_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.