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首页 » 经典英文小说 » The Economy of Workshop Mainipulation » CHAPTER XXIII. FORGING.
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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. [101] 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 [102] 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 [103] 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 [104] 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 formul? 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, [105] 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.


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