6. Handling material in machine construction is one of the
principal expenses to be dealt with; each time a piece is moved
its cost is enhanced, and usually in a much greater degree than is
supposed.
(1.) Why has the lifting of weights been made a standard for
the measure of power?--(2.) Name some of the difficulties to
contend with in the operation of machinery for lifting or
handling material.--(3.) What analogy exists between manual
handling and the operation of hydraulic cranes?--(4.) Explain
how the employment of overhead cranes saves room in a fitting
shop.--(5.) Under what circumstances is it expedient to move
material vertically?--(6.) To what circumstances is the danger of
handling mainly attributable?
CHAPTER XVIII.
_MACHINE COMBINATION._
The combination of several functions in one machine, although
it may not seem an important matter to be considered here, is
nevertheless one that has much to do with the manufacture of
machines, and constitutes what may be termed a principle of
construction.
The reasons that favour combination of functions in machines, and
the effects that such combinations may produce, are so various that
the problem has led to a great diversity of opinions and practice
among both those who construct and even those who employ machines.
It may be said, too, that a great share of the combinations found
in machines, such as those to turn, mill, bore, slot, and drill in
iron fitting, are not due to any deliberate plan on the part of
the makers, so much as to an opinion that such machines represent
a double or increased capacity. So far has combination in machines
been carried, that in one case that came under the writer's
notice, a machine was arranged to perform nearly every operation
required in finishing the parts of machinery; completely organised,
and displaying a high order of mechanical ability in design and
arrangement, but practically of no more value than a single machine
tool, because but one operation at a time could be performed.
To direct the attention of learners to certain rules that
will guide them in forming opinions in this matter of machine
combination, I will present the following propositions, and
afterwards consider them more in detail:--
_First._ By combining two or more operations in one machine, the
only objects gained are a slight saving in first cost, one frame
answering for two or more machines, and a saving of floor room.
_Second._ In a machine where two or more operations are combined,
the capacity of such a machine is only as a single one of these
operations, unless more than one can be carried on at the same time
without interfering one with another.
_Third._ Combination machines can only be employed with success
when one attendant performs all the operations, and when the
change from one to another requires but little adjustment and
re-arrangement.
_Fourth._ The arrangement of the parts of a combination machine
have to be modified by the relations between them, instead of being
adapted directly to the work to be performed.
_Fifth._ The cost of special adaptation, and the usual
inconvenience of fitting combination machines when their parts
operate independently, often equals and sometimes exceeds what is
saved in framing and floor space.
Referring first to the saving effected by combining several
operations in one machine, there is perhaps not one constructor
in twenty that ever stops to consider what is really gained, and
perhaps not one purchaser in a hundred that does the same thing.
The impression is, that when one machine performs two operations it
saves a second machine. A remarkable example of this exists in the
manufacture of combination machines in Europe for working wood,
where it is common to find complicated machines that will perform
all the operations of a joiner's shop, but as a rule only one thing
at a time, and usually in an inconvenient manner, each operation
being hampered and interfered with by another; and in changing from
one kind of work to another the adjustments and changes generally
equal and sometimes exceed the work to be done. What is stranger
still is, that such machines are purchased, when their cost often
equals that of separate machines to perform the same work.
In metal working, owing to a more perfect division of labour, and a
more intelligent manipulation than in wood-working, there is less
combination in machines--in fact, a combination machine for metal
work is rarely seen at this day, and never under circumstances
where it occasions actual loss. The advantage of combination, as
said, can only be in the framing and floor space occupied by the
machines, but these considerations, to be estimated by a proper
standard, are quite insignificant when compared with other items in
the cost of machine operating, such as the attendance, interest on
the invested cost of the machine, depreciation of value by wear,
repairing, and so on.
Assuming, for example, that a machine will cost as much as the
wages of an attendant for one year, which is not far from an
average estimate for iron working machine tools, and that interest,
wear, and repairs amount to ten per cent. on this sum, then the
attendance would cost ten times as much as the machine; in other
words, the wages paid to a workman to attend a machine is, on an
average, ten times as much as the other expenses attending its
operation, power excepted. This assumed, it follows that in machine
tools any improvement directed to labour saving is worth _ten
times_ as much as an equal improvement directed to the economy of
first cost.
This mode of reasoning will lead to proper estimates of the
difference in value between good tools and inferior tools; the
results of performance instead of the investment being first
considered, because the expenses of operating are, as before
assumed, usually ten times as great as the interest on the value of
a machine.
In view of these propositions, I need hardly say to what object
machine improvements should be directed, nor which of the
considerations named are most affected by a combination of machine
functions; the fact is, that if estimates could be prepared,
showing the actual effect of machine combinations, it would
astonish those who have not investigated the matter, and in many
cases show a loss of the whole cost of such machines each year. The
effect of combination machines is, however, by no means uniform;
the remarks made apply to standard machines employed in the regular
work of an engineering or other establishment. In exceptional cases
it may be expedient to use combined machines. In the tool-room of
machine-shops, for instance, where one man can usually perform the
main part of the work, and where there is but little space for
machines, the conditions are especially favourable to combination
machines, such as may be used in milling, turning, drilling, and so
on; but wherever there is a necessity or an opportunity to carry
on two or more of these operations at the same time, the cost of
separate machines is but a small consideration when compared with
the saving of labour that may be effected by independent tools to
perform each operation. The tendency of manufacturing processes
of every kind, at this time, is to a division of labour, and to a
separation of each operation into as many branches as possible, so
that study spent in "segregating" instead of "aggregating" machine
functions is most likely to produce profitable results.
This article has been introduced, not only to give a true
understanding of the effect and value of machine combination,
but to caution against a common error of confounding machine
combination with invention.
A great share of the alleged improvements in machinery, when
investigated will be found to consist in nothing more than the
combination of several functions in one machine, the novelty of
their arrangement leading to an impression of utility and increased
effect.
(1.) What is gained by arranging a machine to perform
several different operations?--(2.) What may be lost by such
combination?--(3.) What is the main expense attending the
operation of machine tools?--(4.) What kind of improvement in
machine tools produces the most profitable result?--(5.) What are
the principal causes which have led to machine combinations.
CHAPTER XIX.
_THE ARRANGEMENT OF ENGINEERING ESTABLISHMENTS._
The first and, perhaps, the most important matter of all in
founding engineering works is that of arrangement. As a commercial
consideration affecting the cost of manipulation, and the expense
of handling material, the arrangement of an establishment may
determine, in a large degree, the profits that may be earned, and,
as explained in a previous place, upon this matter of profits
depends the success of such works.
Aside from the cost or difficulty of obtaining ground sufficient to
carry out plans for engineering establishments, the diversity of
their arrangement met with, even in those of modern construction,
is no doubt owing to a want of reasoning from general premises.
There is always a strong tendency to accommodate local conditions,
and not unfrequently the details of shop manipulation are quite
overlooked, or are not understood by those who arrange buildings.
The similarity of the operations carried on in all works directed
to the manufacture of machinery, and the kind of knowledge that is
required in planning and conducting such works, would lead us to
suppose that at least as much system would exist in machine shops
as in other manufacturing establishments, which is certainly not
the case. There is, however, this difference to be considered:
that whereas many kinds of establishments can be arranged at
the beginning for a specific amount of business, machine shops
generally grow up around a nucleus, and are gradually extended as
their reputation and the demands for their productions increase;
besides, the variety of operations required in an engineering
establishment, and change from one class of work to another, are
apt to lead to a confusion in arrangement, which is too often
promoted, or at least not prevented, by insufficient estimates of
the cost of handling and moving material.
Materials consumed in an engineering establishment consist
mainly of iron, fuel, sand, and lumber. These articles, or their
products, during the processes of manipulation, are continually
approaching the erecting shop, from which finished machinery is
sent out after its completion. This constitutes the erecting
shop, as a kind of focal centre of a works, which should be the
base of a general plan of arrangement. This established, and the
foundry, smithy, finishing, and pattern shops regarded as feeding
departments to the erecting shop, it follows that the connections
between the erecting shop and other departments should be as short
as possible, and such as to allow free passage for material and
ready communication between managers and workmen in the different
rooms. These conditions would suggest a central room for erecting,
with the various departments for casting, forging, and finishing,
radiating from the erecting shop like the spokes of a wheel, or,
what is nearly the same, branching off at right angles on either
side and at one end of a hollow square, leaving the fourth side of
the erecting room to front on a street or road, permitting free
exit for machinery when completed.
The material when in its crude state not only consists of various
things, such as iron, sand, coal, and lumber, that must be kept
separate, but the bulk of such materials is much greater than their
finished product. It is therefore quite natural to receive such
material on the outside or "periphery" of the works where there is
the most room for entrances and for the separate storing of such
supplies. Such an arrangement is of course only possible where
there can be access to a considerable part of the boundary of a
works, yet there are but few cases where a shop cannot be arranged
in general upon the plan suggested. By receiving material on the
outside, and delivering the finished product from the centre,
communications between the departments of an establishment are the
shortest that it is possible to have; by observing the plans of
the best establishments of modern arrangement, especially those in
Europe, it may be seen that this system is approximated in many of
them, especially in establishments devoted to the manufacture of
some special class of work.
Handling and moving material is the principal matter to be
considered in the arrangement of engineering works. The
constructive manipulation can be watched, estimated, and faults
detected by comparison, but handling, like the designs for
machinery, is a more obscure matter, and may be greatly at fault
without its defects being apparent to any but those who are highly
skilled, and have had their attention especially directed to the
matter.
Presuming an engineering establishment to consist of one-storey
buildings, and the main operations to be conducted on the ground
level, the only vertical lifting to be performed will be in the
erecting room, where the parts of machines are assembled. This room
should be reached in every part by over-head travelling cranes,
that cannot only be used in turning, moving, and placing the
work, but in loading it upon cars or waggons. One result of the
employment of over-head travelling cranes, often overlooked, is
a saving of floor-room; in ordinary fitting, from one-third more
to twice the number of workmen will find room in an erecting shop
if a travelling-crane is employed, the difference being that, in
moving pieces they may pass over the top of other pieces instead
of requiring long open passages on the floor. So marked is this
saving of room effected by over-head cranes, that in England, where
they are generally employed, handling is not only less expensive
and quicker, but the area of erecting floors is usually one-half as
much as in America, where travelling-cranes are not employed.
Castings, forgings, and general supplies for erecting can be easily
brought to the erecting shop from the other departments on trucks
without the aid of motive power; so that the erecting and foundry
cranes will do the entire lifting duty required in any but very
large establishments.
The auxiliary departments, if disposed about an erecting shop in
the centre, should be so arranged that material which has to pass
through two or more departments can do so in the order of the
processes, and without having to cross the erecting shop. Casting,
boring, planing, drilling, and fitting, for example, should follow
each other, and the different departments be arranged accordingly;
whenever a casting is moved twice over the same course, it shows
fault of arrangement and useless expense. The same rule applies to
all kinds of material.
A great share of the handling about an engineering establishment is
avoided, if material can be stored and received on a higher level
than the working floors; if, for instance, coal, iron, and sand is
received from railway cars at an elevation sufficient to allow it
to be deposited where it is stored by gravity, it is equivalent
to saving the power and expense required to raise the material to
such a height, or move it and pile it up, which amounts to the
same thing in the end. It is not proposed to follow the details of
shop arrangement, farther than to furnish a clue to some of the
general principles that should be regarded in devising plans of
arrangement. Such principles are much more to be relied upon than
even experience in suggesting the arrangement of shops, because
all experience must be gained in connection with special local
conditions, which often warp and prejudice the judgment, and lead
to error in forming plans under circumstances different from those
where the experience was gained.
(1.) How may the arrangement of an establishment affect
its earnings?--(2.) Why is the arrangement of engineering
establishments generally irregular?--(3.) Why should an
erecting shop be a base of arrangement in engineering
establishments?--(4.) What are the principal materials consumed
in engineering works?--(5.) Why is not special experience a safe
guide in forming plans of shop arrangement?
CHAPTER XX.
_GENERALISATION OF SHOP PROCESSES._
Having thus far treated of such general principles and facts
connected with practical mechanics as might properly precede, and
be of use in, the study of actual manipulation in a workshop, we
come next to casting, forging, and finishing, with other details
that involve manual as well as mental skill, and to which the term
"processes" will apply.
As these shop processes or operations are more or less connected,
and run one into the other, it will be necessary at the beginning
to give a short summary of them, stating the general object
of each, that may serve to render the detailed remarks more
intelligible to the reader as he comes to them in their consecutive
order.
Designing, or generating the plans of machinery, may be considered
the leading element in engineering manufactures or machine
construction, that one to which all others are subordinate, both
in order and importance, and is that branch to which engineering
knowledge is especially directed. Designing should consist, first,
in assuming certain results, and, secondly, in conceiving of
mechanical agents to produce these results. It comprehends the
geometry of movements, the disposition and arrangement of material,
the endurance of wearing surfaces, adjustments, symmetry; in short,
all the conditions of machine operation and machine construction.
This subject will be again treated of at more length in another
section.
Draughting, or drawing, as it is more commonly called, is a means
by which mental conceptions are conveyed from one person to
another; it is the language of mechanics, and takes the place of
words, which are insufficient to convey mechanical ideas in an
intelligible manner.
Drawings represent and explain the machinery to which they relate
as the symbols in algebra represent quantities, and in a degree
admit of the same modifications and experiments to which the
machinery itself could be subjected if it were already constructed.
Drawings are also an important aid in developing designs or
conceptions. It is impossible to conceive of, and retain in the
mind, all the parts of a complicated machine, and their relation
to each other, without some aid to fix the various ideas as they
arise, and keep them in sight for comparison; like compiling
statistics, the footings must be kept at hand for reference, and to
determine the relation that one thing may bear to another.
In the workshop, the objects of drawing are to communicate plans
and dimensions to the workmen, and to enable a division of the
labour, so that the several parts of a machine may be operated
upon by different workmen at the same time--also to enable
classification and estimates of cost to be made, and records kept.
Drawings are, in fact, the base of shop system, upon which depends
not only the accuracy and uniformity of what is produced, but
also, in a great degree, its cost. Complete drawings of whatever
is made are now considered indispensable in the best regulated
establishments; yet we are not so far removed from a time when
most work was made without drawings, but what we may contrast the
present system with that which existed but a few years ago, when
to construct a new machine was a great undertaking, involving
generally many experiments and mistakes.
Pattern-making relates to the construction of duplicate models
for the moulded parts of machinery, and involves a knowledge of
shrinkage and cooling strains, the manner of moulding and proper
position of pieces, when cast, to ensure soundness in particular
parts. As a branch of machine manufacture, pattern-making requires
a large amount of special knowledge, and a high degree of skill;
for in no other department is there so much that must be left to
the discretion and judgment of workmen.
Pattern-makers have to thoroughly understand drawings, in order to
reproduce them on the trestle boards with allowance for shrinkage,
and to determine the cores; they must also understand moulding,
casting, fitting, and finishing. Pattern-making as a branch of
machine manufacture, should rank next to designing and drafting.
Founding and casting relate to forming parts of machinery by
pouring melted metal into moulds, the force of gravity alone being
sufficient to press or shape it into even complicated forms. As a
process for shaping such metal as is not injured by the high degree
of heat required in melting, moulding is the cheapest and most
expeditious of all means, even for forms of regular outline, while
the importance of moulding in producing irregular forms is such
that without this process the whole system of machine construction
would have to be changed. Founding operations are divided into
two classes, known technically as green sand moulding, and loam
or dry sand moulding; the first, when patterns or duplicates are
used to form the moulds, and the second, when the moulds are built
by hand without the aid of complete patterns. Founding involves a
knowledge of mixing and melting metals such as are used in machine
construction, the preparing and setting of cores for the internal
displacement of the metal, cooling and shrinking strains, chills,
and many other things that are more or less special, and can only
be learned and understood from actual observation and practice.
Forging relates to shaping metal by compression or blows when
it is in a heated and softened condition; as a process, it is
an intermediate one between casting and what may be called the
cold processes. Forging also relates to welding or joining pieces
together by sudden heating that melts the surface only, and then by
forcing the pieces together while in this softened or semi-fused
state. Forging includes, in ordinary practice, the preparation of
cutting tools, and tempering them to various degrees of hardness
as the nature of the work for which they are intended may require;
also the construction of furnaces for heating the material, and
mechanical devices for handling it when hot, with the various
operations for shaping, which, as in the case of casting, can only
be fully understood by experience and observation.
Finishing and fitting relates to giving true and accurate
dimensions to the parts of machinery that come in contact with
each other and are joined together or move upon each other, and
consists in cutting away the surplus material which has to be
left in founding and forging because of the heated and expanded
condition in which the material is treated in these last processes.
In finishing, material is operated upon at its normal temperature,
in which condition it can be handled, gauged, or measured, and
will retain its shape after it is fitted. Finishing comprehends
all operations of cutting and abrading, such as turning, boring,
planing and grinding, also the handling of material; it is
considered the leading department in shop manipulation, because
it is the one where the work constructed is organised and brought
together. The fitting shop is also that department to which
drawings especially apply, and other preparatory operations are
usually made subservient to the fitting processes.
Shop system may also be classed as a branch of engineering work;
it relates to the classification of machines and their parts by
symbols and numbers, to records of weight, the expense of cast,
forged, and finished parts, and apportions the cost of finished
machinery among the different departments. Shop system also
includes the maintenance of standard dimensions, the classification
and cost of labour, with other matters that partake both of a
mechanical and a commercial nature.
In order to render what is said of shop processes more easily
understood, it will be necessary to change the order in which they
have been named. Designing, and many matters connected with the
operation of machines, will be more easily learned and understood
after having gone through with what may be called the constructive
operations, such as involve manual skill.
(1.) Name the different departments of an engineering
establishment.--(2.) What does the engineering establishment
include?--(3.) What does the commercial department include?--(4.)
The foundry department?--(5.) The forging department?--(6.) The
fitting department?--(7.) What does the term shop system mean as
generally employed?
CHAPTER XXI.
_MECHANICAL DRAWING._
Machine-drawing may in some respects be said to bear the same
relation to mechanics that writing does to literature; persons
may copy manuscript, or write from dictation, of what they do not
understand; or a mechanical draughtsman may make drawings of a
machine he does not understand; but neither such writing or drawing
can have any value beyond that of ordinary labour. It is both
necessary and expected that a draughtsman shall understand all the
various processes of machine construction, and be familiar with the
best examples that are furnished by modern practice.
Geometrical drawing is not an artistic art so much as it is a
constructive mechanical one; displaying the parts of machinery
on paper, is much the same in practice, and just the same in
principle, as measuring and laying out work in the shop.
Artistic drawing is addressed to the senses, geometrical drawing is
addressed to the understanding. Geometrical drawing may, however,
include artistic skill not in the way of ornamentation, but to
convey an impression of neatness and completeness, that has by
common custom been assumed among engineers, and which conveys to
the mind an idea of competent construction in the drawing itself,
as well as of the machinery which is represented. Artistic effect,
so far as admissible in mechanical drawing, is easy to learn, and
should be understood, yet through a desire to make pictures, a
beginner is often led to neglect that which is more important in
the way of accuracy and arrangement.
It is easy to learn "how" to draw, but it is far from easy to
learn "what" to draw. Let this be kept in mind, not in the way of
disparaging effort in learning "how" to draw, for this must come
first, but in order that the objects and true nature of the work
will be understood.
The engineering apprentice, as a rule, has a desire to make
drawings as soon as he begins his studies or his work, and there
is not the least objection to his doing so; in fact, there is a
great deal gained by illustrating movements and the details of
machinery at the same time of studying the principles. Drawings if
made should always be finished, carefully inked in, and memoranda
made on the margin of the sheets, with the date and the conditions
under which the drawings were made. The sheets should be of uniform
size, not too large for a portfolio, and carefully preserved, no
matter how imperfect they may be. An apprentice who will preserve
his first drawings in this manner will some day find himself in
possession of a souvenir that no consideration would cause him to
part with.
For implements procure two drawing-boards, forty-two inches long
and thirty inches wide, to receive double elephant paper; have the
boards plain without cleets, or ingenious devices for fastening
the paper; they should be made from thoroughly seasoned lumber, at
least one and one-fourth inches thick; if thinner they will not be
heavy enough to resist the thrust of the T squares.
It is better to have two boards, so that one may be used for
sketching and drawing details, which, if done on the same sheet
with elevations, dirties the paper, and is apt to lower the
standard of the finished drawing by what may be called bad
association.
Details and sketches, when made on a separate sheet, should be to a
larger scale than elevations. By changing from one scale to another
the mind is schooled in proportion, and the conception of sizes and
dimensions is more apt to follow the finished work to which the
drawings relate.
In working to regular scales, such as one-half, one-eighth,
or one-sixteenth size, a good plan is to use a common rule,
instead of a graduated scale. There is nothing more convenient
for a mechanical draughtsman than to be able to readily resolve
dimensions into various scales, and the use of a common rule for
fractional scales trains the mind, so that computations come
naturally, and after a time almost without effort. A plain T
square, with a parallel blade fastened on the side of the head,
but not imbedded into it, is the best; in this way set squares can
pass over the head of a T square in working at the edges of the
drawing. It is strange that a draughting square should ever have
been made in any other manner than this, and still more strange,
that people will use squares that do not allow the set squares to
pass over the heads and come near to the edge of the board.
A bevel square is often convenient, but should be an independent
one; a T square that has a movable blade is not suitable for
general use. Combinations in draughting instruments, no matter what
their character, should be avoided; such combinations, like those
in machinery, are generally mistakes, and their effect the reverse
of what is intended.
For set squares, or triangles, as they are sometimes called, no
material is so good as ebonite; such squares are hard, smooth,
impervious to moisture, and contrast with the paper in colour;
besides they wear longer than those made of wood. For instruments,
it is best to avoid everything of an elaborate or fancy kind;
such sets are for amateurs, not engineers. It is best to procure
only such instruments at first as are really required, of the
best quality, and then to add others as necessity may demand; in
this way, experience will often suggest modifications of size or
arrangement that will add to the convenience of a set.
One pair each of three and one-half inch and five inch compasses,
two ruling pens, two pairs of spring dividers, one for pens and one
for pencils, a triangular boxwood scale, a common rule, and a hard
pencil, are the essential instruments for machine-drawing. At the
beginning, when "scratching out" will probably form an item in the
work, it is best to use Whatman's paper, or the best roll paper,
which, of the best manufacture, is quite as good as any other for
drawings that are not water-shaded.
In mounting sheets that are likely to be removed and replaced,
for the purpose of modification, as working drawings generally
are, they can be fastened very well by small copper tacks driven
along the edges at intervals of two inches or less. The paper can
be very slightly dampened before fastening in this manner, and if
the operation is carefully performed the paper will be quite as
smooth and convenient to work upon as though it were pasted down;
the tacks can be driven down so as to be flush with, or below the
surface of, the paper, and will offer no obstruction to squares.
If a drawing is to be elaborate, or to remain long upon a board,
the paper should be pasted down. To do this, first prepare thick
mucilage, or what is better, glue, and have it ready at hand, with
some slips of absorbent paper an inch or so wide. Dampen the sheet
on both sides with a sponge, and then apply the mucilage along the
edge, for a width of one-fourth or three-eighths of an inch. It
is a matter of some difficulty to place a sheet upon a board; but
if the board is set on its edge, the paper can be applied without
assistance. Then, by placing the strips of paper along the edge,
and rubbing over them with some smooth hard instrument, the edges
of the sheet can be pasted firmly to the board, the paper slips
taking up a part of the moisture from the edges, which are longest
in drying. If left in this condition, the centre will dry first,
and the paper be pulled loose at the edges by contraction before
the paste has time to dry. It is therefore necessary to pass over
the centre of the sheet with a wet sponge at intervals to keep the
paper slightly damp until the edges adhere firmly, when it can
be left to dry, and will be tight and smooth. In this operation
much will be learned by practice, and a beginner should not be
discouraged by a few failures. One of the most common difficulties
in mounting sheets is in not having the gum or glue thick enough;
when thin, it will be absorbed by the wood or the paper, or is too
long in drying; it should be as thick as it can be applied with a
brush, and made from clean Arabic gum, tragacanth, or fine glue.
Thumb-tacks are of but little use in mechanical drawing except
for the most temporary purposes, and may very well be dispensed
with altogether; they injure the draughting-boards, obstruct the
squares, and disfigure the sheets.
Pencilling is the first and the most important operation in
draughting; more skill is required to produce neat pencil-work than
to ink in the lines after the pencilling is done.
A beginner, unless he exercises great care in the pencil-work of
a drawing, will have the disappointment to find the paper soon
becoming dirty from plumbago, and the pencil-lines crossing each
other everywhere, so as to give the whole a slovenly appearance. He
will also, unless he understands the nature of the operations in
which he is engaged, make the mistake of regarding the pencil-work
as an unimportant part, instead of constituting, as it does, the
main drawing, and thereby neglect that accuracy which alone can
make either a good-looking or a valuable one.
Pencil-work is indeed the main operation, the inking being merely
to give distinctness and permanency to the lines. The main thing in
pencilling is accuracy of dimensions and stopping the lines where
they should terminate without crossing others. The best pencils
only are suitable for draughting; if the plumbago is not of the
best quality, the points require to be continually sharpened, and
the pencil is worn away at a rate that more than makes up the
difference in cost between the finer and cheaper grades of pencils,
to say nothing of the effect upon a drawing.
It is common to use a flat point for draughting pencils, but a
round one will often be found quite as good if the pencils are
fine, and some convenience is gained by a round point for free-hand
use in making rounds and fillets. A Faber pencil, that has
detachable points which can be set out as they are worn away, is
convenient for draughting.
For compasses, the lead points should be cylindrical, and fit into
a metal sheath without paper packing or other contrivance to hold
them; and if a draughtsman has instruments not arranged in this
manner, he should have them changed at once, both for convenience
and economy.
Ink used in drawing should always be the best that can be procured;
without good ink a draughtsman is continually annoyed by an
imperfect working of pens, and the washing of the lines if there
is shading to be done. The quality of ink can only be determined
by experiment; the perfume that it contains, or tinfoil wrappers
and Chinese labels, are no indication of quality; not even the
price, unless it be with some first-class house. To prepare ink,
I can recommend no better plan of learning than to ask some one
who understands the matter. It is better to waste a little time in
preparing it slowly than to be at a continual trouble with pens,
which will occur if the ink is ground too rapidly or on a rough
surface. To test ink, a few lines can be drawn on the margin of
a sheet, noting the shade, how the ink flows from the pen, and
whether the lines are sharp; after the lines have dried, cross
them with a wet brush; if they wash readily, the ink is too soft;
if they resist the water for a time, and then wash tardily, the
ink is good. It cannot be expected that inks soluble in water can
permanently resist its action after drying; in fact, it is not
desirable that drawing inks should do so, for in shading, outlines
should be blended into the tints where the latter are deep, and
this can only be effected by washing.
Pens will generally fill by capillary attraction; if not, they
should be made wet by being dipped into water; they should not be
put into the mouth to wet them, as there is danger of poison from
some kinds of ink, and the habit is not a neat one.
In using ruling pens, they should be held nearly vertical, leaning
just enough to prevent them from catching on the paper. Beginners
have a tendency to hold pens at a low angle, and drag them on their
side, but this will not produce clean sharp lines, nor allow the
lines to be made near enough to the edges of square blades or set
squares.
In regard to the use of the T square and set squares, no useful
rules can be given except to observe others, and experiment until
convenient customs are attained. A beginner should be careful of
adopting unusual plans, and above all things, of making important
discoveries as to new plans of using instruments, assuming that
common practice is all wrong, and that it is left for him to
develop the true and proper way of drawing. This is a kind of
discovery which is very apt to intrude itself at the beginning of
an apprentice's course in many matters besides drawing, and often
leads him to do and say many things which he will afterwards wish
to recall.
It is generally a safe rule to assume that any custom long and
uniformly followed by intelligent people is right; and, in the
absence of that experimental knowledge which alone enables one to
judge, it is safe to receive such customs, at least for a time, as
being correct.
Without any wish to discourage the ambition of an apprentice to
invent, which always inspires him to laudable exertion, it is
nevertheless best to caution him against innovations. The estimate
formed of our abilities is very apt to be inversely as our
experience, and old engineers are not nearly so confident in their
deductions and plans as beginners are.
A drawing being inked in, the next things are tints, dimension,
and centre lines. The centre lines should be in red ink, and pass
through all points of the drawing that have an axial centre, or
where the work is similar and balanced on each side of the line.
This rule is a little obscure, but will be best understood
if studied in connection with a drawing, and perhaps as well
remembered without further explanation.
Dimension lines should be in blue, but may be in red. Where to
put them is a great point in draughting. To know where dimensions
are required involves a knowledge of fitting and pattern-making,
and cannot well be explained; it must be learned in practice. The
lines should be fine and clear, leaving a space in their centre
for figures when there is room. The distribution of centre lines
and dimensions over a drawing must be carefully studied, for
the double purpose of giving it a good appearance and to avoid
confusion. Figures should be made like printed numerals; they are
much better understood by the workman, look more artistic, and
when once learned require but little if any more time than written
figures. If the scale employed is feet and inches, dimensions to
three feet should be in inches, and above this in feet and inches;
this corresponds to shop custom, and is more comprehensive to the
workman, however wrong it may be according to other standards.
In sketches and drawings made for practice, such as are not
intended for the shop, it is suggested that metrical scales be
employed; it will not interfere with feet and inches, and will
prepare the mind for the introduction of this system of lineal
measurement, which may in time be adopted in England and America,
as it has been in many other countries.
In shading drawings, be careful not to use too deep tints, and
to put the shades in the right place. Many will contend, and not
without good reasons, that working drawings require no shading;
yet it will do no harm to learn how and where they can be shaded:
it is better to omit the shading from choice than from necessity.
Sections must, of course, be shaded--not with lines, although I
fear to attack so old a custom, yet it is certainly a tedious and
useless one: sections with light ink shading of different colours,
to indicate the kind of material, are easier to make, and look
much better. By the judicious arrangement of a drawing, a large
share of it may be in sections, which in almost every case are the
best views to work by. The proper colouring of sections gives a
good appearance to a drawing, and conveys an idea of an organised
machine, or, to use the shop term, "stands out from the paper." In
shading sections, leave a margin of white between the tints and
the lines on the upper and left-hand sides of the section: this
breaks the connection or sameness, and the effect is striking; it
separates the parts, and adds greatly to the clearness and general
appearance of a drawing.
Cylindrical parts in the plane of sections, such as shafts and
bolts, should be drawn full, and have a 'round shade,' which
relieves the flat appearance--a point to be avoided as much as
possible in sectional views.
Conventional custom has assigned blue as a tint for wrought iron,
neutral or pale pink for cast iron, and purple for steel. Wood is
generally distinguished by "graining," which is easily done, and
looks well.
The title of a drawing is a feature that has much to do with its
appearance, and the impression conveyed to the mind of an observer.
While it can add nothing to the real value of a drawing, it is so
easy to make plain letters, that the apprentice is urged to learn
this as soon as he begins to draw; not to make fancy letters, nor
indeed any kind except plain block letters, which can be rapidly
laid out and finished, and consequently employed to a greater
extent. By drawing six parallel lines, making five spaces, and
then crossing them with equidistant lines, the points and angles
in block letters are determined; after a little practice, it
becomes the work of but a few minutes to put down a title or other
matter on a drawing so that it can be seen and read at a glance in
searching for sheets or details.
In the manufacture of machines, there are usually so many sizes
and modifications, that drawings should assist and determine in
a large degree the completeness of classification and record.
Taking the manufacture of machine tools, for example: we cannot
well say, each time they are to be spoken of, a thirty-six inch
lathe without screw and gearing, a thirty-two inch lathe with screw
and gearing, a forty-inch lathe triple geared or double geared,
with a twenty or thirty foot frame, and so on. To avoid this it
is necessary to assume symbols for machines of different classes,
consisting generally of the letters of the alphabet, qualified by
a single number as an exponent to designate capacity or different
modifications. Assuming, in the case of engine lathes, A to be the
symbol for lathes of all sizes, then those of different capacity
and modification can be represented in the drawings and records
as A¹, A², A³, A⁴, and so on, requiring but two characters to
indicate a lathe of any kind. These symbols should be marked in
large plain letters on the left-hand lower corner of sheets, so
that the manager, workman, or any one else, can see at a glance
what the drawings relate to. This symbol should run through the
time-book, cost account, sales record, and be the technical name
for machines to which it applies; in this way machines will always
be spoken of in the works by the name of their symbol.
In making up the time charged to different machines during their
construction, a good plan is to supply each workman with a slate
and pencil, on which he can enter his time as so many hours or
fractions of hours charged to the respective symbols. Instead of
interfering with his time, this will increase a workman's interest
in what he is doing, and naturally lead to a desire to diminish the
time charged to the various symbols. This system leads to emulation
among workmen where any operation is repeated by different persons,
and creates an interest in classification which workmen will
willingly assist in.
When the dimensions and symbols are added to a drawing, the next
thing is pattern or catalogue numbers. These should be marked in
prominent, plain figures on each piece of casting, either in red
or other colour that will contrast with the general face of the
drawing. These numbers, to avoid the use of symbols in connection
with them, must include consecutively all patterns employed in the
business, and can extend to thousands without inconvenience.
A book containing the pattern record should be kept, in which
these catalogue numbers are set down, with a short description
to identify the different parts to which the numbers belong, so
that all the various details of any machine can at any time be
referred to. Besides this description, there should be opposite the
catalogue of pattern numbers, ruled spaces, in which to enter the
weight of castings, the cost of the pattern, and also the amount of
turned, planed, or bored surface on each piece when it is finished,
or the time required in fitting, which is the same thing. In this
book the assembled parts of each machine should be set down in a
separate list, so that orders for castings can be made from the
list without other references. This system is the best one known
to the writer, and is in substance a plan now adopted in many of
the best engineering establishments. A complete system in all
things pertaining to drawings and classifications should be rigidly
adhered to; any plan is better than none, and the schooling of the
mind to be had in the observance of systematic rules is a matter
not to be neglected. New plans for promoting system may at any
time arise, but such plans cannot be at any time understood and
adopted except by those who have cultivated a taste for order and
regularity.
In regard to shaded elevations, it may be said that photography has
superseded them for the purpose of illustrating completed machines,
and but few establishments care to incur the expense of ink-shaded
elevations. Shaded elevations cannot be made with various degrees
of care, and in a longer or shorter time; there is but one standard
for them, and that is that such drawings should be made with great
care and skill. Imperfect shaded elevations, although they may
surprise and please the unskilled, are execrable in the eyes of a
draughtsman or an engineer; and as the making of shaded elevations
can be of but little assistance to an apprentice draughtsman, it is
better to save the time that must be spent in order to make such
drawings, and apply the same study and time to other matters of
greater importance.
It is not assumed that shaded elevations should not be made, nor
that ink shading should not be learned, but it is thought best to
point out the greater importance of other kinds of drawing, too
often neglected to gratify a taste for picture-making, which has
but little to do with practical mechanics.
Isometrical perspective is often useful in drawing, especially in
wood structures, when the material is of rectangular section, and
disposed at right angles, as in machine frames. One isometrical
view, which can be made nearly as quickly as a true elevation,
will show all the parts, and may be figured for dimensions the
same as plane views. True perspective, although rarely necessary
in mechanical drawing, may be studied with advantage in connection
with geometry; it will often lead to the explanation of problems
in isometric drawing, and will also assist in free-hand lines that
have sometimes to be made to show parts of machinery oblique to
the regular planes. Thus far the remarks on draughting have been
confined to manipulation mainly. As a branch of engineering work,
draughting must depend mainly on special knowledge, and is not
capable of being learned or practised upon general principles or
rules. It is therefore impossible to give a learner much aid by
searching after principles to guide him; the few propositions that
follow comprehend nearly all that may be explained in words.
1. Geometrical drawings consist in plans, elevations, and sections;
plans being views on the top of the object in a horizontal plane;
elevations, views on the sides of the object in vertical planes;
and sections, views taken on bisecting planes, at any angle through
an object.
2. Drawings in true elevation or in section are based upon flat
planes, and give dimensions parallel to the planes in which the
views are taken.
3. Two elevations taken at right angles to each other, fix all
points, and give all dimensions of parts that have their axis
parallel to the planes on which the views are taken; but when a
machine is complex, or when several parts lie in the same plane,
three and sometimes four views are required to display all the
parts in a comprehensive manner.
4. Mechanical drawings should be made with reference to all the
processes that are required in the construction of the work, and
the drawings should be responsible, not only for dimensions, but
for unnecessary expense in fitting, forging, pattern-making,
moulding, and so on.
5. Every part laid down has something to govern it that may be
termed a "base"--some condition of function or position which, if
understood, will suggest size, shape, and relation to other parts.
By searching after a base for each and every part and detail, the
draughtsman proceeds upon a regular system, continually maintaining
a test of what is done. Every wheel, shaft, screw or piece of
framing should be made with a clear view of the functions it has
to fill, and there are, as before said, always reasons why such
parts should be of a certain size, have such a speed of movement,
or a certain amount of bearing surface, and so on. These reasons
or conditions may be classed as _expedient_, _important_, or
_essential_, and must be estimated accordingly. As claimed at the
beginning, the designs of machines can only in a limited degree be
determined by mathematical data. Leaving out all considerations
of machine operation with which books have scarcely attempted to
deal, we have only to refer to the element of strains to verify the
general truth of the proposition.
Examining machines made by the best designers, it will be found
that their dimensions bear but little if any reference to
calculated strains, especially in machines involving rapid motion.
Accidents destroy constants, and a draughtsman or designer who does
not combine special and experimental knowledge with what he may
learn from general sources, will find his services to be of but
little value in actual practice.
I now come to note a matter in connection with draughting to
which the attention of learners is earnestly called, and which,
if neglected, all else will be useless. I allude to indigestion,
and its resultant evils. All sedentary pursuits more or less give
rise to this trouble, but none of them so much as draughting. Every
condition to promote this derangement exists. When the muscles
are at rest, circulation is slow, the mind is intensely occupied,
robbing the stomach of its blood and vitality, and, worse than all,
the mechanical action of the stomach is usually arrested by leaning
over the edge of a board. It is regretted that no good rule can
be given to avoid this danger. One who understands the evil may
in a degree avert it by applying some of the logic which has been
recommended in the study of mechanics. If anything tends to induce
indigestion, its opposite tends the other way, and may arrest
it; if stooping over a board interferes with the action of the
digestive organs, leaning back does the opposite; it is therefore
best to have a desk as high as possible, stand when at work, and
cultivate a constant habit of straightening up and throwing the
shoulders back, and if possible, take brief intervals of vigorous
exercise. Like rating the horse-power of a steam-engine, by
multiplying the force into the velocity, the capacity of a man
can be estimated by multiplying his mental acquirements into his
vitality.
Physical strength, bone and muscle, must be elements in successful
engineering experience; and if these things are not acquired at the
same time with a mechanical education, it will be found, when ready
to enter upon a course of practice, that an important element, the
"propelling power," has been omitted.
(1.) What is the difference between geometric and artistic
drawing?--(2.) What is the most important operation in making a
good drawing?--(3.) Into what three classes can working drawings
be divided?--(4.) Explain the difference between elevations
and plans.--(5.) To what extent in general practice is the
proportion of parts and their arrangement in machines determined
mathematically?
CHAPTER XXII.
_PATTERN-MAKING AND CASTING._
Patterns and castings are so intimately connected that it would
be difficult to treat of them separately without continually
confounding them together; it is therefore proposed to speak of
pattern-making and moulding under one head.
Every operation in a pattern-shop has reference to some operation
in the foundry, and patterns considered separately from moulding
operations would be incomprehensible to any but the skilled.
Next to designing and draughting, pattern-making is the most
intellectual of what may be termed engineering processes--the
department that must include an exercise of the greatest amount of
personal judgment on the part of the workman, and at the same time
demands a high grade of hand skill.
For other kinds of work there are drawings furnished, and the plans
are dictated by the engineering department of machinery-building
establishments, but pattern-makers make their own plans for
constructing their work, and have even to reproduce the drawings
of the fitting-shop to work from. Nearly everything pertaining to
patterns is left to be decided by the pattern-maker, who, from the
same drawings, and through the exercise of his judgment alone, may
make patterns that are durable and expensive, or temporary and
cheap, as the probable extent of their use may determine.
The expense of patterns should be divided among and charged to the
machines for which the patterns are employed, but there can be
no constant rules for assessing or dividing this cost. A pattern
may be employed but once, or it may be used for years; it is
continually liable to be superseded by changes and improvements
that cannot be predicted beforehand; and in preparing patterns,
the question continually arises of how much ought to be expended
on them--a matter that should be determined between the engineer
and the pattern-maker, but is generally left to the pattern-maker
alone, for the reason that but few mechanical engineers understand
pattern-making so well as to dictate plans of construction.
To point out some of the leading points or conditions to be taken
into account in pattern-making, and which must be understood
in order to manage this department, I will refer to them in
consecutive order.
_First._--Durability, plans of construction and cost, which all
amount to the same thing. To determine this point, there is to
be considered the amount of use that the patterns are likely to
serve, whether they are for standard or special machines, and the
quality of the castings so far as affected by the patterns. A
first-class pattern, framed to withstand moisture and rapping, may
cost twice as much as another that has the same outline, yet the
cheaper pattern may answer almost as well to form a few moulds as
an expensive one.
_Second._--The manner of moulding and its expense, so far as
determined by the patterns, which may be parted so as to be 'rammed
up' on fallow boards or a level floor, or the patterns may be
solid, and have to be bedded, as it is termed; pieces on the top
may be made loose, or fastened on so as to 'cope off;' patterns may
be well finished so as to draw clean, or rough so that a mould may
require a great deal of time to dress up after a pattern is removed.
_Third._--The soundness of such parts as are to be planed, bored,
and turned in finishing; this is also a matter that is determined
mainly by how the patterns are arranged, by which is the top
and which the bottom or drag side, the manner of drawing, and
provisions for avoiding dirt and slag.
_Fourth._--Cores, where used, how vented, how supported in the
mould, and I will add how made, because cores that are of an
irregular form are often more expensive than external moulds,
including the patterns. The expense of patterns is often greatly
reduced, but is sometimes increased, by the use of cores, which may
be employed to cheapen patterns, add to their durability, or to
ensure sound castings.
_Fifth._--Shrinkage; the allowance that has to be made for the
contraction of castings in cooling, in other words, the difference
between the size of a pattern and the size of the casting. This is
a simple matter apparently, which may be provided for in allowing
a certain amount of shrinkage in all directions, but when the
inequalities of shrinkage both as to time and degree are taken into
account, the allowance to be made becomes a problem of no little
complication.
_Sixth._--Inherent, or cooling strains, that may either spring and
warp castings, or weaken them by maintained tension in certain
parts--a condition that often requires a disposition of the metal
quite different from what working strains demand.
_Seventh._--Draught, the bevel or inclination on the sides of
patterns to allow them to be withdrawn from the moulds without
dragging or breaking the sand.
_Eighth._--Rapping plates, draw plates, and lifting irons for
drawing the patterns out of the moulds; fallow and match boards,
with other details that are peculiar to patterns, and have no
counterparts, neither in names nor uses, outside the foundry.
This makes a statement in brief of what comprehends a knowledge
of pattern-making, and what must be understood not only by
pattern-makers, but also by mechanical engineers who undertake to
design machinery or manage its construction successfully.
As to the manner of cutting out or planing up the lumber for
patterns, and the manner of framing them together, it is useless to
devote space to the subject here; one hour's practical observation
in a pattern-shop, and another hour spent in examining different
kinds of patterns, is worth more to the apprentice than a whole
volume written to explain how these last-named operations are
performed. A pattern, unless finished with paint or opaque varnish,
will show the manner in which the wood is disposed in framing the
parts together.
I will now proceed to review these conditions or principles in
pattern-making and casting in a more detailed way, furnishing
as far as possible reasons for different modes of constructing
patterns, and the various plans of moulding and casting.
In regard to the character or quality of wood patterns, they can
be made, as already stated, at greater or less expense, and if
necessary, capable of almost any degree of endurance. The writer
has examined patterns which had been used more than two hundred
times, and were apparently good for an equal amount of use. Such
patterns are expensive in their first cost, but are the cheapest in
the end, if they are to be employed for a large number of castings.
Patterns for special pieces, or such as are to be used for a few
times only, do not require to be strong nor expensive, yet with
patterns, as with everything else pertaining to machinery, the
safest plan is to err on the side of strength.
For pulleys, gear wheels, or other standard parts of machinery
which are not likely to be modified or changed, iron patterns are
preferable; patterns for gear wheels and pulleys, when made of
wood, aside from their liability to spring and warp, cannot be made
sufficiently strong to withstand foundry use; besides, the greatest
accuracy that can be attained, even by metal patterns, is far from
producing true castings, especially for tooth wheels. The more
perfect patterns are, the less rapping is required in drawing them;
and the less rapping done, the more perfect castings will be.
The most perfect castings for gear wheels and pulleys and other
pieces which can be so moulded, are made by drawing the patterns
through templates without rapping. These templates are simply
plates of metal perforated so that the pattern can be forced
through them by screws or levers, leaving the sand intact. Such
templates are expensive to begin with, because of the accurate
fitting that is required, especially around the teeth of wheels,
and the mechanism that is required in drawing the patterns, but
when a large number of pieces are to be made from one pattern, such
as gear wheels and pulleys, the saving of labour will soon pay for
the templates and machinery required, to say nothing of the saving
of metal, which often amounts to ten per cent., and the increased
value of the castings because of their accuracy.
Mr Ransome of Ipswich, England, where this system of template
moulding originated, has invented a process of fitting templates
for gear wheels and other kinds of casting by pouring melted white
metal around to mould the fit instead of cutting it through the
templates; this effects a great saving in expense, and answers in
many cases quite as well as the old plan.
The expense of forming pattern-moulds may be considered as divided
between the foundry and pattern-shop. What a pattern-maker saves
a moulder may lose, and what a pattern-maker spends a moulder may
save; in other words, there is a point beyond which saving expense
in patterns is balanced by extra labour and waste in moulding--a
fact that is not generally realised because of inaccurate records
of both pattern and foundry work. What is lost or saved by
judicious or careless management in the matter of patterns and
moulding can only be known to those who are well skilled in both
moulding and pattern-making. A moulder may cut all the fillets in
a mould with a trowel; he may stop off, fill up, and print in, to
save pattern-work, but it is only expedient to do so when it costs
very much less than to prepare proper patterns, because patching
and cutting in moulds seldom improves them.
The reader may notice how everything pertaining to patterns and
moulding resolves itself into a matter of judgment on the part of
workmen, and how difficult it would be to apply general rules.
The arrangement of patterns with reference to having certain parts
of castings solid and clean is an important matter, yet one that is
comparatively easy to understand. Supposing the iron in a mould to
be in a melted state, and to contain, as it always must, loose sand
and 'scruff,' and that the weight of the dirt is to melted iron as
the weight of cork is to water, it is easy to see where this dirt
would lodge, and where it would be found in the castings. The top
of a mould or cope, as it is called, contains the dirt, while the
bottom or drag side is generally clean and sound: the rule is to
arrange patterns so that the surfaces to be finished will come on
the bottom or drag side.
Expedients to avoid dirt in such castings as are to be finished all
over or on two sides are various. Careful moulding to avoid loose
sand and washing is the first requisite; sinking heads, that rise
above the moulds, are commonly employed when castings are of a form
which allows the dirt to collect at one point. Moulds for sinking
heads are formed by moulders as a rule, but are sometimes provided
for by the patterns.
The quality of castings is governed by a great many things besides
what have been named, such as the manner of gating or flowing the
metal into the moulds, the temperature and quality of the iron, the
temperature and character of the mould--things which any skilled
foundryman will take pleasure in explaining in answer to courteous
and proper questions.
Cores are employed mainly for what may be termed the displacement
of metal in moulds. There is no clear line of distinction between
cores and moulds, as founding is now conducted; cores may be
of green sand, and made to surround the exterior of a piece,
as well as to make perforations or to form recesses within it.
The term 'core,' in its technical sense, means dried moulds, as
distinguished from green sand. Wheels or other castings are said
to be cast in cores when the moulds are made in pieces and dried.
Supporting and venting cores, and their expansion, are conditions
to which especial attention is called. When a core is surrounded
with hot metal, it gives off, because of moisture and the burning
of the 'wash,' a large amount of gas which must have free means
of escape. In the arrangement of cores, therefore, attention must
be had to some means of venting, which is generally attained
by allowing them to project through the sides of the mould and
communicate with the air outside.
An apprentice may get a clear idea of this venting process by
inspecting tubular core barrels, such as are employed in moulding
pipes or hollow columns, or by examining ordinary cores about a
foundry. Provision of some kind to 'carry off the vent,' as it is
termed by moulders, will be found in every case. The venting of
moulds is even more important than venting cores, because core
vents only carry off gas generated within the core itself, while
the gas from its exterior surface, and from the whole mould, has to
find means of escaping rapidly from the flasks when the hot metal
enters.
A learner will no doubt wonder why sand is used for moulding,
instead of some more adhesive material like clay. If he is not
too fastidious for the experiment, and will apply a lump of damp
moulding sand to his mouth and blow his breath through the mass,
the query will be solved. If it were not for the porous nature of
sand-moulds they would be blown to pieces as soon as the hot metal
entered them; not only because of the mechanical expansion of the
gas, but often from explosion by combustion. Gas jets from moulds,
as may be seen at any time when castings are poured, will take fire
and burn the same as illuminating gas.
If it were not for securing vent for gas, moulds could be made from
plastic material so as to produce fine castings with clear sharp
outlines.
The means of supporting cores must be devised, or at least
understood, by pattern-makers; these supports consist of 'prints'
and 'anchors.' Prints are extensions of the cores, which project
through the casting and extend into the sides of the mould, to
be held by the sand or by the flask. The prints of cores have
duplicates on the patterns, called core prints, which are, or
should be, of a different colour from the patterns, so as to
distinguish one from the other. The amount of surface required to
support cores is dependent upon their weight, or rather upon their
cubic contents, because the weight of a core is but a trifling
matter compared to its floating force when surrounded by melted
metal. An apprentice in studying devices for supporting cores must
remember that the main force required is to hold them down, and
not to bear their weight. The floating force of a core is as the
difference between its weight and that of a solid of metal of the
same size--a matter moulders often forget to consider. It is often
impossible, from the nature of castings, to have prints large
enough to support the cores, and it is then effected by anchors,
pieces of iron that stand like braces between the cores and the
flasks or pieces of iron imbedded in the sand to receive the strain
of the anchors.
In constructing patterns where it is optional whether to employ
cores or not, and in preparing drawings for castings which may
have either a ribbed or a cored section, it is nearly always best
to employ cores. The usual estimate of the difference between the
cost of moulding rib and cored sections, as well as of skeleton and
cored patterns, is wrong. The expense of cores is often balanced
by the advantage of having an 'open mould,' that is accessible for
repairs or facing, and by the greater durability and convenience of
the solid patterns. Taking, for example, a column, or box frame for
machinery, that might be made either with a rib or a cored section,
it would at first thought seem that patterns for a cored casting
would cost much more by reason of the core-boxes; but it must be
remembered that in most patterns labour is the principal expense,
and what is lost in the extra lumber required for a core-box or in
making a solid pattern is in many cases more than represented in
the greater amount of labour required to construct a rib pattern.
Cores expand when heated, and require an allowance in their
dimensions the reverse from patterns; this is especially the
case when the cores are made upon iron frames. For cylindrical
cores less than six inches diameter, or less than two feet long,
expansion need not be taken into account by pattern-makers, but for
large cores careful calculation is required. The expansion of cores
is as the amount of heat imparted to them, and the amount of heat
taken up is dependent upon the quantity of metal that may surround
the core and its conducting power.
Shrinkage, or the contraction of castings in cooling, is provided
for by adding from one-tenth to one-eighth of an inch to each
foot in the dimensions of patterns. This is a simple matter,
and is accomplished by employing a shrink rule in laying down
pattern-drawings from the figured dimensions of the finished work;
such rules are about one-hundredth part longer than the standard
scale.
This matter of shrinkage is indeed the only condition in
pattern-making which is governed by anything near a constant rule,
and even shrinkage requires sometimes to be varied to suit special
cases. For small patterns whose dimensions do not exceed one foot
in any direction, rapping will generally make up for shrinkage, and
no allowance is required in the patterns, but pattern-makers are so
partial to the rule of shrinkage, as the only constant one in their
work, that they are averse to admitting exceptions, and usually
keep to the shrink rule for all pieces, whether large or small.
Inherent or cooling strains in castings is much more intricate than
shrinkage: it is, in fact, one of the most uncertain and obscure
matters that pattern-makers and moulders have to contend with.
Inherent strains may weaken castings, or cause them to break while
cooling, or sometimes even after they are finished; and in many
kinds of works such strains must be carefully guarded against, both
in the preparation of designs and the arrangement of patterns,
especially for wheels and pulleys with spokes, and for struts or
braces with both ends fixed. The main difficulty resulting from
cooling strains, however, is that of castings being warped and
sprung; this difficulty is continually present in the foundry and
machine-shop, and there is perhaps no problem in the whole range
of mechanical manipulation of which there exists more diversity
of opinion and practice than of means to prevent the springing of
castings. This being the case, an apprentice can hardly hope for
much information here. There is no doubt of springing and strains
in castings being the result of constant causes that might be fully
understood if it were not for the ever-changing conditions which
exist in casting, both as to the form of pieces, the temperature
and quality of metal, mode of cooling, and so on.
Castings are of course sprung by the action of unequal strains,
caused by one part cooling or 'setting' sooner than another. That
far all is clear, but the next step takes us into the dark. What
are the various conditions which induce irregular cooling, and how
is it to be avoided?
Irregularity of cooling may be the result of unequal conducting
power in different parts of a mould or cores, or it may be from
the varying dimensions of the castings, which contain heat as
their thickness, and give it off in the same ratio. As a rule,
the drag or bottom side of a casting cools first, especially if a
mould rests on the ground, and there is not much sand between the
castings and the earth; this is a common cause of unequal cooling,
especially in large flat pieces. Air being a bad conductor of heat,
and the sand usually thin on the cope or top side, the result is
that the top of moulds remain quite hot, while at the bottom the
earth, being a good conductor, carries off the heat and cools that
side first, so that the iron 'sets' first on the bottom, afterwards
cooling and contracting on the top, so that castings are warped and
left with inherent strains.
These are but a few of many influences which tend to irregular
cooling, and are described with a view of giving a clue from
which other causes may be traced out. The want of uniformity in
sections which tends to irregular cooling can often be avoided
without much loss by a disposition of the metal with reference
to cooling strains. This, so far as the extra metal required to
give uniformity to or to balance the different sides of a casting,
is a waste which engineers are sometimes loth to consent to,
and often neglect in designs for moulded parts; yet, as before
said, the difficulty of irregular cooling can in a great degree
be counteracted by a proper distribution of the metal, without
wasting, if the matter is properly understood. No one is prepared
to make designs for castings who has not studied the subject of
cooling strains as thoroughly as possible, from practical examples
as well as by theoretical deductions.
Draught, or the taper required to allow patterns to be
drawn readily, is another of those indefinite conditions in
pattern-making that must be constantly decided by judgment and
experience. It is not uncommon to find rules for the draught of
patterns laid down in books, but it would be difficult to find
such rules applied. The draught may be one-sixteenth of an inch to
each foot of depth, or it may be one inch to a foot of depth, or
there may be no draught whatever. Any rule, considered aside from
specified conditions, will only confuse a learner. The only plan to
understand the proper amount of draught for patterns is to study
the matter in connection with patterns and foundry operations.
Patterns that are deep, and for castings that require to be
parallel or square when finished, are made with the least possible
amount of draught. If a pattern is a plain form, that affords
facilities for lifting or drawing, it may be drawn without taper
if its sides are smooth and well finished. Pieces that are shallow
and moulded often should, as a matter of convenience, have as much
taper as possible; and as the quantity of draught can be as the
depth of a pattern, we frequently see them made with a taper that
exceeds one inch to the foot of depth.
Moulders generally rap patterns as much as they will stand, often
more than they will stand; and in providing for draught it is
necessary to take these customs into account. There is no use
in making provision to save rapping unless the rapping is to be
omitted.
Rapping plates, draw-irons, and other details of pattern-making are
soon understood by observation. Perhaps the most useful suggestion
which can be given in reference to draw-irons is to say they should
be set on the under or bottom side of patterns, instead of on the
top, where they are generally placed. A draw-plate set in this way,
with a hole bored through the pattern so as to insert draw-irons
from the top, cannot pull off, which it is apt to do if set on the
top side. Every pattern no matter how small, should be ironed,
unless it is some trifling piece, with dowel-pins, draw and rapping
plates. If a system of draw-irons is not rigidly carried out,
moulders will not trouble themselves to take care of patterns.
In conclusion, I will say on the subject of patterns and castings,
that a learner must depend mainly upon what he can see and what is
explained to him in the pattern-shop and foundry. He need never
fear an uncivil answer to a proper question, applied at the right
time and place. Mechanics who have enough knowledge to give useful
information of their business, have invariably the courtesy and
good sense to impart such information to those who require it.
An apprentice should never ask questions about simple and obvious
matters, or about such things as he can easily learn by his own
efforts. The more difficult a question is, the more pleasure a
skilled man will take in answering it. In short, a learner should
carefully consider questions before asking them. A good plan is
to write them down, and when information is wanted about casting,
never go to a foundry to interrupt a manager or moulder at melting
time, nor in the morning, when no one wants to be annoyed with
questions.
I will, in connection with this subject of patterns and castings,
suggest a plan of learning especially applicable in such cases,
that of adopting a habit of imagining the manner of moulding, and
the kind of pattern used in producing each casting that comes under
notice. Such a habit becomes easy and natural in a short time, and
is a sure means of acquiring an extended knowledge of patterns and
moulding.
A pattern-maker no sooner sees a casting than he imagines the kind
of pattern employed in moulding it; a moulder will imagine the plan
of moulding and casting a piece; while an engineer will criticise
the arrangement, proportions, adaptation, and general design, and
if skilled, as he ought to be, will also detect at a glance any
useless expense in patterns or moulding.
(1.) Why cannot the regular working drawings of a machine be
employed to construct patterns by?--(2.) What should determine
the quality or durability of patterns?--(3.) How can the
arrangement of patterns affect certain parts of a casting?--(4.)
What means can be employed to avoid inherent strain in
castings?--(5.) Why is the top of a casting less sound than the
bottom or drag side?--(6.) What are cores employed for?--(7.)
What is meant by venting a mould?--(8.) Explain the difference
between green and dry sand mouldings.--(9.) Why is sand employed
for moulds?--(10.) What generally causes the disarrangement
of cores in casting?--(11.) Why are castings often sprung or
crooked?--(12.) What should determine the amount of draught given
to patterns?--(13.) What are the means generally adopted to avoid
cooling strains in castings?
CHAPTER XXIII.
_FORGING._
Workshop processes which are capable of being systematised are the
most easy to learn. When a process is reduced to a system it is no
longer a subject of special knowledge, but comes within general
rules and principles, which enable a learner to use his reasoning
powers to a greater extent in mastering it.
To this proposition another may be added, that shop processes may
be systematised or not, as they consist in duplication, or the
performance of certain operations repeatedly in the same manner. It
has been shown in the case of patterns that there could be no fixed
rules as to their quality or the mode of constructing them, and
that how to construct patterns is a matter of special knowledge and
skill.
These rules apply to forging, but in a different way from other
processes. Unlike pattern-making or casting, the general processes
in forging are uniform; and still more unlike pattern-making or
casting, there is a measurable uniformity in the articles produced,
at least in machine-forging, where bolts, screws, and shafts are
continually duplicated.
A peculiarity of forging is that it is a kind of hand process,
where the judgment must continually direct the operations, one blow
determining the next, and while pieces forged may be duplicates,
there is a lack of uniformity in the manner of producing them.
Pieces may be shaped at a white welding heat or at a low red heat,
by one or two strong blows or by a dozen lighter blows, the whole
being governed by the circumstances of the work as it progresses. A
smith may not throughout a whole day repeat an operation precisely
in the same manner, nor can he, at the beginning of an operation,
tell the length of time required to execute it, nor even the
precise manner in which he will perform it. Such conditions are
peculiar, and apply to forging alone.
I think proper to point out these peculiarities, not so much from
any importance they may have in themselves, but to suggest critical
investigation, and to dissipate any preconceived opinions of
forging being a simple matter, easy to learn, and involving only
commonplace operations.
The first impressions an apprentice forms of the smith-shop as a
department of an engineering establishment is that it is a black,
sooty, dirty place, where a kind of rough unskilled labour is
performed--a department which does not demand much attention.
How far this estimate is wrong will appear in after years, when
experience has demonstrated the intricacies and difficulties of
forging, and when he finds the skill in this department is more
difficult to obtain, and costs more relatively than in any other.
Forging as a branch of work requires, in fact, the highest skill,
and is one where the operation continually depends upon the
judgment of the workman, which neither power nor machines can to
any extent supplant. Dirt, hard labour, and heat deter men from
learning to forge, and create a preference for the finishing shop,
which in most places makes a disproportion between the number of
smiths and finishers.
Forging as a process in machine-making includes the forming and
shaping of the malleable parts of machinery, welding or joining
pieces together, the preparation of implements for forging and
finishing, tempering of steel tools, and usually case-hardening.
Considered as a process, forging may be said to relate to shaping
malleable material by blows or compression when it is rendered soft
by heating. So far as hand-tools and the ordinary hand operations
in forging, there can be nothing said that will be of much use
to a learner. In all countries, and for centuries past, hand
implements for forging have remained quite the same; and one has
only to visit any machine forging-shop to see samples and types
of standard tools. There is no use in describing tongs, swages,
anvils, punches, and chisels, when there is nothing in their form
nor use that may not be seen at a glance; but tools and machines
for the application of motive power in forging processes deserve
more careful notice.
Forging plant consists of rolling mills, trip-hammers,
steam-hammers, drops, and punches, with furnaces, hearths, and
blowing apparatus for heating. A general characteristic of all
forging machines is that of a great force acting throughout a short
distance. Very few machines, except the largest hammers, exceed a
half-inch of working range, and in average operations not one-tenth
of an inch.
These conditions of short range and great force are best attained
by what may be termed percussion, and by machines which act by
blows instead of positive and gradual pressure; hence we find that
hand and power hammers are the most common tools among those of the
smith-shop.
To exert a powerful force acting through but a short distance,
percussive devices are much more effective and simple than those
acting by maintained or direct pressure. A hammer-head may give a
blow equal to many tons by its momentum, and absorb the reactive
force which is equal to the blow; but if an equal force was to be
exerted by screws, levers, or hydraulic apparatus, we can easily
see that an abutment would be required to withstand the reactive
force, and that such an abutment would require a strength perhaps
beyond what ingenuity could devise.
This principle is somewhat obscure, and the nature of percussive
forces not generally considered--a matter which may be illustrated
by considering the action of a simple hand-hammer. Few people, in
witnessing the use of a hammer, or in using one themselves, ever
think of it as an engine giving out tons of force, concentrating
and applying power by functions which, if performed by other
mechanism, would involve trains of gearing, levers, or screws; and
that such mechanism, if employed instead of a hammer, must lack
that important function of applying force in any direction as the
will and hands may direct. A simple hand-hammer is in the abstract
one of the most intricate of mechanical agents--that is, its action
is more difficult to analyse than that of many complex machines
involving trains of mechanism; yet our familiarity with hammers
causes this fact to be overlooked, and the hammer has even been
denied a place among those mechanical contrivances to which there
has been applied the name of "mechanical powers."
Let the reader compare a hammer with a wheel and axle, inclined
plane, screw, or lever, as an agent for concentrating and applying
power, noting the principles of its action first, and then
considering its universal use, and he will conclude that, if there
is a mechanical device that comprehends distinct principles, that
device is the common hammer. It seems, indeed, to be one of those
provisions to meet a human necessity, and without which mechanical
industry could not be carried on. In the manipulation of nearly
every kind of material, the hammer is continually necessary in
order to exert a force beyond what the hands may do, unaided by
mechanism to multiply their force. A carpenter in driving a spike
requires a force of from one to two tons; a blacksmith requires a
force of from five pounds to five tons to meet the requirements
of his work; a stonemason applies a force of from one hundred to
one thousand pounds in driving the edge of his tools; chipping,
calking, in fact nearly all mechanical operations, consist more
or less in blows, such blows being the application of accumulated
force expended throughout a limited distance.
Considered as a mechanical agent, a hammer concentrates the power
of the arms, and applies it in a manner that meets the requirements
of various purposes. If great force is required, a long swing and
slow blows accomplish tons; if but little force is required, a
short swing and rapid blows will serve--the degree of force being
not only continually at control, but also the direction in which
it is applied. Other mechanism, if employed instead of hammers
to perform a similar purpose, would require to be complicated
machines, and act in but one direction or in one plane.
These remarks upon hammers are not introduced here as a matter
of curiosity, nor with any intention of following mechanical
principles beyond where they will explain actual manipulation, but
as a means of directing attention to percussive acting machines
generally, with which forging processes, as before explained, have
an intimate connection.
Machines and tools operating by percussive action, although they
comprise a numerous class, and are applied in nearly all mechanical
operations, have never received that amount of attention in
text-books which the importance of the machines and their extensive
use calls for. Such machines have not even been set off as a class
and treated of separately, although the distinction is quite clear
between machines with percussive action, and those with what may
be termed direct action, both in the manner of operating and in
the general plans of construction. There is, of course, no lack of
formulæ for determining the measure of force, and computing the
dynamic effect of percussive machines acting against a measured or
assumed resistance, and so on; but this is not what is meant. There
are certain conditions in the operation of machines, such as the
strains which fall upon supporting frames, the effect produced upon
malleable material when struck or pressed, and more especially of
conditions which may render percussive or positive acting machines
applicable to certain purposes; but little explanation has been
given which is of value to practical men.
Machines and tools that operate by blows, such as hammers and
drops, produce effect by the impact of a moving mass by force
accumulated throughout a long range, and expending the sum of
this accumulated force on an object. The reactive force not being
communicated to nor resisted by the machine frames, is absorbed by
the inertia of the mass which gave the blow; the machinery required
in such operations being only a weight, with means to guide or
direct it, and mechanism for connection with motive power. A
hand-hammer, for example, accumulates and applies the force of the
arm, and performs all the functions of a train of mechanism, yet
consists only of a block of metal and a handle to guide it.
Machines with direct action, such as punches, shears, or rolls,
require first a train of mechanism of some kind to reduce the
motion from the driving power so as to attain force; and secondly,
this force must be balanced or resisted by strong framing, shafts,
and bearings. A punching-machine, for example, must have framing
strong enough to resist a thrust equal to the force applied to the
work; hence the frames of such machines are always a huge mass,
disposed in the most advantageous way to meet and resist this
reactive force, while the main details of a drop-machine capable
of exerting an equal force consist only of a block and a pair of
guides to direct its course.
Leaving out problems of mechanism in forging machines, the
adaptation of pressing or percussive processes is governed mainly
by the size and consequent inertia of the pieces acted upon. In
order to produce a proper effect, that is, to start the particles
of a piece throughout its whole depth at each blow, a certain
proportion between a hammer and the piece acted upon must be
maintained. For heavy forging, this principle has led to the
construction of enormous hammers for the performance of such work
as no pressing machinery can be made strong enough to execute,
although the action of such machinery in other respects would best
suit the conditions of the work. The greater share of forging
processes may be performed by either blows or compression, and no
doubt the latter process is the best in most cases. Yet, as before
explained, machinery to act by pressure is much more complicated
and expensive than hammers and drops. The tendency in practice is,
however, to a more extensive employment of press-forging processes.
(1.) What peculiarity belongs to the operation of forging to
distinguish it from most others?--(2.) Describe in a general way
what forging operations consist in.--(3.) Name some machines
having percussive action.--(4.) What may this principle of
operating have to do with the framing of a machine?--(5.) If a
steam-hammer were employed as a punching-machine, what changes
would be required in its framing?--(6.) Explain the functions
performed by a hand-hammer.
CHAPTER XXIV.
_TRIP-HAMMERS._
Trip-hammers employed in forging bear a close analogy to, and were
no doubt first suggested by, hand-hammers. Being the oldest of
power-forging machines, and extensively employed, it will be proper
to notice trip-hammers before steam-hammers.
As remarked in the case of other machines treated of, there is no
use of describing the mechanism of trip-hammers; it is presumed
that every engineer apprentice has seen trip-hammers, or can do so;
and the plan here is to deal especially with what he cannot see,
and would not be likely to learn by casual observation.
One of the peculiarities of trip-hammers as machines is the
mechanical difficulties in connecting them with the driving power,
especially in cases where there are a number of hammers to be
driven from one shaft.
The sudden and varied resistance to line shafts tends to loosen
couplings, destroy gearing, and produce sudden strains that are
unknown in other cases; and shafting arranged with the usual
proportions for transmitting power will soon fail if applied to
driving trip-hammers. Rigid connections or metal attachments ace
impracticable, and a slipping belt arranged so as to have the
tension varied at will is the usual and almost the only successful
means of transmitting power to hammers. The motion of trip-hammers
is a curious problem; a head and die weighing, together with the
irons for attaching them, one hundred pounds, will, with a helve
eight feet long, strike from two to three hundred blows a minute.
This speed exceeds anything that could be attained by a direct
reciprocal motion given to the hammer-head by a crank, and far
exceeds any rate of speed that would be assumed from theoretical
inference. The hammer-helve being of wood, is elastic, and acts
like a vibrating spring, its vibrations keeping in unison with the
speed of the tripping points. The whole machine, in fact, must be
constructed upon a principle of elasticity throughout, and in this
regard stands as an exception to almost every other known machine.
The framing for supporting the trunnions, which one without
experience would suppose should be very rigid and solid, is found
to answer best when composed of timber, and still better when this
timber is laid up in a manner that allows the structure to spring
and yield. Starting at the dies, and following back through the
details of a trip-hammer to the driving power, the apprentice may
note how many parts contribute to this principle of elasticity:
First--the wooden helve, both in front of and behind the trunnion;
next--the trunnion bar, which is usually a flat section mounted
on pivot points; third--the elasticity of the framing called the
'husk,' and finally the frictional belt. This will convey an idea
of the elasticity required in connecting the hammer-head with the
driving power, a matter to be borne in mind, as it will be again
referred to.
Another peculiar feature in trip-hammers is the rapidity with which
crystallisation takes place in the attachments for holding the die
blocks to the helves, where no elastic medium can be interposed to
break the concussion of the dies. Bolts to pass through the helve,
although made from the most fibrous Swedish iron, will on some
kinds of work not last for more than ten days' use, and often break
in a single day. The safest mode of attaching die blocks, and the
one most common, is to forge them solid, with an eye or a band to
surround the end of the helve.
At the risk of laying down a proposition not warranted by science,
I will mention, in connection with this matter of crystallisation,
that metal when disposed in the form of a ring, for some strange
reason seems to evade the influences which produce crystalline
change. A hand-hammer, for example, may be worn away and remain
fibrous; the links of chains and the tires of waggon wheels do not
become crystallised; even the tires on locomotive wheels seem to
withstand this influence, although the conditions of their use are
such as to promote crystallisation.
Among exceptions to the ordinary plans of constructing
trip-hammers, may be mentioned those employed in the American
Armoury at Springfield, U.S., where small hammers with rigid frames
and helves, the latter thirty inches long, forged from Lowmoor
iron, are run at a speed of 'six hundred blows a minute.' As an
example, however, they prove the necessity for elasticity, because
the helves and other parts have to be often renewed, although the
duty performed is very light, such as making small screws.
(1.) What limits the speed at which the reciprocating parts of
machines may act?--(2.) What is the nature of reciprocal motion
produced by cranks?--(3.) Can reciprocating movement be uniform
in such machines as power-hammers, saws, or pumps?--(4.) What
effect as to the rate of movement is produced by the elastic
connections of a trip-hammer?
CHAPTER XXV.
_CRANK-HAMMERS._
Power-hammers operated by crank motion, adapted to the lighter
kinds of work, are now commonly met with in the forging-shops
of engineering establishments. They are usually of very simple
construction, and I will mention only two points in regard to such
hammers, which might be overlooked by an apprentice in examining
them.
The faces of the dies remain parallel, no matter how large the
piece may be that is operated upon, while with a trip-hammer,
the top die moves in an arc described from the trunnions of the
helve, and the faces of the dies can only be parallel when in one
position, or when operating on pieces of a certain depth. This
feature of parallel movement with the dies of crank-hammers is
of great importance on some kinds of work, and especially so for
machine-forgings where the size or depth of the work is continually
being varied.
A second point to be noticed in hammers of this class is the
nature of the connection with the driving power. In all cases
there will be found an equivalent for the elastic helve of the
trip-hammer--either air cylinders, deflecting springs, or other
yielding attachments,--interposed between the crank and the
hammer-head, also a slipping frictional belt or frictional clutches
for driving, as in the case of trip-hammers.
CHAPTER XXVI.
_STEAM-HAMMERS._
The direct application of steam to forging-hammers is without
doubt the greatest improvement that has ever been made in forging
machinery; not only has it simplified operations that were carried
on before this invention, but has added many branches, and extended
the art of forging to purposes which could never have been attained
except for the steam-hammer.
The general principles of hammer-action, so far as already
explained, apply as well to hammers operated by direct steam; and
a learner, in forming a conception of steam-hammers, must not fall
into the common error of regarding them as machines distinct from
other hammers, or as operating upon new principles. A steam-hammer
is nothing more than the common hammer driven by a new medium,
a hammer receiving power through the agency of steam instead of
belts, shafts, and cranks. The steam-hammer in its most improved
form is so perfectly adapted to fill the different conditions
required in power-hammering, that there seems nothing left to be
desired.
Keeping in view what has been said about an elastic connection
for transmitting motion and power to hammers, and cushioning the
vibratory or reciprocating parts, it will be seen that steam as a
driving medium for hammers fills the following conditions:--
_First._--The power is connected to the hammer by means of the
least possible mechanism, consisting only of a cylinder, a piston,
and slide valve, induction pipe and throttle valve; these few
details taking the place of a steam-engine, shafts, belts, cranks,
springs, pulleys, gearing, in short, all such details as are
required between the hammer-head and the steam-boiler in the case
of trip-hammers or crank-hammers.
_Second._--The steam establishes the greatest possible elasticity
in the connection between a hammer and the driving power, and at
the same time serves to cushion the blows at both the top and
bottom of the stroke, or on the top only, as occasion may require.
_Third._--Each blow given is an independent operation, and can
be repeated at will, while in other hammers such changes can only
be made throughout a series of blows by gradually increasing or
diminishing their force.
_Fourth._--There is no direct connection between the moving parts
of the hammer and the framing, except lateral guides for the
hammer-head; the steam being interposed as a cushion in the line
of motion, this reduces the required strength and weight of the
framing to a minimum, and avoids positive strains and concussion.
_Fifth._--The range and power of the blows, as well as the time in
which they are delivered, is controlled at will; this constitutes
the greatest distinction between steam and other hammers, and the
particular advantage which has led to their extended use.
_Sixth._--Power can be transmitted to steam-hammers through a small
pipe, which may be carried in any direction, and for almost any
distance, at a moderate expense, so that hammers may be placed
in such positions as will best accommodate the work, and without
reference to shafts or other machinery.
_Seventh._--There is no waste of power by slipping belts or other
frictional contrivances to graduate motion; and finally, there is
no machinery to be kept in motion when the hammer is not at work.
Keeping these various points in mind, an apprentice will derive
both pleasure and advantage from tracing their application
in steam-hammers, which may come under notice, and various
modifications of the mechanism will only render investigation more
interesting.
One thing more must be noticed, a matter of some intricacy, but
without which, all that has been explained would fail to give a
proper idea of steam-hammer-action. The valve motions are alluded
to.
Steam-hammers are divided into two classes--one having the valves
moved by hand, and the other class with automatic valve movement.
The action of steam-hammers may also be divided into what is termed
elastic blows, and dead blows.
In operating by elastic blows, the steam piston is cushioned at
both the up and down stroke, and the action of a steam-hammer
corresponds to that of a helve trip-hammer, the steam filling the
office of a vibrating spring; in this case a hammer gives a quick
rebounding blow, the momentum being only in part spent upon the
work, and partly arrested by cushioning on the steam in the bottom
of the cylinder under the piston.
Aside from the greater rapidity with which a hammer may operate
when working on this principle, there is nothing gained, and much
lost; and as this kind of action is imperative in any hammer that
has a 'maintained or positive connection' between its reciprocating
parts and the valve, it is perhaps fair to infer that one reason
why most automatic hammers act with elastic blows is either because
of a want of knowledge as to a proper valve arrangement, or the
mechanical difficulties in arranging valve gear to produce dead
blows.
In working with dead blows, no steam is admitted under the piston
until the hammer has finished its down stroke, and expended its
momentum upon the work. So different is the effect produced by
these two plans of operating, that on most kinds of work a hammer
of fifty pounds, working with dead blows, will perform the same
duty that one of a hundred pounds will, when acting by elastic or
cushioned blows.
This difference between dead and elastic strokes is so important
that it has served to keep hand-moved valves in use in many cases
where much could be gained by employing automatic acting hammers.
Some makers of steam-hammers have so perfected the automatic class,
that they may be instantly changed so as to work with either dead
blows or elastic blows at pleasure, thereby combining all the
advantages of both principles. This brings the steam-hammer where
it is hard to imagine a want of farther improvement.
The valve gearing of automatic steam-hammers to fill the two
conditions of allowing a dead or an elastic blow, furnishes one of
the most interesting examples of mechanical combination.
It was stated that to give a dead or stamp stroke, the valve must
move and admit steam beneath the piston after the hammer has made
a blow and stopped on the work, and that such a movement of the
valve could not be imparted by any maintained connection between
the hammer-head and valve. This problem is met by connecting the
drop or hammer-head with some mechanism which will, by reason of
its momentum, continue to 'move after the hammer-head stops.' This
mechanism may consist of various devices. Messrs Massey in England,
and Messrs Ferris & Miles in America, employ a swinging wiper bar,
which is by reason of its weight or inertia retarded, and does not
follow the hammer-head closely on the down stroke, but swings into
contact and opens the valve after the hammer has come to a full
stop.
By holding this wiper bar continuously in contact with the
hammer-drop, elastic or rebounding blows are given, and by adding
weight in certain positions to the wiper bar its motion is so
retarded that a hammer will act as a stamp or drop. A German firm
employs the concussion of the blow to disengage valve gear, so
that it may fall and effect this after movement of the valves.
Other engineers effect the same end by employing the momentum of
the valve itself, having it connected to the drop by a slotted or
yielding connection, which allows an independent movement of the
valve after the hammer stops.
(1.) In comparing steam-hammers with trip or crank hammers
what mechanism does steam supplant or represent?--(2.) What
can be called the chief distinction between steam and other
hammers?--(3.) Under what circumstances is an automatic valve
motion desirable?--(4.) Why is a dead or uncushioned blow most
effective?--(5.) Will a hammer operate with air the same as with
steam?
CHAPTER XXVII.
_COMPOUND HAMMERS._
Another principle to be noticed in connection with hammers and
forging processes is that of the inertia of the piece operated
upon--a matter of no little importance in the heavier kinds of work.
When a piece is placed on an anvil, and struck on the top side
with a certain force, the bottom or anvil side of the piece does
not receive an equal force. A share of the blow is absorbed by the
inertia of the piece struck, and the effect on the bottom side is,
theoretically, as the force of the blow, less the cushioning effect
and the inertia of the pieces acted upon.
In practice this difference of effect on the top and bottom, or
between the anvil and hammer sides of a piece, is much greater
than would be supposed. The yielding of the soft metal on the top
cushions the blow and protects the under side from the force. The
effect produced by a blow struck upon hot iron cannot be estimated
by the force of the blow; it requires, to use a technical term,
a certain amount of force to "start" the iron, and anything less
than this force has but little effect in moving the particles and
changing the form of a piece.
From this it may be seen that there must occur a great loss of
power in operating on large pieces, for whatever force is absorbed
by inertia has no effect on the underside. By watching a smith
using a hand hammer it will be seen that whenever a piece operated
upon is heavier than the hammer employed, but little if any effect
is produced on the anvil or bottom surface, nor is this loss of
effect the only one. The expense of heating, which generally
exceeds that of shaping forgings, is directly as the amount of
shaping that may be done at each heat; and consequently, if the
two sides of a piece, instead of one, can be equally acted upon,
one-half the heating will be saved.
Another object gained by equal action on both sides of large
pieces is the quality of the forgings produced, which is generally
improved by the rapidity of the shaping processes, and injured by
too frequent heating.
The loss of effect by the inertia of the pieces acted upon
increases with the weight of the work; not only the loss of power,
but also the expense of heating increases with the size of the
pieces. There is, however, such a difference in the mechanical
conditions between light and heavy forging that for any but a heavy
class of work there would be more lost than gained in attempting to
operate on both sides of pieces at the same time.
To attain a double effect, and avoid the loss pointed out, Mr
Ramsbottom designed what may be called compound hammers, consisting
of two independent heads or rams moving in opposite directions, and
acting simultaneously upon pieces held between them.
It would be inferred that the arrangement of these double acting
hammers must necessarily be complicated and expensive, but the
contrary is the fact. The rams are simply two masses of iron
mounted on wheels that run on ways, like a truck, and the impact
of the hammers, so far as not absorbed in the work, is neutralised
by each other. No shock or jar is communicated to framing or
foundations as in the case of single acting hammers that have fixed
anvils. The same rule applies in the back stroke of the hammers as
the links which move them are connected together at the centre,
where the power is applied at right angles to the line of the
hammer movement. The links connecting the two hammers constitute,
in effect, a toggle joint, the steam piston being attached where
they meet in the centre.
The steam cylinder which moves the hammers is set in the earth at
some depth below the plane upon which they move, and even when
the heaviest work is done there is no perceptible jar when one is
standing near the hammers, as there always is with those which have
a vertical movement and are single acting.
(1.) Why is the effect produced different on the top and
bottom of a piece when struck by a hammer?--(2.) Why does not
a compound hammer create jar and concussion?--(3.) What would
be a mechanical difficulty in presenting the material to such
hammers?--(4.) Which is most important, speed or weight, in the
effect produced on the under side of pieces, when struck by
single acting hammers?
CHAPTER XXVIII.
_TEMPERING STEEL._
Tempering may be called a mystery of the smith-shop; this operation
has that attraction which characterises every process that is
mysterious, especially such as are connected with, or belong to
mechanical manipulation. A strange and perhaps fortunate habit
of the mind is to be greatly interested in what is not well
understood, and to disregard what is capable of plain demonstration.
An old smith who has stood at the forge for a score of years will
take the same interest in tempering processes that a novice will.
When a piece is to be tempered which is liable to spring or break,
and the risk is great, he will enter upon it with the same zeal and
interest that he would have done when learning his trade.
No one has been able to explain clearly why a sudden change of
temperature hardens steel, nor why it assumes various shades of
colour at different degrees of hardness; we only know the fact, and
that steel fortunately has such properties.
Every one who uses tools should understand how to temper them,
whether they be for iron or wood. Experiments with tempered tools
is the only means of determining the proper degree of hardness,
and as smiths, except with their own tools, have to rely upon the
explanations of others as to proper hardening, it follows that
tempering is generally a source of complaint.
Tempering, as a term, is used to comprehend both hardening and
drawing; as a process it depends mainly upon judgment instead of
skill, and has no such connection with forging as to be performed
by smiths only. Tempering requires a different fire from those
employed in forging, and also more care and precision than
blacksmiths can exercise, unless there are furnaces and baths
especially arranged for tempering tools.
A difficulty which arises in hardening tools is because of the
contraction of the steel which takes place in proportion to the
change of temperature; and as the time of cooling is in proportion
to the thickness or size of a piece, it follows, of course, that
there is a great strain and a tendency to break the thinner parts
before the thicker parts have time to cool; this strain may take
place either from cooling one side first, or more rapidly than
another.
The following propositions in regard to tempering, comprehend the
main points to be observed:
The permanent contraction of steel in tempering is as the degree of
hardness imparted to it by the bath.
The time in which the contraction takes place is as the temperature
of the bath and the cross section of the piece; in other words the
heat passes off gradually from the surface to the centre.
Thin sections of steel tools being projections from the mass which
supports the edges, are cooled first, and if provision is not made
to allow for contraction they are torn asunder.
The main point in hardening and the most that can be done to avoid
irregular contraction, is to apply the bath so that it will act
first and strongest on the thickest parts. If a piece is tapering
or in the form of a wedge, the thick end should enter the bath
first; a cold chisel for instance that is wide enough to endanger
cracking should be put into the bath with the head downward.
The upflow of currents of warmed water are a common cause of
irregular cooling and springing of steel tools in hardening; the
water that is heated, rises vertically, and the least inclination
of a piece from a perpendicular position, allows a warm current to
flow up one side.
The most effectual means of securing a uniform effect from a
tempering bath is by violent agitation, either of the bath or the
piece; this also adds to the rapidity of cooling.
The effect of tempering baths is as their conducting power;
chemicals except as they may contribute to the conducting
properties of a bath, may safely be disregarded. For baths, cold or
ice water loaded with salt for extreme hardness, and warm oil for
tools that are thin and do not require to be very hard, are the two
extremes outside of which nothing is required in ordinary practice.
In the case of tools composed partly of iron and partly of steel,
steel laid as it is called, the tendency to crack in hardening
may be avoided in most cases by hammering the steel edge at a low
temperature until it is so expanded that when cooled in hardening
it will only contract to a state of rest and correspond to the iron
part; the same result may be produced by curving a piece, giving
convexity to the steel side before hardening.
Tools should never be tempered by immersing their edges or cutting
parts in the bath, and then allowing the heat to "run down" to
attain a proper temper at the edge. I am well aware that this is
attacking a general custom, but it is none the less wrong for that
reason. Tools so hardened have a gradually diminishing temper from
their point or edge, so that no part is properly tempered, and they
require continual re-hardening, which spoils the steel; besides,
the extreme edge, the only part which is tempered to a proper
shade, is usually spoiled by heating and must be ground away to
begin with. No latheman who has once had a set of tools tempered
throughout by slow drawing, either in an oven, or on a hot plate,
will ever consent to point hardening afterwards. A plate of iron,
two to two and one-half inches thick, placed over the top of a
tool dressing fire, makes a convenient arrangement for tempering
tools, besides adding greatly to the convenience of slow heating,
which is almost as important as slow drawing. The writer has by
actual experiment determined that the amount of tool dressing and
tempering, to say nothing of time wasted in grinding tools, may in
ordinary machine fitting be reduced one-third by "oven tempering."
As to the shades that appear in drawing temper, or tempering it
is sometimes called, it is quite useless to repeat any of the old
rules about "straw colour, violet, orange, blue," and so on; the
learner knows as much after such instruction as before. The shades
of temper must be seen to be learned, and as no one is likely to
have use for such knowledge before having opportunities to see
tempering performed, the following plan is suggested for learning
the different shades. Procure eight pieces of cast steel about two
inches long by one inch wide and three-eighths of an inch thick,
heat them to a high red heat and drop them into a salt bath;
preserve one without tempering to show the white shade of extreme
hardness, and polish one side of each of the remaining seven
pieces; then give them to an experienced workman to be drawn to
seven varying shades of temper ranging from the white piece to the
dark blue colour of soft steel. On the backs of these pieces labels
can be pasted describing the technical names of the shades and the
general uses to which tools of corresponding hardness are adapted.
This will form an interesting collection of specimens and accustom
the eye to the various tints, which after some experience will be
instantly recognised when seen separately.
It may be remarked as a general rule that the hardness of cutting
tools is "inverse as the hardness of the material to be cut,"
which seems anomalous, and no doubt is so, if nothing but the
cutting properties of edges is considered; but all cutting edges
are subjected to transverse strain, and the amount of this strain
is generally as the hardness of the material acted upon; hence the
degree of temper has of necessity to be such as to guard against
breaking the edges. Tools for cutting wood, for example, can be
much harder than for cutting iron, or to state it better, tools for
cutting wood are harder than those usually employed for cutting
iron; for if iron tools were always as carefully formed and as
carefully used as those employed in cutting wood, they could be
equally hard.
Forges, pneumatic machinery for blast, machinery for handling
large pieces, and other details connected with forging, are easily
understood from examples.
(1.) What causes tools to bend or break in hardening?--(2.)
What means can be employed to prevent injury to tools in
hardening?--(3.) Can the shades of temper be produced on a piece
of steel without hardening?--(4.) What forms a limit of hardness
for cutting tools?--(5.) What are the objects of steel-laying
tools instead of making them of solid steel?
CHAPTER XXIX.
_FITTING AND FINISHING._
The fitting or finishing department of engineering establishments
is generally regarded as the main one.
Fitting processes, being the final ones in constructing machinery,
are more nearly in connection with its use and application; they
consist in the organisation or bringing together the results of
other processes carried on in the draughting room, pattern shop,
foundry, and smith shop.
To the unskilled, or to those who do not take a comprehensive view
of an engineering business as a whole, the finishing and fitting
department seems to constitute the whole of machine manufacture--an
impression which a learner should guard against, because nothing
but a true understanding of the importance and relations of the
different divisions of an establishment can enable them to be
thoroughly or easily learned.
Finishing, therefore, it must be borne in mind, is but one among
several processes, and that the fitting department is but one out
of four or more among which attention is to be divided.
Finishing as a process is a secondary and not always an essential
one; many parts of machinery are ready for use when forged or cast
and do not require fitting; yet a finishing shop must in many
respects be considered the leading department of an engineering
establishment. Plans, drawings and estimates are always based on
finished work, and when the parts have accurate dimensions; hence
designs, drawings and estimates may be said to pass through the
fitting shop and follow back to the foundry and smith shop, so
that finishing, although the last process in the order of the
work, is the first one after the drawings in every other sense;
even the dimensions in pattern-making which seems farthest removed
from finishing, are based upon fitting dimensions, and to a great
extent must be modified by the conditions of finishing.
In casting and forging operations the material is treated while in
a heated and expanded condition; the nature of these operations
is such that accurate dimensions cannot be attained, so that both
forgings and castings require to be made enough larger than their
finished dimensions to allow for shrinkage and irregularities.
Finishing as a process consists in cutting away this surplus
material, and giving accurate dimensions to the parts of machinery
when the material is at its natural temperature. Finishing
operations being performed as said upon material at its normal
temperature permits handling, gauging and fitting together of the
parts of machinery, and as nearly all other processes involve
heating, finishing may be called the cold processes of metal
work. The operations of a fitting shop consist almost entirely of
cutting, and grinding or abrading; a proposition that may seem
novel, yet these operations comprehend nearly all that is performed
in what is called fitting.
Cutting processes may be divided into two classes: cylindrical
cutting, as in turning, boring, and drilling, to produce circular
forms; and plane cutting, as in planing, shaping, slotting and
shearing, to produce plane or rectangular forms. Abrading or
grinding processes may be applied to forms of any kind.
To classify further--cutting machines may be divided into those
wherein the tools move and the material is fixed, and those wherein
the material is moved and the tools fixed, and machines which
involve a compound movement of both the tools and the material
acted upon.
There is also a distinction between machine and hand cutting
that may be noted. In machine cutting it is performed in true
geometrical lines, the tools or material being moved by positive
guides as in planing and turning; in hand operations, such as
filing, scraping or chipping, the tools are moved without positive
guidance, and act in irregular lines.
To attempt a generalisation of the operations of the fitting shop
in this manner may not seem a very practical means of understanding
them, yet the application will be better understood as we go
farther on.
Cutting tools include nearly all that are employed in finishing;
lathes, planing machines, drilling and boring machines, shaping,
slotting and milling machines, come within this class. The
machines named make up what are called standard tools, such as are
essential and are employed in all establishments where general
machine manufacture is carried on. Such machines are constructed
upon principles substantially the same in all countries, and have
settled into a tolerably uniform arrangement of movements and parts.
Besides the machine tools named, there are special machines to
be found in most works, machines directed to the performance of
certain work; by a particular adaptation such machines are rendered
more effective, but they are by such adaptation unfitted for
general purposes.
General engineering work cannot consist in the production of
duplicate pieces, nor in operations performed constantly in the
same manner as in ordinary manufacturing; hence there has been much
effort expended in adapting machines to general purposes--machines,
which seldom avoid the objections of combination, pointed out in a
previous chapter.
The principal improvements and changes in machine fitting at the
present time is in the application of special tools. A lathe, a
planing machine, or drilling machine as a standard machine, must
be adapted to a certain range of work, but it is evident that if
such tools were specially arranged for either the largest or the
smallest pieces that come within their capacity, more work could be
performed in a given time and consequently at less expense. It is
also evident that machine tools must be kept constantly at work in
order to be profitable, and when there are not sufficient pieces
of one kind to occupy a machine, it must be employed on various
kinds of work; but whenever there are sufficient pieces of the
same size upon which certain processes of a uniform character are
to be performed, there is a gain by having machines constructed to
conform as nearly as possible to the requirements of special work,
and without reference to any other.
It is now proposed to review the standard tools of a fitting
shop, noticing the general principles of their construction and
especially of their operation; not by drawings nor descriptions to
show what a lathe or a planing machine is, nor how some particular
engineer has constructed such tools, but upon the plan explained
in the introduction, presuming the reader to be familiar with the
names and purposes of standard machine tools. If he has not learned
this much, and does not understand the names and general objects
of the several operations carried on in a fitting shop, he should
proceed to acquaint himself thus far before troubling himself with
books of any kind.
(1.) Why cannot the parts of machinery be made to accurate
dimensions by forging or casting?--(2.) What is the difference
between hand tool and machine tool operation as to truth?--(3.)
Why cannot hand-work be employed in duplicating the parts of
machinery?--(4.) What is the difference between standard and
special machine tools?
CHAPTER XXX.
_TURNING LATHES._
In machinery the ruling form is cylindrical; in structures other
than machinery, those which do not involve motion, the ruling form
is rectangular.
Machine motion is mainly rotary; and as rotary motion is
accomplished by cylindrical parts such as shafts, bearings, pulleys
and wheels, we find that the greater share of machine tools are
directed to preparing cylindrical forms. If we note the area of
the turned, bored and drilled surface in ordinary machinery, and
compare with the amount of planed surface, we will find the former
not less than as two to one in the finer class of machinery, and
as three to one in the coarser class; from this may be estimated
approximately the proportion of tools required for operating on
cylindrical surfaces and plane surfaces; assuming the cutting tools
to have the same capacity in the two cases, the proportion will be
as three to one. This difference between the number of machines
required for cylindrical and plane surfaces is farther increased,
when we consider that tools act continually on cylindrical surfaces
and intermittently on plane surfaces.
In practice, the truth of this proposition is fully demonstrated by
the excess in the number of lathes and boring tools compared with
those for planing.
An engine lathe is for many reasons called the master tool in
machine fitting. It is not only the leading tool so far as
performing a greater share of the work; but an engine lathe as an
organised machine combines, perhaps, a greater number of useful
and important functions, than any machine which has ever been
devised. A lathe may be employed to turn, bore, drill, mill, or
cut screws, and with a strong screw-feed may be employed to some
extent for planing; what is still more strange, notwithstanding
these various functions, a lathe is comparatively a simple
machine without complication or perishable parts, and requires no
considerable change in adapting it to the various purposes named.
For milling, drilling or boring ordinary work within its range, a
lathe is by no means a makeshift tool, but performs these various
operations with nearly all the advantages of machines adapted to
each purpose. An ingenious workman who understands the adaptation
of a modern engine lathe can make almost any kind of light
machinery without other tools, except for planing, and may even
perform planing when the surfaces are not too large; in this way
machinery can be made at an expense not much greater than if a full
equipment of different tools is employed. This of course can only
be when no division of labour is required, and when one man is to
perform all the several processes of turning, drilling, and so on.
The lathe as a tool for producing heliacal forms would occupy
a prominent place among machine tools, if it were capable of
performing no other work; the number of parts of machinery which
have screw-threads is astonishing; clamping-bolts to hold parts
together include a large share of the fitting on machinery of all
kinds, while screws are the most common means for increasing power,
changing movements and performing adjustments.
A finisher's engine lathe consists essentially of a strong
inflexible shear or frame, a running spindle with from eight to
sixteen changes of motion, a sliding head, or tail stock, and a
sliding carriage to hold and move the tools.
For a half century past no considerable change has been made in
engine lathes, at least no new principle of operation has been
added, but many improvements have been made in their adaptation and
capacity for special kinds of work. Improvements have been made in
the facilities for changing wheels in screw cutting and feeding,
by frictional starting gear for the carriages, an independent feed
movement for turning, arrangements to adjust tools, cross feeding
and so on, adding something, no doubt, to the efficiency of lathes;
but the improvements named have been mainly directed to supplanting
the skill of lathemen.
A proof of this last proposition is found in the fact that a
thorough latheman will perform nearly as much work and do it as
well on an old English lathe with plain screw feed, as can be
performed on the more complicated lathes of modern construction;
but as economy of skill is sometimes an equal or greater object
than a saving of manual labour, estimates of tool capacity should
be made accordingly. The main points of a lathe, such as may most
readily affect its performance, are first--truth in the bearings
of the running spindle which communicates a duplicate of its shape
to pieces that are turned,--second, coincidence between the line
of the spindle and the movement of the carriage,--third, a cross
feed of the tool at a true right angle to the spindle and carriage
movement,--fourth, durability of wearing surfaces, especially the
spindle bearings and sliding ways. To these may be added many other
points, such as the truth of feeding screws, rigidity of frames,
and so on, but such requirements are obvious.
To avoid imperfection in the running spindles of lathes, or any
lateral movement which might exist in the running bearings, there
have been many attempts to construct lathes with still centres at
both ends for the more accurate kinds of work. Such an arrangement
would produce a true cylindrical rotation, but must at the same
time involve mechanical complication to outweigh the object gained.
It has besides been proved by practice that good fitting and good
material for the bearings and spindles of lathes will insure all
the accuracy which ordinary work demands.
It may be noticed that the carriages of some lathes move on what
are termed V tracks which project above the top of lathe frames,
and that in other lathes the carriages slide on top of the frames
with a flat bearing. As these two plans of mounting lathe carriages
have led to considerable discussion on the part of engineers, and
as its consideration may suggest a plan of analysing other problems
of a similar nature, I will notice some of the conditions existing
in the two cases, calling the different arrangements by the names
of flat shears and track shears.
These different plans will be considered first in reference to
the effect produced upon the movement of carriages; this includes
friction, endurance of wear, rigidity of tools, convenience of
operating and the cost of construction. The cutting point in both
turning and boring on a slide lathe is at the side of a piece,
or nearly level with the lathe centres, and any movement of a
carriage horizontally across the lathe affects the motion of the
tool and the shape of the piece acted upon, directly to the extent
of such deviation, so that parallel turning and boring depend
mainly upon avoiding any cross movement or side play of a carriage.
This, in both theory and practice, constitutes the greatest
difference between flat top and track shears; the first is arranged
especially to resist deviation in a vertical plane, which is of
secondary importance, except in boring with a bar; the second is
arranged to resist horizontal deviation, which in nine-tenths of
the work done on lathes becomes an exact measure of the inaccuracy
of the work performed.
A true movement of carriages is dependent upon the amount or
wearing power of their bearing surface, how this surface is
disposed in reference to the strain to be resisted, and the
conditions under which the sliding surfaces move; that is, how kept
in contact. The cutting strain which is to be mainly considered,
falls usually at an angle of thirty to forty degrees downward
toward the front, from the centre of the lathe. To resist such
strain a flat top shear presents no surface at right angles to the
strain; the bearings are all oblique, and not only this, but all
horizontal strain falls on one side of the shear only; for this
reason, flat top shears have to be made much heavier than would be
required if the sum of their cross section could be employed to
resist transverse strain. This difficulty can, however, be mainly
obviated by numerous cross girts, which will be found in most lathe
frames having flat tops.
A carriage moving on angular ways always moves steadily and
easily, without play in any direction until lifted from its
bearing, which rarely happens, and its lifting is easily opposed
by adjustable gibs. A carriage on a flat shear is apt to have play
in a horizontal direction because of the freedom which must exist
to secure easy movement. In the case of tracks, it may also be
mentioned that the weight of a carriage acts as a constant force to
hold it steady, while with a flat shear the weight of a carriage
is in a sense opposed to the ways, and has no useful effect in
steadying or guiding. The rigidity and steadiness of tool movement
is notoriously in favour of triangular tracks, so much so that
nearly all American machine tool-makers construct lathes in this
manner, although it adds no inconsiderable cost in fitting.
It may also be mentioned that lathes constructed with angular
guides, have usually such ways for the moving heads as well as for
the carriages; this gives the advantage of firmly binding the two
sides of the frame together in fastening the moving head, which in
effect becomes a strong girt across the frame; the carriages also
have an equal and independent hold on both sides of a shear. In
following this matter thus far, it may be seen how many conditions
may have to be considered in reasoning about so apparently simple a
matter as the form of ways for lathe carriages; we might even go on
to many more points that have not been mentioned; but what has been
explained will serve to show that the matter is not one of opinion
alone, and that without practical advantages, machine tool-makers
will not follow the most expensive of these two modes of mounting
lathe carriages.
Lathes in common use for machine fitting are screw-cutting engine
lathes, lathes for turning only, double-geared, single-geared,
and back-geared lathes, lathes for boring, hand-lathes, and
pulley-turning lathes; also compound lathes with double heads and
two tool carriages.
These various lathes, although of a widely varied construction
and adapted to uses more or less dissimilar, are still the engine
lathe either with some of its functions omitted to simplify and
adapt it to some special work, or with some of the operative parts
compounded to attain greater capacity.
In respect to lathe manipulation, which is perhaps the most
difficult to learn of all shop operations, the following hints are
given, which may prove of service to a learner: At the beginning
the form of tools should be carefully studied; this is one of the
great points in lathe work; the greatest distinction between a
thorough and indifferent latheman is that one knows the proper
form and temper of tools and the other does not. The adjustment
and presenting of tools is soon learned by experience, but the
proper form of tools is a matter of greater difficulty. One of the
first things to study is the shape of cutting edges, both as to
clearance below the edge of the tool, and the angle of the edge,
with reference to both turning and boring, because the latter
is different from turning. The angle of lathe tools is clearly
suggested by diagrams, and there is no better first lesson in
drawing than to construct diagrams of cutting angles for plane and
cylindrical surfaces.
A set of lathe tools should consist of all that are required for
every variety of work performed, so that no time will be lost
by waiting to prepare tools after they are wanted. An ordinary
engine lathe, operating on common work not exceeding twenty inches
of diameter, will require from twenty-five to thirty-five tools,
which will serve for every purpose if they are kept in order and in
place. A workman may get along with ten tools or even less, but not
to his own satisfaction, nor in a speedy way. Each tool should be
properly tempered and ground, ready for use 'when put away;' if a
tool is broken, it should at once be repaired, no matter when it is
likely to be again used. A workman who has pride in his tools will
always be supplied with as many as he requires, because it takes no
computation to prove that fifty pounds of extra cast steel tools,
as an investment, is but a small matter compared to the gain in
manipulation by having them at hand.
To an experienced mechanic a single glance at the tools on a lathe
is a sufficient clue to the skill of the operator. If the tools are
ground ready to use, of the proper shape, and placed in order so as
to be reached without delay, the latheman may at once be set down
as having two of the main qualifications of a first-class workman,
which are order, and a knowledge of tools; while on the contrary, a
lathe board piled full of old waste, clamp bolts, and broken tools,
shows a want of that system and order, without which no amount of
hand skill can make an efficient workman.
It is also necessary to learn as soon as possible the
technicalities pertaining to lathe work, and still more important
to learn the conventional modes of performing various operations.
Although lathe work includes a large range of operations which
are continually varied, yet there are certain plans of performing
each that has by long custom become conventional; to gain an
acquaintance with these an apprentice should watch the practice of
the best workmen, and follow their plans as near as he can, not
risking any innovation or change until it has been very carefully
considered. Any attempt to introduce new methods, modes of chucking
work, setting and grinding tools, or other of the ordinary
operations in turning, may not only lead to awkward mistakes, but
will at once put a stop to useful information that might otherwise
be gained from others. The technical terms employed in describing
lathe work are soon learned, generally sooner than they are needed,
and are often misapplied, which is worse than to be ignorant of
them.
In cutting screws it is best not to refer to that mistaken
convenience called a wheel list, usually stamped on some part of
engine lathes to aid in selecting wheels. A screw to be cut is to
the lead screw on a lathe as the wheel on the screw is to the wheel
on the spindle, and every workman should be familiar with so simple
a matter as computing wheels for screw cutting, when there is but
one train of wheels. Wheels for screw cutting may be computed not
only quite as soon as read from an index, but the advantage of
being familiar with wheel changes is very important in other cases,
and frequently such combinations have to be made when there is not
an index at hand.
The following are suggested as subjects which may be studied in
connection with lathes and turning: the rate of cutting movement
on iron, steel, and brass; the relative speed of the belt cones,
whether the changes are by a true ascending scale from the slowest;
the rate of feed at different changes estimated like the threads
of a screw at so many cuts per inch; the proportions of cone or
step pulleys to insure a uniform belt tension, the theory of
the following rest as employed in turning flexible pieces, the
difference between having three or four bearing points for centre
or following rests; the best means of testing the truth of a lathe.
All these matters and many more are subjects not only of interest
but of use in learning lathe manipulation, and their study will
lead to a logical method of dealing with problems which will
continually arise.
The use of hand tools should be learned by employing them on every
possible occasion. A great many of the modern improvements in
engine lathes are only to evade hand tool work, and in many cases
effect no saving except in skill. A latheman who is skilful with
hand tools will, on many kinds of light work, perform more and do
it better on a hand lathe than an engine lathe; there is always
more or less that can be performed to advantage with hand tools
even on the most elaborate engine lathes.
It is no uncommon thing for a skilled latheman to lock the slide
rest, and resort to hand tools on many kinds of work when he is in
a hurry.
(1.) Why does machinery involve so many cylindrical forms?--(2.)
Why is it desirable to have separate feed gear for turning and
screw cutting?--(3.) What is gained by the frictional starting
gearing now applied to the finer class of lathes?--(4.) How
may the alignment of a lathe be tested?--(5.) What kind of
deviation with a lathe carriage will most affect the truth of
work performed?--(6.) How may an oval hole be bored on a common
slide lathe?--(7.) How can the angular ways of a lathe and the
corresponding grooves in a carriage be planed to fit without
employing gauges?--(8.) Give the number of teeth in two wheels to
cut a screw of ten threads, when a leading screw is four threads
per inch?
CHAPTER XXXI.
_PLANING OR RECIPROCATING MACHINES._
The term planing should properly be applied only to machines that
produce planes or flat surfaces, but the technical use of the term
includes all cutting performed in right lines, or by what may be
called a straight movement of tools.
As no motion except rotary can be continuous, and as rotary
movement of tools is almost exclusively confined to shaping
cylindrical pieces, a proper distinction between machine tools
which operate in straight lines, and those which operate with
circular movement, will be to call them by the names of rotary and
reciprocating.
It may be noticed that all machines, except milling machines, which
act in straight lines and produce plane surfaces have reciprocating
movement; the class includes planing, slotting and shaping
machines; these, with lathes, constitute nearly the whole equipment
of an ordinary fitting shop.
It is strange, considering the simplicity of construction and the
very important office filled by machines for cutting on plane
surfaces, that they were not sooner invented and applied in metal
work. Many men yet working at finishing, can remember when all flat
surfaces were chipped and filed, and that long after engine lathes
had reached a state of efficiency and were generally employed,
planing machines were not known. This is no doubt to be accounted
for in the fact that reciprocal movement, except that produced by
cranks or eccentrics, was unknown or regarded as impracticable for
useful purposes until late years, and when finally applied it was
thought impracticable to have such movements operate automatically.
This may seem quite absurd to even an apprentice of the present
time, yet such reciprocating movement, as a mechanical problem, is
by no means so simple as it may at first appear.
A planing machine platen, for instance, moves at a uniform rate
of speed each way, and by its own motion shifts or reverses the
driving power at each extreme of the stroke. Presuming that there
were no examples to be examined, an apprentice would find many
easier problems to explain than how a planing machine can shift
its own belts. If a platen or table disengages the power that is
moving it, the platen stops; if the momentum carries it enough
farther to engage or connect other mechanism to drive the platen in
the opposite direction, the moment such mechanism comes into gear
the platen must stop, and no movement can take place to completely
engage clutches or shift belts. This is a curious problem that will
be referred to again.
Reciprocating tools are divided into those wherein the cutting
movement is given to the tools, as in shaping and slotting
machines, and machines wherein the cutting movement is given to
the material to be planed, as in a common planing machine. Very
strangely we find in general practice that machine tools for both
the heaviest and the lightest class of work, such as shaping, and
butting, operate upon the first principle, while pieces of a medium
size are generally planed by being moved in contact with stationary
tools.
This problem of whether to move the material or to move the tools
in planing, is an old one; both opinion and practice vary to some
extent, yet practice is fast settling down into constant rules.
Judged upon theoretical grounds, and leaving out the mechanical
conditions of operation, it would at once be conceded that a proper
plan would be to move the lightest body; that is, if the tools
and their attachments were heavier than the material to be acted
upon, then the material should be moved for the cutting action,
and _vice versa_. But in practice there are other conditions to
be considered more important than a question of the relative
weight of reciprocating parts; and it must be remembered that in
solving any problem pertaining to machine action, the conditions
of operation are to be considered first and have precedence over
problems of strain, arrangement, or even the general principles of
construction; that is, the conditions of operating must form a base
from which proportions, arrangements, and so on, must be deduced.
A standard planing machine, such as is employed for most kinds of
work, is arranged with a running platen or carriage upon which the
material is fastened and traversed beneath the cutting tools. The
uniformity of arrangement and design in machines of this kind in
all countries wherever they are made, must lead to the conclusion
that there are substantial reasons for employing running platens
instead of giving a cutting movement to the tools.
A planing machine with a running platen occupies nearly twice as
much floor space, and requires a frame at least one-third longer
than if the platen were fixed and the tools performed the cutting
movement. The weight which has to be traversed, including the
carriage, will in nearly all cases exceed what it would be with a
tool movement; so that there must exist some very strong reasons in
favour of a moving platen, which I will now attempt to explain, or
at least point out some of the more prominent causes which have led
to the common arrangement of planing machines.
Strains caused by cutting action, in planing or other machines,
fall within and are resisted by the framing; even when the tools
are supported by one frame and the material by another, such
frames have to be connected by means of foundations which become a
constituent part of the framing in such cases.
Direct action and reaction are equal; if a force is exerted in
any direction there must be an equal force acting in the opposite
direction; a machine must absorb its own strains.
Keeping this in view, and referring to an ordinary planing machine
with which the reader is presumed to be familiar, the focal point
of the cutting strain is at the edge of the tools, and radiates
from this point as from a centre to the various parts of the
machine frame, and through the joints fixed and movable between the
tools and the frame; to follow back from this cutting point through
the mechanism to the frame proper; first starting with the tool and
its supports and going to the main frame; then starting from the
material to be planed, and following back in the other direction,
until we reach the point where the strains are absorbed by the main
frame, examining the joints which intervene in the two cases, there
will appear some reasons for running carriages.
Beginning at the tool there is, first, a clamped joint between the
tool and the swing block; second, a movable pivoted joint between
the block and shoe piece; third, a clamped joint between the shoe
piece and the front saddle; fourth, a moving joint where the front
saddle is gibed to the swing or quadrant plate; fifth, a clamp
joint between the quadrant plate and the main saddle; sixth, a
moving joint between the main saddle and the cross head; seventh,
a clamp joint between the cross head and standards; and eighth,
bolted joints between the standards and the main frame; making in
all eight distinct joints between the tool and the frame proper,
three moving, four clamped, and one bolted joint.
Starting again from the cutting point, and going the other way from
the tool to the frame, there is, first, a clamped and stayed joint
between the material and platen, next, a running joint between
the platen and frame; this is all; one joint that is firm beyond
any chance of movement, and a moving joint that is not held by
adjustable gibs, but by gravity; a force which acts equally at
all times, and is the most reliable means of maintaining a steady
contact between moving parts.
Reviewing these mechanical conditions, we may at once see
sufficient reasons for the platen movement of planing machines;
and that it would be objectionable, if not impossible, to add a
traversing or cutting action to tools already supported through the
medium of eight joints. To traverse for cutting would require a
moving gib joint in place of the bolted one, between the standards
and main frame, leading to a complication of joints and movements
quite impracticable.
These are, however, not the only reasons which have led to a
running platen for planing machines, although they are the most
important.
If a cutting movement were performed by the tool supports, it
would necessarily follow that the larger a piece to be planed, and
the greater the distance from the platen to the cutting point,
the farther a tool must be from its supports; a reversal of the
conditions required; because the heavier the work the greater
the cutting strain will be, and the tool supports less able to
withstand the strains to be resisted.
It may be assumed that the same conditions apply to the standards
of a common planing machine, but the case is different; the upright
framing is easily made strong enough by increasing its depth; but
the strain upon running joints is as the distance from them at
which a force is applied, or to employ a technical phrase, as the
amount of overhang. With a moving platen the larger and heavier
a piece to be planed, the more firmly a platen is held down; and
as the cross section of pieces usually increases with their depth,
the result is that a planing machine properly constructed will act
nearly as well on thick as thin pieces.
The lifting strain at the front end of a platen is of course
increased as the height at which the cutting is done above its
top, but this has not in practice been found a difficulty of any
importance, and has not even required extra length or weight of
platens beyond what is demanded to receive pieces to be planed and
to resist flexion in fastening heavy work. The reversing movement
of planing machine platens already alluded to is one of the most
complex problems in machine tool movement.
Platens as a rule run back at twice the forward or cutting
movement, and as the motion is uniform throughout each stroke, it
requires to be stopped at the extremes by meeting some elastic or
yielding resistance which, to use a steam phrase, "cushions" or
absorbs the momentum, and starts the platen back for the return
stroke.
This object is attained in planing machines by the friction of
the belts, which not only cushions the platen like a spring, but
in being shifted opposes a gradually increasing resistance until
the momentum is overcome and the motion reversed. By multiplying
the movement of the platen with levers or other mechanism, and
by reason of the movement that is attained by momentum after the
driving power ceases to act, it is found practicable to have a
platen 'shift its own belts,' a result that would never have been
reached by theoretical deductions, and was no doubt discovered by
experiment, like the automatic movement of engine valves is said to
have been.
It is not intended to claim that this platen-reversing motion
cannot, like any other mechanical movement, be resolved
mathematically, but that the mechanical conditions are so obscure
and the invention made at a time that warrants the supposition of
accidental discovery.
In the driving gearing of planing machines, conditions which favour
the reversing movement are high speed and narrow driving belts. The
time in which belts may be shifted is as their speed and width; to
be shifted a belt must be deflected or bent edgewise, and from this
cause wind spirally in order to pass from one pulley to another. To
bend or deflect a belt edgewise there will be required a force in
proportion to its width, and the time of passing from one pulley to
another is as the number of revolutions made by the pulleys.
Planing machines of the most improved construction are driven by
two belts instead of one, and many mechanical expedients have been
adopted to move the belts differentially, so that both should not
be on the driving pulley at the same time, but move one before
the other in alternate order. This is easily attained by simply
arranging the two belts with the distance between them equal to one
and one-half or one and three-fourth times the width of the driving
pulley. The effect is the same as that accomplished by differential
shifting gearing, with the advantage of permitting an adjustment of
the relative movement of the belts.
Another principle in planing machines which deserves notice is the
manner of driving carriages or platens; this is usually performed
by means of spur wheels and a rack. A rack movement is smooth
enough, and effective enough so far as a mechanical connection
between the driving gearing and a platen, but there is a difficulty
met with from the torsion and elasticity of cross-shafts and a
train of reducing gearing. In all other machines for metal cutting,
it has been a studied object to have the supports for both the
tools and the material as rigid as possible; but in the common type
of planing machines, such as have rack and pinion movement, there
is a controversion of this principle, inasmuch as a train of wheels
and several cross-shafts constitute a very effective spring between
the driving power and the point of cutting, a matter that is easily
proved by planing across the teeth of a rack, or the threads of
a screw, on a machine arranged with spur wheels and the ordinary
reducing gearing. It is true the inertia of a platen is interposed
and in a measure overcomes this elasticity, but in no degree that
amounts to a remedy.
A planing machine invented by Mr Bodmer in 1841, and since improved
by Mr William Sellers of Philadelphia, is free from this elastic
action of the platen, which is moved by a tangent wheel or screw
pinion. In Bodmer's machine the shaft carrying the pinion was
parallel to the platen, but in Sellers' machine is set on a shaft
with its axis diagonal to the line of the platen movement, so
that the teeth or threads of the pinion act partly by a screw
motion, and partly by a progressive forward movement like the
teeth of wheels. The rack on the platen of Mr Sellers' machine is
arranged with its teeth at a proper angle to balance the friction
arising from the rubbing action of the pinion, which angle has
been demonstrated as correct at 5°, the ordinary coefficient of
friction; as the pinion-shaft is strongly supported at each side of
the pinion, and the thrust of the cutting force falls mainly in the
line of the pinion shaft, there is but little if any elasticity, so
that the motion is positive and smooth.
The gearing of these machines is alluded to here mainly for the
purpose of calling attention to what constitutes a new and singular
mechanical movement, one that will furnish a most interesting
study, and deserves a more extended application in producing slow
reciprocating motion.
(1.) Can the driving power be employed directly to shift the
belts of a planing machine?--(2.) Why are planing machines
generally constructed with a running carriage instead of running
tools?--(3.) What objection exists in employing a train of spur
wheels to drive a planing machine carriage?--(4.) What is gained
by shifting the belts of a planing machine differentially?--(5.)
What produces the screeching of belts so common with planing
machines?--(6.) What conditions favour the shifting of planing
machine belts?
CHAPTER XXXII.
_SLOTTING MACHINES._
Slotting machines with vertical cutting movement differ from
planing machines in several respects, to which attention may be
directed. In slotting, the tools are in most cases held rigidly and
do not swing from a pivot as in planing machines. The tools are
held rigidly for two reasons; because the force of gravity cannot
be employed to hold them in position at starting, and because the
thrust or strain of cutting falls parallel, and not transverse to
the tools as in planing. Another difference between slotting and
planing is that the cutting movement is performed by the tools
and not by movement of the material. The cutting strains are also
different, falling at right angles to the face of the table, in the
same direction as the force of gravity, and not parallel to the
face of the table, as in planing and shaping machines.
The feed motion in slotting machines, because of the tools being
held rigidly, has to operate differently from that of planing
machines. The cross-feed of a planing machine may act during the
return stroke, but in slotting machines, the feed movement should
take place at the end of the up-stroke, or after the tools are
clear of the material; so much of the stroke as is made during the
feeding action is therefore lost; and because of this, mechanism
for operating the feed usually has a quick abrupt action so as to
save useless movement of the cutter bar.
The relation between the feeding and cutting motion of
reciprocating machines is not generally considered, and forms an
interesting problem for investigation.
(1.) Name some of the differences between planing and slotting
machines.--(2.) Why should the feed motion of a slotting machine
act abruptly?--(3.) To what class of work are slotting machines
especially adapted?
CHAPTER XXXIII.
_SHAPING MACHINES._
Shaping machines as machine tools occupy a middle place between
planing and slotting machines; their movements correspond more to
those of slotting machines, while the operation of the tools is the
same as in planing. Some of the advantages of shaping over planing
machines for certain kinds of work are, because of the greater
facilities afforded for presenting and holding small pieces, or
those of irregular shape; the supports or tables having both
vertical and horizontal faces to which pieces may be fastened, and
the convenience of the mechanism for adjusting and feeding tools.
Shaping machines are generally provided with adjustable vices,
devices for planing circular forms, and other details which cannot
be so conveniently employed with planing machines. Another feature
of shaping machines is a positive range of the cutting stroke
produced by crank motion, which permits tools to be stopped with
precision at any point; this admits of planing slots, keyways, and
such work as cannot well be performed upon common planing machines.
Shaping machines are divided into two classes, one modification
with a lateral feed of the tools and cutter bar, technically called
"travelling head machines," the other class with a feed motion of
the table which supports the work, called table-feeding machines.
The first modification is adapted for long pieces to be planed
transversely, such as toothed racks, connecting rods, and similar
work; the second class to shorter pieces where much hand adjustment
is required.
An interesting study in connection with modern shaping machines is
the principle of various devices called 'quick return' movements.
Such devices consist of various modifications of slotted levers,
and what is known as Whitworth's quick return motion.
The intricacy of the subject renders it a difficult one to deal
with except by the aid of diagrams, and as such mechanism may
be inspected in almost any machine fitting shop, attention is
called to the subject as one of the best that can be chosen for
demonstration by diagrams. Problems of these variable speed
movements are not only of great interest, but have a practical
importance not found in many better known problems which take up
time uselessly and have no application in a practical way.
The remarks, given in a former place, relating to tools for
turning, apply to those for planing as well, except that in planing
tools greater rigidity and strength are required.
(1.) Why are shaping machines better adapted than planing
machines for planing slots, key-ways, and so on?--(2.) What
objects are gained by a quick return motion of the cutter bar of
shaping machines?
CHAPTER XXXIV.
_BORING AND DRILLING._
Boring, as distinguished from drilling, consists in turning out
annular holes to true dimensions, while the term drilling is
applied to perforating or sinking holes in solid material. In
boring, tools are guided by axial support independent of the
bearing of their edges on the material, while in drilling, the
cutting edges are guided and supported mainly from their contact
with and bearing on the material drilled.
Owing to this difference in the manner of guiding and supporting
the cutting edges, and the advantages of an axial support for tools
in boring, it becomes an operation by which the most accurate
dimensions are attainable, while drilling is a comparatively
imperfect operation; yet the ordinary conditions of machine fitting
are such that nearly all small holes can be drilled with sufficient
accuracy.
Boring may be called internal turning, differing from external
turning, because of the tools performing the cutting movement,
and in the cut being made on concave instead of convex surfaces;
otherwise there is a close analogy between the operations of
turning and boring. Boring is to some extent performed on lathes,
either with boring bars or by what is termed chuck-boring, in the
latter the material is revolved and the tools are stationary.
Boring may be divided into three operations as follows:
chuck-boring on lathes; bar-boring, when a boring bar runs
on points or centres, and is supported at the ends only; and
bar-boring when a bar is supported in and fed through fixed
bearings. The principles are different in these operations, each
one being applicable to certain kinds of work. A workman who can
distinguish between these plans of boring, and can always determine
from the nature of a certain work which is the best to adopt, has
acquired considerable knowledge of fitting operations.
Chuck-boring is employed in three cases; for holes of shallow
depth, taper holes, and holes that are screw-threaded. As pieces
are overhung in lathe-boring there is not sufficient rigidity
neither of the lathe spindle nor of the tools to admit of deep
boring. The tools being guided in a straight line, and capable of
acting at any angle to the axis of rotation, the facilities for
making tapered holes are complete; and as the tools are stationary,
and may be instantly adjusted, the same conditions answer for
cutting internal screw-threads; an operation corresponding to
cutting external screws, except that the cross motions of the tool
slide are reversed.
The second plan of boring by means of a bar mounted on points or
centres is one by which the greatest accuracy is attainable;
it is like chuck-boring a lathe operation, and one for which no
better machine than a lathe has been devised, at least for the
smaller kinds of work. It is a problem whether in ordinary machine
fitting there is not a gain by performing all boring in this
manner whenever the rigidity of boring bars is sufficient without
auxiliary supports, and when the bars can pass through the work.
Machines arranged for this kind of boring can be employed in
turning or boring as occasion may require.
When a tool is guided by turning on points, the movement is
perfect, and the straightness or parallelism of holes bored in this
manner is dependent only on the truth of the carriage movement.
This plan of boring is employed for small steam cylinders,
cylindrical valve seats, and in cases where accuracy is essential.
The third plan of boring with bars resting in bearings is more
extensively practised, and has the largest range of adaptation. A
feature of this plan of boring is that the form of the boring-bar,
or any imperfection in its bearings, is communicated to the work;
a want of straightness in the bar makes tapering holes. This, of
course, applies to cases where a bar is fed through fixed bearings
placed at one or both ends of a hole to be bored. If a boring-bar
is bent, or out of truth between its bearings, the diameter of the
hole being governed by the extreme sweep of the cutters is untrue
to the same extent, because as the cutters move along and come
nearer to the bearings, the bar runs with more truth, forming a
tapering hole diminishing toward the rests or bearings. The same
rule applies to some extent in chuck-boring, the form of the lathe
spindle being communicated to holes bored; but lathe spindles are
presumed to be quite perfect compared with boring bars.
The prevailing custom of casting machine frames in one piece, or
in as few pieces as possible, leads to a great deal of bar-boring,
most of which can be performed accurately enough by boring bars
supported in and fed through bearings. By setting up temporary
bearings to support boring-bars, and improvising means of driving
and feeding, most of the boring on machine frames can be performed
on floors or sole plates and independent of boring machines and
lathes. There are but few cases in which the importance of studying
the principles of tool action is more clearly demonstrated than
in this matter of boring; even long practical experience seldom
leads to a thorough understanding of the various problems which it
involves.
Drilling differs in principle from almost every other operation
in metal cutting. The tools, instead of being held and directed
by guides or spindles, are supported mainly by the bearing of the
cutting edges against the material.
A common angular-pointed drill is capable of withstanding a greater
amount of strain upon its edges, and rougher use than any other
cutting implement employed in machine fitting. The rigid support
which the edges receive, and the tendency to press them to the
centre, instead of to tear them away as with other tools, allows
drills to be used when they are imperfectly shaped, improperly
tempered, and even when the cutting edges are of unequal length.
Most of the difficulties which formerly pertained to drilling are
now removed by machine-made drills which are manufactured and
sold as an article of trade. Such drills do not require dressing
and tempering or fitting to size after they are in use, make true
holes, are more rigid than common solid shank drills, and will
drill to a considerable depth without clogging.
A drilling machine, adapted to the usual requirements of a machine
fitting establishment, consists essentially of a spindle arranged
to be driven at various speeds, with a movement for feeding the
drills; a firm table set at right angles to the spindle, and
arranged with a vertical adjustment to or from the spindle, and a
compound adjustment in a horizontal plane. The simplicity of the
mechanism required to operate drilling tools is such that it has
permitted various modifications, such as column drills, radial
drills, suspended drills, horizontal drills, bracket drills,
multiple drills, and others.
Drilling, more than any other operation in metal cutting, requires
the sense of feeling, and is farther from such conditions as admit
of power feeding. The speed at which a drill may cut without
heating or breaking is dependent upon the manner in which it
is ground and the nature of the material drilled, the working
conditions may change at any moment as the drilling progresses;
so that hand feed is most suitable. Drilling machines arranged
with power feed for boring should have some means of permanently
disengaging the feeding mechanism to prevent its use in ordinary
drilling.
I am well aware how far this opinion is at variance with practice,
especially in England; yet careful observation in a workshop will
prove that power feed in ordinary drilling effects no saving of
time or expense.
(1.) What is the difference between boring and drilling?--(2.)
Why will drills endure more severe use than other tools?--(3.)
Why is hand feeding best suited for drills?--(4.) What is the
difference between boring with a bar supported on centres and one
fed through journal bearings?
CHAPTER XXXV.
_MILLING._
Milling relates to metal cutting with serrated rotary cutters,
and differs in many respects from either planing or turning. The
movement of the cutting edges can be more rapid than with tools
which act continuously, because the edges are cooled during the
intervals between each cut; that is, if a milling tool has twenty
teeth, any single tooth or edge acts only from a fifteenth to a
twentieth part of the time; and as the cutting distance or time
of cutting is rarely long enough to generate much heat, the speed
of such tools may be one-half greater than for turning, drilling,
or planing tools. Another distinction between milling and other
tools is the perfect and rigid manner in which the cutting edges
are supported; they are short and blunt, besides being usually
carried on short rigid mandrils. A result of this rigid support of
the tools is seen in the length of the cutting edges that can be
employed, which are sometimes four inches or more in length. It
is true the amount of material cut away in milling is much less
than the edge movement will indicate when compared with turning
or planing; yet the displacing capacity of a milling machine
exceeds that of either a lathe or a planing machine. Theoretically
the cutting or displacing capacity of any metal or wood cutting
machine, is as the length of the edges multiplied into the speed
of their cutting movement; a rule which applies very uniformly in
wood cutting, and also in metal cutting within certain limits; but
the strains that arise in metal cutting are so great that they may
exceed all means of resisting them either in the material acted
upon, or in the means of supporting tools, so that the length of
cutting edges is limited. In turning chilled rolls at Pittsburg,
tools to six inches wide are employed, and the effect produced is
as the length of the edge; but the depth of the cut is slight, and
the operation is only possible because of the extreme rigidity of
the pieces turned, and the tools being supported without movable
joints as in common lathes.
Under certain conditions a given quantity of soft iron or steel may
be cut away at less expense, and with greater accuracy, by milling
than by any other process.
A milling tool with twenty edges should represent as much wearing
capacity as a like number of separate tools, and may be said to
equal twenty duplicate tools; hence, in cutting grooves, notches,
or similar work, a milling tool is equivalent to a large number of
duplicate single tools, which cannot be made or set with the same
truth; so that milling secures accuracy and duplication, objects
which are in many cases more important than speed.
Milling, as explained, being a more rapid process than either
planing or turning, it seems strange that so few machines of
this kind are employed in engineering shops. This points to some
difficulty to be contended with in milling, which is not altogether
apparent, because economic reasons would long ago have led to a
more extended use of milling processes, if the results were as
profitable as the speed of cutting indicates. This is, however, not
the case, except on certain kinds of material, and only for certain
kinds of work.
The advantages gained by milling, as stated, are speed,
duplication, and accuracy; the disadvantages are the expense of
preparing tools and their perishability.
A solid milling cutter must be an accurately finished piece of
work, made with more precision than can be expected in the work
it is to perform. This accuracy cannot be attained by ordinary
processes, because such tools, when tempered, are liable to become
distorted in shape, and frequently break. When hardened they must
be finished by grinding processes, if intended for any accurate
work; in fact, no tools, except gauging implements, involve more
expense to prepare, and none are so liable to accident when in use.
Such tools consist of a combination of cutting edges, all of which
may be said to depend on each one; because if one breaks, the
next in order will have a double duty to perform, and will soon
follow--a reversal of the old adage, that 'union is strength,' if
by strength is meant endurance.
In planing and turning, the tools require no exact form; they can
be roughly made, except the edge, and even this, in most cases, is
shaped by the eye. Such tools are maintained at a trifling expense,
and the destruction of an edge is a matter of no consequence.
The form, temper, and strength can be continually adapted to the
varying conditions of the work and the hardness of material. The
line of division between planing and milling is fixed by two
circumstances--the hardness and uniformity of the material to be
cut, and the importance of duplication. Brass, clean iron, soft
steel, or any homogeneous metal not hard enough to cause risk to
the tools, can be milled at less expense than planed, provided
there is enough work of a uniform character to justify the expense
of milling tools. Cutting the teeth of wheels is an example where
milling is profitable, but not to the extent generally supposed.
In the manufacture of small arms, sewing machines, clocks, and
especially watches, where there is a constant and exact duplication
of parts, milling is indispensable. Such manufactures are in some
cases founded on milling operations, as will be pointed out in
another chapter.
Milling tools large enough to admit of detachable cutters being
employed, are not so expensive to maintain as solid tools. Edge
movement can sometimes be multiplied in this way, so as to greatly
exceed what a single tool will perform.
Milling tools are employed at Crewe for roughing out the slots in
locomotive crank axles. A number of detachable tools are mounted on
a strong disc, so that four to six will act at one time; in this
way the displacement exceeds what a lathe can perform when acting
continuously with two tools. Rotary planing machines constructed on
the milling principle, have been tried for plane surfaces, but with
indifferent success, except for rough work.
There is nothing in the construction or operation of milling
machines but what will be at once understood by a learner who sees
them in operation. The whole intricacy of the process lies in its
application or economic value, and but very few, even among the
most skilled, are able in all cases to decide when milling can
be employed to advantage. Theoretical conclusions, aside from
practical experience, will lead one to suppose that milling can be
applied in nearly all kinds of work, an opinion which has in many
cases led to serious mistakes.
(1.) If milling tools operate faster than planing or turning
tools, why are they not more employed?--(2.) How may the effect
produced by cutting tools generally be computed?--(3.) To what
class of work are milling machines especially suited?--(4.) Why
do milling processes produce more accurate dimensions than are
attainable by turning or planing?--(5.) Why can some branches of
manufacture be said to depend on milling processes?
CHAPTER XXXVI.
_SCREW-CUTTING._
The tools employed for cutting screw threads constitute a separate
class among the implements of a fitting shop, and it is considered
best to notice them separately.
Screw-cutting is divided into two kinds, one where the blanks or
pieces to be threaded are supported on centres, the tools held
and guided independently of their bearing at the cutting edges,
called chasing; the other process is where the blanks have no axial
support, and are guided only by dies or cutting tools, called
die-cutting.
The first of these operations includes all threading processes
performed on lathes, whether with a single tool, by dies carried
positively by slide rests, or by milling.
The second includes what is called threading in America and
screwing in England. Machines for this purpose consist essentially
of mechanism to rotate either the blank to be cut or the dies, and
devices for holding and presenting the blanks.
Chasing produces screws true with respect to their axis, and is the
common process of threading all screws which are to have a running
motion in use, either of the screw itself, or the nut.
Die-cutting produces screws which may not be true, but are still
sufficiently accurate for most uses, such as clamping and joining
together the parts of machinery or other work.
Chasing operations being lathe work, and involving no principles
not already noticed, what is said further will be in reference to
die-cutting or bolt-threading machines, which, simple as they may
appear to the unskilled, involve, nevertheless many intricacies
which will not appear upon superficial examination.
Screw-cutting machines may be divided into modifications as
follows:--(1) Machines with running dies mounted in what is called
the head; (2) Machines with fixed dies, in which motion is given to
the rod or blank to be threaded; (3) Machines with expanding dies
which open and release the screws when finished without running
back; (4) Machines with solid dies, in which the screws have to be
withdrawn by changing the motion of the driving gearing; making in
all four different types.
If these various plans of arranging screw-cutting machines had
reference to different kinds of work, it might be assumed that all
of them are correct, but they are as a rule all applied to the
same kind of work; hence it is safe to conclude that there is one
arrangement better than the rest, or that one plan is right and
the others wrong. This matter may in some degree be determined by
following through the conditions of use and application.
Between a running motion of the dies, or a running motion of the
blanks, there are the following points which may be noticed.
If dies are fixed, the clamping mechanism to hold the rods has to
run with the spindle; such machines must be stopped while fastening
the rods or blanks. Clamping jaws are usually as little suited
for rotation on a spindle as dies are, and generally afford more
chances for obstruction and accident. To rotate the rods, if they
are long, they must pass through the driving spindle, because
machines cannot well be made of sufficient length to receive
long rods. In machines of this class, the dies have to be opened
and closed by hand instead of by the driving power, which can be
employed for the purpose when the dies are mounted in a running
head.
With running dies, blanks may be clamped when a machine is in
motion, and as the blank does not revolve, it may, when long, be
supported in any temporary manner. The dies can be opened and
closed by the driving power also, and no stopping of a machine is
necessary; so that several advantages of considerable importance
may be gained by mounting the dies in a running head, a plan which
has been generally adopted in late years by machine tool makers
both in England and America.
In respect to the difference between expanding and solid dies it
consists mainly in the time required to run back, and the injury
to dies which this operation occasions. Uniformity of size is
within certain limits insured by solid dies, but they are more
liable to derangement and less easy to repair than expanding or
independent dies.
Another difference between solid and expanding dies, which may be
pointed out, is in the firmness with which the cutting edges are
held. With a solid die, the edges or teeth being all combined in
one solid piece, are firmly held in a fixed position; while with
expanding dies their position has to be maintained by mechanical
devices which are liable to yield under the pressure which arises
in cutting. The result is, that the precision with which a
screwing machine with movable dies will act, is dependent upon the
strength of the 'abutment' behind the dies, which should be a hard
unyielding surface with as much area as possible.
Connected with screw dies, there are various problems, such as
clearance behind the cutting edge; whether an odd or even number
of edges are best; how many threads require to be bevelled at the
starting point; and many other matters about which there are no
determined rules. The diversity of opinion that will be met with on
these points, and in reference to taps, the form of screw-threads,
and so on, will convince a learner of the intricacies in this
apparently simple matter of cutting screw-threads.
(1.) Describe the different modifications of screw-cutting
machines.--(2.) What is gained by revolving the dies instead of
the rod?--(3.) What is gained by expanding dies?--(4.) What is
the difference between screws cut by chasing and those cut on a
screw-cutting machine?
CHAPTER XXXVII.
_STANDARD MEASURES._
Machines are composed of parts connected together by rigid and
movable joints; rigid joints are necessary because of the expense,
and in most cases the impossibility, of constructing framing and
other fixed detail in one piece.
All moving parts must of course be independent of fixed parts, the
relation between the two being maintained by what has been called
running joints.
It is evident that when the parts of a machine are joined
together, each piece which has contact on more than one side must
have specific dimensions; it is farther evident that as many of
the joints in a machine as are to accommodate the exigencies
of construction must be without space, that is, they represent
continued sections of what should be solid material, if it were
possible to construct the parts in that manner. This also demands
specific dimensions.
In arranging the details of machines, it is impossible to have a
special standard of dimensions for each case, or even for each
shop; the dimensions employed are therefore made to conform to some
general standard, which by custom becomes known and familiar to
workmen and to a country, or as we may now say to all countries.
A standard of lineal measures, however, cannot be taken from one
country to another, or even transferred from one shop to another
without the risk of variation; and it is therefore necessary
that such a standard be based upon something in nature to which
reference can be made in cases of doubt.
In ages past, various attempts were made to find some constant in
nature on which measures could be based. Some of these attempts
were ludicrous, and all of them failures, until the vibrations of
a pendulum connected length and space with time. The problem was
then more easy. The changes of seasons and the movement of heavenly
bodies had established measures of time, so that days, hours, and
minutes became constants, proved and maintained by the unerring
laws of nature.
A pendulum vibrating in uniform time regardless of distance, but
always as its length, if arranged to perform one vibration in a
given time, gave a constant measure of length. Thus lineal measure
comes from time; cubic or solid measures from lineal measure, and
standards of weight from the same source; because when a certain
quantity of a substance of any kind could be determined by lineal
measurement, and this quantity was weighed, a standard of weight
would be reached, provided there was some substance sufficiently
uniform, to which reference could be made in different countries.
Such a substance is sea or pure water; weighed in vacuo, or with
the air at an assumed density, water gives a result constant
enough for a standard of weight.
It is a strange thought that with all the order, system, and
regularity, existing in nature, there is nothing but the movements
of the heavenly bodies constant enough to form a base for gauging
tests. The French standard based upon the calculated length of the
meridian may be traced to this source.
Nothing animate or inanimate in nature is uniform; plants,
trees, animals, are all different; even the air we breathe and
the temperature around us is constantly changing; only one thing
is constant, that is time, and to this must we go for all our
standards.
I am not aware that the derivation of our standard measures has
been, in an historical way, as the foregoing remarks will indicate,
nor is it the purpose here to follow such history. A reader, whose
attention is directed to the subject, will find no trouble in
tracing the matter from other sources. The present object is to
show what a wonderful series of connections can be traced from so
simple a tool as a measuring gauge, and how abstruse, in fact,
are many apparently simple things, often regarded as not worth a
thought beyond their practical application.
(1.) Why are machine frames constructed in sections, instead of
being in one piece?--(2.) Why must parts which have contact on
opposite sides have specific dimensions?--(3.) What are standards
of measure based upon in England, America, and France?--(4.) How
can weight be measured by time?--(5.) Has the French metre proved
a standard admitting of test reference?
CHAPTER XXXVIII.
_GAUGING IMPLEMENTS._
Among the improvements in machine fitting which have in recent
years come into general use, is the employment of standard gauges,
by means of which uniform dimensions are maintained, and within
certain limits, an interchange of the parts of machinery is
rendered possible.
Standard gauging implements were introduced about the year 1840,
by the celebrated Swiss engineer, John G. Bodmer, a man who for
many reasons deserves to be considered as the founder of machine
tool manufacture. He not only employed gauges in his works to
secure duplicate dimensions, but also invented and put in use
many other reforms in manipulation; among these may be mentioned
the decimal or metrical division of measures, a system of detail
drawings classified by symbols, the mode of calculating wheels by
diametric pitch, with many other things which characterise the best
modern practice.
The importance of standard dimensions, and the effect which a
system of gauging may have in the construction of machines, will
be a matter of some difficulty for a learner to understand. The
interchangeability of parts, which is the immediate object in
employing gauges, is plain enough, and some of the advantages at
once apparent, yet the ultimate effects of such a system extend
much farther than will at first be supposed.
The division of labour, that system upon which we may say our
great industrial interests are founded, is in machine fitting
promoted in a wonderful degree by the use of gauging implements. If
standard dimensions can be maintained, it is easy to see that the
parts of a machine can be constructed by different workmen, or in
different shops, and these parts when assembled all fit together,
without that tedious and uncertain plan of try-fitting which was
once generally practised. There are, it is true, certain kinds
of fitting which cannot well be performed by gauges; moving flat
surfaces, such as the bearings of lathe slides or the faces of
steam engine valves, are sooner and better fitted by trying them
together and scraping off the points of contact; but even in such
cases the character of the work will be improved, if one or both
surfaces have been first levelled by gauging or surface plates.
In cylindrical fitting, which as before pointed out, constitutes
the greater part in machine fitting, gauges are especially
important, because trial-fitting is in most cases impossible.
Flat or plane joints nearly always admit of adjustment between the
fitted surfaces; that is, the material scraped or ground away in
fitting can be compensated by bringing the pieces nearer together;
but parallel cylindrical joints cannot even be tried together
until finished, consequently, there can be nothing cut away in
trying them together. Tapering, or conical joints, can of course be
trial-fitted, and even parallel fits are sometimes made by trial,
but it is evident that the only material that can be cut away in
such cases, is what makes the difference between a fit too close,
and one which will answer in practice.
As to the practical results which may be attained by a gauging
system, it may be said that they are far in advance of what is
popularly supposed, especially in Europe, where gauges were first
employed.
The process of milling, which has been so extensively adopted in
the manufacture of guns, watches, sewing-machines, and similar work
in America, has, on principles explained in the chapter on milling,
enabled a system of gauging which it is difficult to comprehend
without seeing the processes carried on. And so important is the
effect due to this duplicating or gauging system, that several
important branches of manufacture have been controlled in this way,
when other elements of production, such as the price of labour,
rent, interest, and so on, have been greatly in favour of countries
where the trying system is practised.
As remarked, the gauging system is particularly adapted to,
or enabled by milling processes, and of course must have its
greatest effect in branches of work directed to the production
of uniform articles, such as clocks, watches, sewing-machines,
guns, hand tools, and so on. That is, the direct effect on the
cost of processes will be more apparent and easily understood in
such branches of manufacture; yet in general engineering work,
where each machine is more or less modified, and made to special
plans, the commercial gain resulting from the use of gauges is
considerable.
In respect to repairing alone, the consideration of having the
parts of machinery fitted to standard sizes is often equal to its
whole value.
Machinery subjected to destructive wear, and to be operated at a
distance from machine shops--locomotive engines for example--if not
constructed with standard dimensions, may, by the detention due to
repairing, cause a loss and inconvenience equal to their value; if
a shaft wheel bearing, or even a fitted screw bolt is broken, time
must be allowed to make the parts new; and in order to fit them,
the whole machine, or such of its details as have connection with
the broken parts, must be taken to a shop in order to fit by trial.
The duplicate system has gradually made its way in locomotive
engineering, and will no doubt extend to the whole of railway
equipment, as constants for dimensions are proved and agreed upon.
The gauging system has been no little retarded by a selfish and
mistaken opinion that an engineering establishment may maintain
peculiar standards of its own; in fact, relics of this spirit
are yet to be met with in old machines, where the pitch of
screw-threads has been made to fractional parts of an inch, so that
engineers, other than the original makers, could not well perform
repairing, or replace broken parts.
One of the effects of employing gauges in machine fitting is to
inspire confidence in workmen. Instead of a fit being regarded
as a mysterious result more the work of chance than design, men
accustomed to gauges come to regard precision as something both
attainable and indispensable. A learner, after examining a set of
well fitted cylindrical gauges, will form a new conception of what
a fit is, and will afterwards have a new standard fixed in his mind.
The variation of dimensions which are sensible to the touch at
one ten-thousandth part of an inch, furnishes an example of how
important the human senses are even after the utmost precision
attainable by machine action. Pieces may pass beneath the cutters
of a milling machine under conditions, which so far as machinery
avails will produce uniform sizes, yet there is no assurance of the
result until the work is felt by gauges.
The eye fails to detect variations in size, even by comparison,
long before we reach the necessary precision in common fitting.
Even by comparison with figured scales or measuring with rules, the
difference between a proper and a spoiled fit is not discernible by
sight.
Many of the most accurate measurements are, however, performed
by sight, with vernier calipers for example, the variation being
multiplied hundreds or thousands of times by mechanism, until the
least differences can be readily seen.
In multiplying the variations of a measuring implement by
mechanism, it is obvious that movable joints must be employed; it
is also obvious that no positive joint, whether cylindrical or
flat, could be so accurately fitted as to transmit such slight
movement as occurs in gauging or measuring. This difficulty is in
most measuring instruments overcome by employing a principle not
before alluded to, but common in many machines, that of elastic
compensation.
A pair of spring calipers will illustrate this principle. The
points are always steady, because the spring acting continually
in one direction compensates the loose play that may be in the
screw. In a train of tooth wheels there is always more or less
play between the teeth; and unless the wheels always revolve in
one direction, and have some constant resistance offered to their
motion, 'backlash' or irregular movement will take place; but if
there is some constant and uniform resistance such as a spring
would impart, a train of wheels will transmit the slightest motion
throughout.
The extreme nicety with which gauging implements are fitted seems
at first thought to be unnecessary, but it must be remembered
that a cylindrical joint in ordinary machine fitting involves a
precision almost beyond the sense of feeling, and that any sensible
variation in turning gauges is enough to spoil a fit.
Opposed to the maintenance of standard dimensions are the
variations in size due to temperature. This difficulty applies
alike to gauging implements and to parts that are to be tested; yet
in this, as in nearly every phenomenon connected with matter, we
have succeeded in turning it to some useful purpose. Bands of iron,
such as the tires of wheels when heated, can be 'shrunk' on, and a
compressive force and security attained, which would be impossible
by forcing the parts together both at the same temperature.
Shrinking has, however, been almost entirely abandoned for such
joints as can be accurately fitted.
(1.) How may gauging implements affect the division of
labour?--(2.) In what way do standard dimensions affect the value
of machinery?--(3.) Why cannot cylindrical joints be fitted
by trying them together?--(4.) Under what circumstances is it
most important that the parts of machinery should have standard
dimensions?--(5.) Which sense is most acute in testing accurate
dimensions?--(6.) How may slight variations in dimensions be made
apparent to sight?
CHAPTER XXXIX.
_DESIGNING MACHINES._
It will scarcely be expected that any part of the present work,
intended mainly for apprentice engineers, should relate to
designing machines, yet there is no reason why the subject should
not to some extent be treated of; it is one sure to engage more or
less attention from learners, and the study of designing machines,
if properly directed, cannot fail to be of advantage.
There is, perhaps, no one who has achieved a successful experience
as an engineer but will acknowledge the advantages derived from
early efforts to generate original designs, and none who will not
admit that if their first efforts had been more carefully directed,
the advantages gained would have been greater.
It is exceedingly difficult for an apprentice engineer, without
experimental knowledge, to choose plans for his own education, or
to determine the best way of pursuing such plans when they have
been chosen; and there is nothing that consumes so much time, or is
more useless than attempting to make original designs, if there is
not some systematic method followed.
There is but little object in preparing designs, when their
counterparts may already exist, so that in making original plans,
there should be a careful research as to what has been already
done in the same line. It is not only discouraging, but annoying,
after studying a design with great care, to find that it has been
anticipated, and that the scheme studied out has been one of
reproduction only. For this reason, attempts to design should at
first be confined to familiar subjects, instead of venturing upon
unexplored ground.
Designing is in many respects the same thing as invention, except
that it deals more with mechanism than principles, although it
may, and often does include both. Like invention, designing should
always be attempted for the attainment of some definite object laid
down at the beginning, and followed persistently throughout.
It is not always an easy matter to hit upon an object to which
designs may be directed; and although at first thought it may seem
that any machine, or part of a machine, is capable of improvement,
it will be found no easy matter to detect existing faults or to
conceive plans for their remedy.
A new design should be based upon one of two suppositions--either
that existing mechanism is imperfect in its construction, or that
it lacks functions which a new design may supply; and if those who
spend their time in making plans for novel machinery would stop to
consider this from the beginning, it would save no little of the
time wasted in what may be called scheming without a purpose.
After determining the ultimate objects of an improvement, and
laying down the general principles which should be followed in
the preparation of a design, there is nothing connected with
constructive engineering that can be more nearly brought within
general rules than arranging details. I am well aware of how far
this statement is at variance with popular opinion among mechanics,
and of the very thorough knowledge of machine application and
machine operation required in making designs, and mean that
there are certain principles and rules which may determine the
arrangement and distribution of material, the position and relation
of moving parts, bearings, and so on, and that a machine may be
built up with no more risk of mistakes than in erecting a permanent
structure.
Designing machines must have reference to adaptation, endurance,
and the expense of construction. Adaptation includes the
performance of machinery, its commercial value, or what the
machinery may earn in operating; endurance, the time that machines
may operate without being repaired, and the constancy of their
performance; expense, the investment represented in machinery.
The adaptation, endurance, and cost of machines in designing become
resolved into problems of movements, the arrangement of parts, and
proportions.
Movements and strains may be called two of the leading conditions
upon which designs for machines are based: movements determine
general dimensions, and strains determine the proportions and sizes
of particular parts. Movement and strain together determine the
nature and area of bearings or bearing surfaces.
The range and speed of movement of the parts of machines are
elements in designing that admit of a definite determination
from the work to be accomplished, but arrangement cannot be so
determined, and is the most difficult to find data for. To sum up
these propositions we have:--
1. A conception of certain functions in a machine, and some
definite object which it is to accomplish.
2. Plans of adaptation and arrangement of the component parts of
the machinery, or organisation as it may be called.
3. A knowledge of specific conditions, such as strains, the range
and rate of movements, and so on.
4. Proportions of the various parts, including the framing, bearing
surfaces, shafts, belts, gearing, and other details.
5. Symmetry of appearance, which is often more the result of
obvious adaptation than ornamentation.
To illustrate the practical application of what has preceded, let
it be supposed, for example, that a machine is to be made for
cutting teeth in iron racks ¾ in. pitch and 3 in. face, and that
a design is to be prepared without reference to such machines as
may already be in use for the purpose.
It is not assumed that an actual design can be made which by words
alone will convey a comprehensive idea of an organised machine; it
is intended to map out a course which will illustrate a plan of
reasoning most likely to attain a successful result in such cases.
The reader, in order to better understand what is said, may keep in
mind a common shaping machine with crank motion, a machine which
nearly fills the requirements for cutting tooth racks.
Having assumed a certain work to do, the cutting of tooth racks
¾ in. pitch, and 3 in. face, the first thing to be considered
will be, is the machine to be a special one, or one of general
adaptation? This question has to do, first, with the functions of
the machine in the way of adapting it to the cutting of racks of
various sizes, or to performing other kinds of work, and secondly,
as to the completeness of the machine; for if it were to be a
standard one, instead of being adapted only to a special purpose,
there are many expensive additions to be supplied which can be
omitted in a special machine. It will be assumed in the present
case that a special machine is to be constructed for a particular
duty only.
The work to be performed consists in cutting away the metal between
the teeth of a rack, leaving a perfect outline for the teeth; and
as the shape of teeth cannot well be obtained by an adjustment of
tools, it must be accomplished by the shape of the tools. The
shape of the tools must, therefore, be constantly maintained, and
as the cross section of the displaced metal is not too great, it
may be assumed that the shape of the tools should be a profile of
the whole space between two teeth, and such a space be cut away at
one setting or one operation. By the application of certain rules
laid down in a former place in reference to cutting various kinds
of material, reciprocating or planing tools may be chosen instead
of rotary or milling tools.
Movements come next in order, and consist of a reciprocating
cutting movement of the tools or material, a feed movement to
regulate the cutting action, and a longitudinal movement of the
rack, graduated to pitch or space, the distance between the teeth.
The reciprocating cutting movement being but four inches or less,
a crank is obviously the best means to produce this motion, and
as the movement is transverse to the rack, which may be long and
unwieldy, it is equally obvious that the cutting motion should be
performed by the tools instead of the rack.
The feed adjustment of the tool being intermittent and the amount
of cutting continually varying, this movement should be performed
by hand, so as to be controlled at will by the sense of feeling.
The same rule applies to the adjustment of the rack for spacing;
being intermittent and irregular as to time, this movement should
also be performed by hand. The speed of the cutting movement is
known from ordinary practice to be from sixteen feet to twenty
feet a minute, and a belt two and a half inches wide must move two
hundred feet a minute to propel an ordinary metal cutting tool, so
that the crank movement or cutter movement must be increased by
gearing until a proper speed of the belt is reached; from this the
speed of intermediate movers will be found.
Arrangement comes next; in this the first matter to be considered
is convenience of manipulation. The cutting position should be so
arranged as to admit of an easy inspection of the work. An operator
having to keep his hand on the adjusting or feed mechanism, which
is about twelve inches above the work, it follows that if the
cutting level is four feet from the floor, and the feed handle
five feet from the floor, the arrangement will be convenient for a
standing position. As the work requires continual inspection and
hand adjustments, it will for this reason be a proper arrangement
to overhang both the supports for the rack and the cutting tools,
placing them, as we may say, outside the machine, to secure
convenience of access and to allow of inspection. The position of
the cutting bar, crank, connections, gearing, pulleys, and shafts,
will assume their respective places from obvious conditions, mainly
from the position of the operator and the work.
Next in order are strains. As the cutting action is the source
of strains, and as the resistance offered by the cutting tools
is as the length or width of the edges, it will be found in the
present case that while other conditions thus far have pointed
to small proportions, there is now a new one which calls for
large proportions. In displacing the metal between teeth of
three-quarters of an inch pitch, the cutting edge or the amount of
surface acted upon is equal to a width of one inch and a half. It
is true, the displacement may be small at each cut, but the strain
is rather to be based upon the breadth of the acting edge than the
actual displacement of metal, and we find here strains equal to the
average duty of a large planing machine. This strain radiates from
the cutting point as from a centre, falling on the supports of the
work with a tendency to force it from the framing. Between the rack
and the crank-shaft bearing, through the medium of the tool, cutter
bar, connection, and crank pin, and in various directions and
degrees, this strain may be followed by means of a simple diagram.
Besides this cutting strain, there are none of importance; the
tension of the belt, the side thrust in bearings, the strain from
the angular thrust of the crank, and the end thrust of the tool,
although not to be lost sight of, need not have much to do with
problems of strength, proportion, and arrangement.
Strains suggest special arrangement, which is quite a distinct
matter from general arrangement, the latter being governed mainly
by the convenience of manipulation. Special arrangement deals
with and determines the shape of framing, following the strains
throughout a machine. In the present case we have a cutting strain
which may be assumed as equal to one ton, exerted between the
bracket or jaws which support the work, and the crank-shaft. It
follows that between these two points the metal in the framing
should be disposed in as direct a line as possible, and provision
be made to resist flexion by deep sections parallel with the
cutting motion.
Lastly, proportions; having estimated the cutting force required
at one ton, although less than the actual strain in a machine of
this kind, we proceed upon this to fix proportions, beginning with
the tool shank, and following back through the adjusting saddle,
the cutting bar, connections, crank pins, shafts, and gear wheels
to the belt. Starting again at the tool, or point of cutting,
following through the supports of the rack, the jaws that clamp it,
the saddle for the graduating adjustment, the connections with the
main frame, and so on to the crank-shaft bearing a second time,
dimensions may be fixed for each piece to withstand the strains
without deflection or danger of breaking. Such proportions cannot,
I am aware, be brought within the rules of ordinary practice by
relying upon calculation alone to fix them, and no such course is
suggested; calculation may aid, but cannot determine proportions
in such cases; besides, symmetry, which cannot be altogether
disregarded, modifies the form and sometimes the dimensions of
various parts.
I have in this way imperfectly indicated a methodical plan of
generating a design, as far as words alone will serve, beginning
with certain premises based upon a particular work to be performed,
and then proceeding to consider in consecutive order the general
character of the machine, mode of operation, movements and
adjustments, general arrangement, strains, special arrangement, and
proportions.
With a thorough knowledge of practical machine operation, and
an acquaintance with existing practice, an engineer proceeding
upon such a plan, will, if he does not overlook some of the
conditions, be able to generate designs which may remain without
much modification or change, so long as the purpose to which the
machinery is directed remains the same.
Perseverance is an important trait to be cultivated in first
efforts at designing; it takes a certain amount of study to
understand any branch of mechanism, no matter what natural capacity
may be possessed by a learner. Mechanical operations are not
learned intuitively, but are always surrounded by many peculiar
conditions which must be learned _seriatim_, and it is only by an
untiring perseverance at one thing that there can be any hope of
improving it by new designs.
A learner who goes from gearing and shafts to steam and hydraulics,
from machine tools to cranes and hoisting machinery, will not
accomplish much. The best way is to select at first an easy
subject, one that admits of a great range of modification,
and if possible, one that has not assumed a standard form of
construction. Bearings and supports for shafts and spindles, is a
good subject to begin with.
In designing supports for shafts the strains are easily defined and
followed, while the vertical and lateral adjustment, lubrication
of bearings, symmetry of supports and hangers, and so on, will
furnish grounds for endless modification, both as to arrangement
and mechanism.
In making designs it is best to employ no references except such
as are carried in the memory. The more familiar a person is
with machinery of any class, the more able he may be to prepare
designs, but not by measuring and referring to other people's
plans. Dimensions and arrangement from examples are, by such a
course, unconsciously carried into a new drawing, even by the most
skilled; besides, it is by no means a dignified matter to collect
other people's plans, and by a little combination and modification
produce new designs. It may be an easy plan to acquire a certain
kind of proficiency, but will most certainly hinder an engineer
from ever rising to the dignity of an original designer.
Symmetry, as an element in designs for machinery, is one of those
unsettled matters which may be determined only in connection with
particular cases; it may, however, be said that for all engineering
implements and manufacturing machinery of every kind, there should
be nothing added for ornament, or anything that has no connection
with the functions of the machinery.
Modern engineers of the abler class are so thoroughly in accord in
this matter of ornamentation, both in opinion and practice, that
the subject hardly requires to be mentioned, and it will be no
disadvantage for a learner to commence by cultivating a contempt
for whatever has no useful purpose. Of existing practice it may be
said, that in what may be called industrial machinery, the amount
of ornamentation is inverse as the amount of engineering skill
employed in preparing designs.
A safe rule will be to assume that machinery mainly used and seen
by the skilled should be devoid of ornament, and that machinery
seen mainly by the unskilled, or in public, should have some
ornament. Steam fire engines, sewing machines, and works of a
similar kind, which fall under the inspection of the unskilled, are
usually arranged with more or less ornament.
As a rule, ornament should never be carried further than graceful
proportions; the arrangement of framing should follow as nearly
as possible the lines of strain. Extraneous decoration, such as
detached filagree work of iron, or painting in colours, is so
repulsive to the taste of the true engineer and mechanic that it is
unnecessary to speak against it.
(1.) Name some of the principal points to be kept in view in
preparing designs?--(2.) Why should attempts at designing be
confined to one class of machinery?--(3.) What objection exists
to examining references when preparing designs?
CHAPTER XL.
_INVENTION._
The relation between invention and the engineering arts, and
especially between invention and machines, will warrant a short
review of the matter here; or even if this reason were wanting,
there is a sufficient one in the fact that one of the first aims
of an engineering apprentice is to invent something; and as the
purpose here is, so far as the limits will permit, to say something
upon each subject in which a beginner has an interest, invention
must not be passed over.
It has been the object thus far to show that machines, processes,
and mechanical manipulation generally may be systematised and
generalised to a greater or less extent, and that a failure
to reduce mechanical manipulation and machine construction to
certain rules and principles can mainly be ascribed to our want of
knowledge, and not to any inherent difficulty or condition which
prevents such solution. The same proposition is applicable to
invention, with the difference that invention, in its true sense,
may admit of generalisation more readily than machine processes.
Invention, as applied to mechanical improvements, should not mean
chance discovery. Such a meaning is often, if not generally,
attached to the term invention, yet it must be seen that results
attained by a systematic course of reasoning or experimenting can
have nothing to do with chance or even discovery. Such results
partake more of the nature of demonstrations, a name peculiarly
suitable for such inventions as are the result of methodical
purpose.
In such sciences as rest in any degree upon physical experiment,
like chemistry, to experiment without some definite object may
be a proper kind of research, and may in the future, as it has
in the past, lead to great and useful results; but in mechanics
the case is different; the demonstration of the conservation of
force, and the relation between force and heat, have supplied the
last link in a chain of principles which may be said to comprehend
all that we are called upon to deal with in dynamical science,
and there remains but little hope of developing anything new or
useful by discovery alone. The time has been, and has not yet
passed away, when even the most unskilled thought their ability to
invent improvements in machinery equal with that of an engineer or
skilled mechanic; but this is now changed; new schemes are weighed
and tested by scientific standards, in many cases as reliable
as actual experiments. A veil of mystery which ignorance of the
physical sciences had in former times thrown around the mechanic
arts, has been cleared away; chance discovery, or mechanical
superstition, if the term may be allowed, has nearly disappeared.
Many modern engineers regard their improvements in machinery as the
exercise of their profession only, and hesitate about asking for
protective grants to secure an exclusive use of that which another
person might and often does demonstrate, as often as circumstances
call for such improvement. There are of course new articles of
manufacture to be discovered, and many improvements in machinery
which may be proper subject matter for patent rights; improvements
which in all chance would not be made for the term of a patent,
except by the inventor; but such cases are rare; and it is fair to
assume that unless an invention is one which could not have been
regularly deduced from existing data, and one that would not in all
probability have been made for a long term of years by any other
person than the inventor, such an invention cannot in fairness
become the property of an individual without infringing the rights
of others.
It is not the intention to discuss patent law, nor even to estimate
what benefits have in the past, or may in the future, be gained to
technical industry by the patent system, but to impress engineering
apprentices with a better and more dignified appreciation of their
calling than to confound it with chance invention, and thereby
destroy that confidence in positive results which has in the past
characterised mechanical engineering; also to caution learners
against the loss of time and effort too often expended in searching
after inventions.
It is well for an apprentice to invent or demonstrate all that he
can--the more the better; but as explained in a previous place,
what is attempted should be according to some system, and with a
proper object. Time spent groping in the dark after something of
which no definite conception has been formed, or for any object not
to fill an ascertained want, is generally time lost. To demonstrate
or invent, one should begin methodically, like a bricklayer builds
a wall, as he mortars and sets each brick, so should an engineer
qualify, by careful study, each piece or movement that is added to
a mechanical structure, so that when done, the result may be useful
and enduring.
As remarked, every attempt to generate anything new in machinery
should be commenced by ascertaining a want of improvement. When
such a want has been ascertained, attention should be directed
first to the principles upon which such want or fault is to be
remedied. Proper mechanism can then be supplied like the missing
links in a chain. Propositions thus stated may fail to convey the
meaning intended; this systematic plan of inventing may be better
explained by an example.
Presuming the reader to remember what was said of steam hammers
in another place, and to be familiar with the uses and general
construction of such hammers, let it be supposed steam-hammers,
with the ordinary automatic valve action, those that give an
elastic or steam-cushioned blow, are well known. Suppose further
that by analysing the blows given by hammers of this kind, it is
demonstrated that dead blows, such as are given when a hammer comes
to a full stop in striking, are more effectual in certain kinds of
work, and that steam-hammers would be improved by operating on this
dead-stroke principle.
Such a proposition would constitute the first stage of an invention
by demonstrating a fault in existing hammers, and a want of certain
functions which if added would make an improvement.
Proceeding from these premises, the first thing should be to
examine the action of existing valve gear, to determine where this
want of the dead-stroke function can best be supplied, and to gain
the aid of such suggestions as existing mechanism may offer, also
to see how far the appliances in use may become a part of any new
arrangement.
By examining automatic hammers it will be found that their valves
are connected to the drop by means of links, producing coincident
movement of the piston and valve, and that the movement of one is
contingent upon and governed by the other. It will also be found
that these connections or links are capable of extension, so as
to alter the relative position of the piston and valve, thereby
regulating the range of the blow, but that the movement of the two
is reciprocal or in unison. Reasoning inductively, not discovering
or inventing, it may be determined that to secure a stamp blow of
a hammer-head, the valve must not open or admit steam beneath the
piston until a blow is completed and the hammer has stopped.
At this point will occur one of those mechanical problems which
requires what may be called logical solution. The valve must be
moved by the drop; there is no other moving mechanism available;
the valve and drop must besides be connected, to insure coincident
action, yet the valve requires to move when the drop is still.
Proceeding inductively, it is clear that a third agent must be
introduced, some part moved by the drop, which will in turn move
the valve, but this intermediate agent so arranged that it may
continue to move after the hammer-drop has stopped.
This assumed, the scheme is complete, so far as the relative
movement of the hammer-drop and the valve, but there must be some
plan of giving motion to this added mechanism. In many examples
there may be seen parts of machinery which continue in motion after
the force which propels them has ceased to act; cannon balls are
thrown for miles, the impelling force acting for a few feet only; a
weaver's shuttle performs nearly its whole flight after the driver
has stopped. In the present case, it is therefore evident that an
independent or subsequent movement of the valves may be obtained by
the momentum of some part set in motion during the descent of the
hammer-head.
To sum up, it is supposed to have been determined by inductive
reasoning, coupled with some knowledge of mechanics, that a steam
hammer, to give a dead blow, requires the following conditions in
the valve gearing:
1. That the drop and valve, while they must act relatively, cannot
move in the same time, or in direct unison.
2. The connection between the hammer drop and valve cannot be
positive, but must be broken during the descent of the drop.
3. The valve must move after the hammer stops.
4. To cause a movement of the valve after the hammer stops there
must be an intermediate agent, that will continue to act after the
movement of the hammer drop has ceased.
5. The obvious means of attaining this independent movement of the
valve gear, is by the momentum of some part set in motion by the
hammer-drop, or by the force of gravity reacting on this auxiliary
agent.
The invention is now complete, and as the principles are all
within the scope of practical mechanism, there is nothing left
to do but to devise such mechanical expedients as will carry out
the principles laid down. This mechanical scheming is a second,
and in some sense an independent part of machine improvement, and
should always be subservient to principles; in fact, to separate
mechanical scheming from principles, generally constitutes what has
been called chance invention.
Referring again to the hammer problem, it will be found by
examining the history that the makers of automatic-acting
steam-hammers capable of giving the dead stamp blow, have employed
the principle which has been described. Instead of employing the
momentum, or the gravity of moving parts, to open the valve after
the hammer stops, some engineers have depended upon disengaging
valve gear by the concussion and jar of the blow, so that the valve
gearing, or a portion of it, fell and opened the valve. The 'dead
blow gear,' fitted to the earlier Nasmyth, or Wilson, hammers, was
constructed on the latter plan, the valve spindle when disengaged
being moved by a spring.
I will not consume space to explain the converse of this system
of inventing, nor attempt to describe how a chance schemer would
proceed to hunt after mechanical expedients to accomplish the valve
movement in the example given.
Inventions in machine improvement, no matter what their nature,
must of course consist in and conform to certain fixed modes
of operating, and no plan of urging the truth of a proposition
is so common, even with a chance inventor, as to trace out the
'principles' which govern his discovery.
In studying improvements with a view to practical gain, a learner
can have no reasonable hope of accomplishing much in fields already
gone over by able engineers, nor in demonstrating anything new in
what may be called exhausted subjects, such as steam-engines or
water-wheels; he should rather choose new and special subjects, but
avoid schemes not in some degree confirmed by existing practice.
It has been already remarked that the boldness of young engineers
is very apt to be inversely as their experience, not to say their
want of knowledge, and it is only by a strong and determined effort
towards conservatism, that a true balance is maintained in judging
of new schemes.
The life of George Stephenson proves that notwithstanding the
novelty and great importance of his improvements in steam transit,
he did not "discover" these improvements. He did not discover that
a floating embankment would carry a railway across Chat Moss,
neither did he discover that the friction between the wheels of
a locomotive and the rails would enable a train to be drawn by
tractive power alone. Everything connected with his novel history
shows that all of his improvements were founded upon a method of
reasoning from principles and generally inductively. To say that
he "discovered" our railway system, according to the ordinary
construction of the term, would be to detract from his hard and
well-earned reputation, and place him among a class of fortunate
schemers, who can claim no place in the history of legitimate
engineering.
Count Rumford did not by chance develope the philosophy of forces
upon which we may say the whole science of dynamics now rests; he
set out upon a methodical plan to demonstrate conceptions that were
already matured in his mind, and to verify principles which he had
assumed by inductive reasoning. The greater part of really good and
substantial improvements, such as have performed any considerable
part in developing modern mechanical engineering, have come through
this course of first dealing with primary principles, instead of
groping about blindly after mechanical expedients, and present
circumstances point to a time not far distant when chance discovery
will quite disappear.
(1.) What change has taken place in the meaning of the name
"invention" as applied to machine improvement?--(2.) What should
precede an attempt to invent or improve machinery?--(3.) In what
sense should the name invention be applied to the works of such
men as Bentham, Bodmer, or Stephenson?
CHAPTER XLI.
_WORKSHOP EXPERIENCE._
To urge the necessity of learning practical fitting as a part of
an engineering education is superfluous. A mechanical engineer who
has not been "through the shop" can never expect to attain success,
nor command the respect even of the most inferior workmen; without
a power of influencing and controlling others, he is neither
fitted to direct construction, nor to manage details of any kind
connected with engineering industry. There is nothing that more
provokes a feeling of resentment in the mind of a skilled man than
to meet with those who have attempted to qualify themselves in the
theoretical and commercial details of engineering work, and then
assume to direct labour which they do not understand; nor is a
skilled man long in detecting an engineer of this class; a dozen
words in conversation upon any mechanical subject is generally
enough to furnish a clue to the amount of practical knowledge
possessed by the speaker.
As remarked in a previous place, no one can expect to prepare
successful designs for machinery, who does not understand the
details of its construction; he should know how each piece is
moulded, forged, turned, planed, or bored, and the relative cost of
these processes by the different methods which may be adopted.
An engineer may direct and control work without a knowledge of
practical fitting, but such control is merely a commercial one, and
cannot of course extend to mechanical details which are generally
the vital part; the obedience that may thus be enforced in
controlling others is not to be confounded with the respect which a
superior knowledge of work commands.
A gain from learning practical fitting is the confidence which
such knowledge inspires in either the direction of work or the
preparation of plans for machinery. An engineer who hesitates in
his plans for fear of criticism, or who does not feel a perfect
confidence in them, will never achieve much success.
Improvements, which have totally changed machine fitting during
thirty years past, have been of a character to dispense in a great
measure with hand skill, and supplant it with what may be termed
mental skill. The mere physical effect produced by a man's hands
has steadily diminished in value, until it has now almost come
to be reckoned in foot-pounds; but the necessity for practical
knowledge instead of being diminished is increased.
Formerly an apprentice entered a shop to learn hand skill, and to
acquaint himself with a number of mysterious processes; to learn
a series of arbitrary rules which might serve to place him at a
disadvantage even with those whose capacity was inferior and who
had less education; but now the whole is changed. An engineer
apprentice enters the shop with a confidence that he may learn
whatever the facilities afford if he will put forth the required
efforts; there are no mysteries to be solved; nearly all problems
are reached and explained by science, leaving a greater share
of the shop-time of a learner to be devoted to studying what is
special.
This change in engineering pursuits has also produced a change in
the workmen almost as thorough as in manipulation. A man who deals
with special knowledge only and feels that the secrets of his
calling are not governed by systematic rules, by which others may
qualify themselves without his assistance, is always more or less
narrow-minded and ignorant. The nature of his relations to others
makes him so; of this no better proof is wanted than to contrast
the intelligence of workmen who are engaged in what may be termed
exclusive callings with people whose pursuits are regulated by
general rules and principles. A machinist of modern times, having
outgrown this exclusive idea, has been raised thereby to a social
position confessedly superior to that of most other mechanics, so
that shop association once so dreaded by those who would otherwise
have become mechanics, is no longer an obstacle.
Some hints will now be given relating to apprentice experience in a
workshop, such matters being selected as are most likely to be of
interest and use to a learner.
Upon entering a shop the first thing to be done is to gain the
confidence and the respect of the manager or foreman who has charge
of the work; to gain such confidence and respect is different
from, and has nothing to do with, social relations and must depend
wholly upon what transpires in the works. To inspire the confidence
of a friend one must be kind, faithful, and honourable; but to
command the confidence of a foreman one must be punctual, diligent,
and intelligent. There are no more kindly sentiments than those
which may be founded on a regard for industry and earnest effort.
A learner may have the misfortune to break tools, spoil work,
and fail in every way to satisfy himself, yet if he is punctual,
diligent, and manifests an interest in the work, his misfortunes
will not cause unkind resentment.
It must always be remembered that what is to be learned should not
be estimated according to a learner's ideas of its importance. A
manager and workmen generally look upon fitting as one of the most
honourable and intelligent of pursuits, deserving of the respect
and best efforts of an apprentice; and while a learner may not
think it a serious thing to make a bad fit, or to meet with an
accident, his estimate is not the one to judge from. The least
word or act which will lead workmen to think that an apprentice is
indifferent, at once destroys interest in his success, and cuts off
one of the main sources from which information may be derived.
An apprentice in entering the workshop should avoid everything
tending to an appearance of fastidiousness, either of manner or
dress; nothing is more repulsive to workmen, and it may be added,
nothing is more out of place in a machine shop than to divide one's
time between the work and an attempt to keep clean. An effort to
keep as neat as the nature of the work will admit is at all times
right, but to dress in clothing not appropriate, or to allow a fear
of grease to interfere with the performance of work, is sure to
provoke derision.
The art of keeping reasonably clean even in a machine shop is worth
studying; some men are greased from head to foot in a few hours, no
matter what their work may be; while others will perform almost any
kind of work, and keep clean without sacrificing convenience in the
least. This difference is the result of habits readily acquired and
easily retained.
Punctuality costs nothing, and buys a great deal; a learner who
reaches the shop a quarter of an hour before starting time, and
spends that time in looking about, manifests thereby an interest
in the work, and avails himself of an important privilege, one of
the most effectual in gaining shop knowledge. Ten minutes spent
in walking about, noting the changes wrought in the work from day
to day, furnishes constant material for thought, and acquaints a
learner with many things which would otherwise escape attention.
It requires, however, no little care and discrimination to avoid
a kind of resentment which workmen feel in having their work
examined, especially if they have met with an accident or made
a mistake, and when such inspection is thought to be prompted by
curiosity only. The better plan in such cases is to ask permission
to examine work in such a way that no one will hear the request
except the person addressed; such an application generally will
secure both consent and explanation.
Politeness is as indispensable to a learner in a machine shop as
it is to a gentleman in society. The character of the courtesy may
be modified to suit the circumstances and the person, but still it
is courtesy. An apprentice may understand differential calculus,
but a workman may understand how to bore a steam cylinder; and in
the workman's estimation a problem in calculus is a trivial thing
to understand compared with the boring of a steam engine cylinder.
Under these circumstances, if a workman is not allowed to balance
some of his knowledge against politeness, an apprentice is placed
at a disadvantage.
Questions and answers constitute the principal medium for acquiring
technical information, and engineering apprentices should carefully
study the philosophy of questions and answers, just as he does
the principles of machinery. Without the art of questioning but
slow progress will be made in learning shop manipulation. A proper
question is one which the person asked will understand, and the
answer be understood when it is given; not an easy rule, but a
correct one. The main point is to consider questions before they
are asked; make them relevant to the work in hand, and not too
many. To ask frequent questions, is to convey an impression that
the answers are not considered, an inference which is certainly
a fair one, if the questions relate to a subject demanding some
consideration. If a man is asked one minute what diametrical pitch
means, and the next minute how much cast iron shrinks in cooling,
he is very apt to be disgusted, and think the second question not
worth answering.
It is important, in asking questions, to consider the mood and
present occupation of the person addressed; one question asked
when a man's mind is not too much occupied, and when he is in a
communicative humour, is worth a dozen questions asked when he is
engaged, and not disposed to talk.
It is a matter of courtesy in the usages of a shop, and one of
expediency to a learner, to ask questions from those who are
presumed to be best informed on the subject to which the questions
relate; and it is equally a matter of courtesy to ask questions
of different workmen, being careful, however, never to ask two
different persons the same question, nor questions that may call
out conflicting answers.
There is not a more generous or kindly feeling in the world than
that with which a skilled mechanic will share his knowledge with
those who have gained his esteem, and who he thinks merit and
desire the aid that he can give.
An excellent plan to retain what is learned, is to make notes.
There is nothing will assist the memory more in learning mechanics
than to write down facts as they are learned, even if such
memoranda are never referred to after they are made.
It is not intended to recommend writing down rules or tables
relating to shop manipulation so much as facts which require remark
or comment to impress them on the memory; writing notes not only
assists in committing the subjects to memory, but cultivates a
power of composing technical descriptions, a very necessary part of
an engineering education. Specifications for engineering work are a
most difficult kind of composition and may be made long, tedious,
and irrelevant, or concise and lucid.
There are also a large number of conventional phrases and endless
technicalities to be learned, and to write them will assist in
committing them to memory and decide their orthography.
In making notes, as much as possible of what is written should be
condensed into brief formulæ, a form of expression which is fast
becoming the written language of machine shops. Reading formulæ
is in a great degree a matter of habit, like studying mechanical
drawings; that which at the beginning is a maze of complexity,
after a time becomes intelligible and clear at a glance.
Upon entering the shop, a learner will generally, to use a shop
phrase, "be introduced to a hammer and chisel;" he will, perhaps,
regard these hand tools with a kind of contempt. Seeing other
operations carried on by power, and the machines in charge of
skilled men, he is likely to esteem chipping and filing as of but
little importance and mainly intended for keeping apprentices
employed. But long after, when a score of years has been added to
his experience, the hammer, chisel, and file, will remain the most
crucial test of his hand skill, and after learning to manipulate
power tools of all kinds in the most thorough manner, a few blows
with a chipping hammer, or a half-dozen strokes with a file, will
not only be a more difficult test of skill, but one most likely to
be met with.
To learn to chip and file is indispensable, if for no other
purpose, to be able to judge of the proficiency of others or
to instruct them. Chipping and filing are purely matters of
hand skill, tedious to learn, but when once acquired, are never
forgotten. The use of a file is an interesting problem to study,
and one of no little intricacy; in filing across a surface one inch
wide, with a file twelve inches long, the pressure required at each
end to guide it level may change at each stroke from nothing to
twenty pounds or more; the nice sense of feeling which determines
this is a matter of habit acquired by long practice. It is a wonder
indeed that true surfaces can be made with a file, or even that a
file can be used at all, except for rough work.
If asked for advice as to the most important object for an
apprentice to aim at in beginning his fitting course, nine out
of ten experienced men will say, "to do work well." As power
is measured by force and velocity, work is measured by the two
conditions of skill and time. The first consideration being, how
well a thing may be done, and secondly, in how short a time may it
be performed; the skill spent on a piece of work is the measure of
its worth; if work is badly executed, it makes no difference how
short the time of performance has been; this can add nothing to the
value of what is done although the expense is diminished.
A learner is apt to reverse this proposition at the beginning, and
place time before skill, but if he will note what passes around
him, it will be seen that criticism is always first directed to the
character of work performed. A manager does not ask a workman how
long a time was consumed in preparing a piece of work until its
character has been passed upon; in short, the quality of work is
its mechanical standard, and the time consumed in preparing work
is its commercial standard. A job is never properly done when the
workman who performed it can see faults, and in machine fitting,
as a rule, the best skill that can be applied is no more than the
conditions call for; so that the first thing to be learned is to
perform work well, and afterwards to perform it rapidly.
Good fitting is often not so much a question of skill as of the
standard which a workman has fixed in his mind, and to which all
that he does will more or less conform. If this standard is one
of exactness and precision, all that is performed, whether it be
filing, turning, planing, or drawing, will come to this standard.
This faculty of mind can be defined no further than to say that
it is an aversion to whatever is imperfect, and a love for what
is exact and precise. There is no faculty which has so much to do
with success in mechanical pursuits, nor is there any trait more
susceptible of cultivation. Methodical exactness, reasoning, and
persistence are the powers which lead to proficiency in engineering
pursuits.
There is, perhaps, no more fitting conclusion to these suggestions
for apprentices than a word about health and strength. It was
remarked in connection with the subject of drawing, that the powers
of a mechanical engineer were to be measured by his education
and mental abilities, no more than by his vitality and physical
strength, a proposition which it will be well for an apprentice to
keep in mind.
One not accustomed to manual labour will, after commencing, find
his limbs aching, his hands sore; he will feel exhausted both
at the beginning and at the end of a day's work. These are not
dangerous symptoms. He has only to wait until his system is built
up so as to sustain this new draught upon its resources, and
until nature furnishes a power of endurance, which will in the
end be a source of pride, and add a score of years to life. Have
plenty of sleep, plenty of plain, substantial food, keep the skin
clean and active, laugh at privations, and cultivate a spirit
of self-sacrifice and a pride in endurance that will court the
hardest and longest efforts. An apprentice who has not the spirit
and firmness to endure physical labour, and adapt himself to the
conditions of a workshop, should select some pursuit of a nature
less aggressive than mechanical engineering.
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Rennie, Sir John.
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Practical Treatise on Steam Boilers and Boiler Making,
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The Practice of Hand-turning in Wood, Ivory, Shell,
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Treatise on Valve-Gears, with special consideration of
the Link-Motions of Locomotive Engines, by Dr. Gustav
Zeuner, third edition, revised and enlarged, translated
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author, by Moritz Müller, _plates_, 8vo, cloth 12 6
Ventilation.
Health and Comfort in House Building, or Ventilation
with Warm Air by Self-Acting Suction Power, with
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Tabulated Weights of Angle, T, Bulb, and Flat Iron, for
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On the Arrangement, Care, and Operation of Wood-working
Factories and Machinery, forming a complete
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A Treatise on the Construction and Operation of
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crown 8vo, cloth 5 0
Royal 8vo, cloth, 7_s._ 6_d._
_Spons' Engineers' and Contractors' Illustrated_
_Book of Prices of Machines, Tools, Ironwork, and Contractors'
Material. 1876._
E. & F. N. SPON: LONDON AND NEW YORK.
* * * * *
Transcriber's notes:
Punctuation has been standardized after careful comparison with
other occurrences within the text and consultation of external
sources. Inconsistencies in spelling and word usage have been
retained except as follows:
Pg vii: 1) added "P" to "(P)AGE".
2) ARRANGEMNET altered to ARRANGEMENT.
Pg 5: deserve altered to deserves. "These few words contain a truth
which deserves"
Pg 12: has altered to have. "It is to be regretted that there have
not been books"
Helmotz altered to Helmholtz.
Pg 47: counter shafts altered to countershafts. "intermediate or
countershafts, the expense"
Pg 57: breaks altered to brakes. "to operate railway brakes?"
Pg 59: separated altered to separate "The employment of magnetic
machines to separate iron and brass filings or shop waste"
Pg 60: (4.) was retained as the question seems to be included
after (3.)
Pg 62: breaks altered to brakes "operated by friction brakes"
Pg 64: kind altered to kinds "in high buildings for most kinds of
manufacture"
Pg 97: preventing altered to prevent "to prevent the springing of
castings."
Pg 100: Number of Question (8.) was missing. Questions renumbered
according to the other blocks of questions.
Pg 120: adapation altered to adaptation. "but they are by such
adaptation unfitted for general purposes."
Pg 151: does altered to do "In what way do standard dimensions"
Pg 170: han altered to hand "purely matters of hand skill"
Catalog:
Pg 14: solvlng altered to solving
*** End of this LibraryBlog Digital Book "The Economy of Workshop Mainipulation - A logical method of learning constructive mechanics" ***
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