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.