HAMMER. This well-known tool stands quite without rival for producing the numberless effects which are due to the remarkable force of "impact." Of all the array of hand tools in use in the industrial arts it is undoubtedly the one that could least be dispensed with, and so it must have been from the very earliest days of handicraft. A ham-mer of some rude kind must have been as essential to the shaping of an arrow-head out of a flint, as is its modern repre-sentative to the forging of a bayonet out of a bar of steel.
Hammers to be used by hand are made of an endless variety of shapes, and of weights varying from a small fraction of an ounce to 28 or 30 fix Of the various types now in use the differences are mainly due to the special requirements of the particular trades for which they are intended, but partly also to individual fancy. A few examples are given in fig. 1a being the ordinary car-penter's hammer with a cross pane, b a claw hammer, c an engineer's, and d a shoemaker's hammer, which last has apparently retained its form for many generations. The same may also be said of the two others in the woodcut,
FIG-. 1.Varieties of Hand Hammer.
which are good examples of the roughness of the instruments with which results of almost incredible delicacy can be produced. Of these e is a favourite hammer used in the workshops of a London firm of goldbeaters, with which, together with others not less rude in appearance, gold is hammered out into leaves of such exceeding thinness that 250,000 are required to make up the thickness of an inch. Extraordinarily ill-shaped as /, the file-cutter's hammer, looks, yet with this and a simple chisel the teeth of files are cut by hand with a precision with which no machinery has as yet been able to compete, at least in England.
Hand-hammers and sledge-hammers being rigidly limited as to weight by the trifling power which one manor at most two mencan bring to bear upon them, it long ago became obviously desirable to obtain the force of impact from some more powerful source than human muscles, so that the weight of hammer heads might be largely in-creased and their blows made proportionately more effec-tive. The first step taken in this direction seems to have been the introduction of the " Hercules," a ponderous mass of iron attached to a vertical guide rod, which was lifted originally by a gang of men with ropes, but afterwards by steam power, and allowed to fall by its own weight. This was a fairly efficient tool for forging large anchors and for similar purposes, the strength of the blow and the point at which it was delivered being easily regulated. But as the demand for wrought iron increased, the necessity for more rapid as well as more powerful hammers to aid in its manufacture increased also. The lift or helve hammer (fig. 2) and the tilt hammer (fig. 3) thus came into use, and under these forms hammers may be said to have ranked for
FIG. 2.Lift Hammer.
the first time as true machine tools. Each of these consists of a heavy head attached to a beam mounted on gudgeons, which is lifted at regular intervals by suitable cams or pins
FIG. 3.Tilt Hammer.
carried by a revolving shaft driven by steam or other power, their chief points of difference being the relative position " the gudgeons and the portion of the beam at which the
FIG. 4.Steam Hammer.
power is applied, as shown in the diagrams. Heavy blows are thus obtained with the one, and lighter but much more rapid blows with the other, both, however, invariable in their intensity, the lift being always to one fixed height.
In the steam hammer (fig. 4), which was first patented by Nasmyth in the year 1842, this objection is completely obviated. By the simple device of attaching the hammer head to the lower end of the rod of a piston working in an inverted steam cylinder, he produced a machine cap-able of being made to deliver its blows with a force to which no limit has yet been found, and yet so perfectly under control as to be able to crack a hazel nut without injuring the kernel. To the introduction of this invalu-able tool is due more than to any other single cause the power which we now possess of producing the forgings in iron and steel which are demanded by the arts of modern times; and in one or other of its many forms it is now to be met with in every workshop in which heavy work is carried on. Many modifications of the Nasmyth steam hammer have been introduced. In one of these steam pressure has been applied above the piston to intensify the blow, it being only used in the original form for lifting the " tup " and piston and for regulating their descent. In another the piston and piston-rod have been attached to the framing, and the cylinder made movable, its weight being thus added to that of the hammer.
Duplex hammers differ materially from the ordinary steam hammer, inasmuch as no anvil is necessary,two hammer heads of equal weight (and for some purposes they weigh only a few pounds, for others as much as 30 tons) being made to deliver horizontal blows of equal force simultaneously on opposite sides of the forging, which thus receive perfectly equal treatment. The importance of this may be gathered from the fact that every increase in the weight of the vertical steam hammer necessitates a very much larger increase in the weight and solidity of the anvil which is necessary to afford the requisite inertia for resist-ing the blow. For instance, the anvil of a 40-ton hammer now in use at Woolwich Arsenal weighs upwards of 160 tons, and has nearly 500 tons of iron in its foundation, whereas the anvil of an 80-ton hammer of which a full-sized model was amongst the most striking objects at the Paris Exhibition of 1878, is of no less than 720 tons weight.
The recent growth of steam hammers to the enormous size just mentioned is due chiefly to the large dimensions, independently of the mere weight, of the forgings which have now to be made for heavy ordnance and for other purposes. As long as the thickness of a forging is moder-ate, the reaction of the anvil acting upon its under side has an effect not greatly inferior to that of the hammer on the upper side. But with every increase in thickness some of this reaction is lost and the effect of the blow is more and more confined to the outer portions of the mass, which thus receive more than their share of treatment at the expense of the central portions. On this account the difficulty of obtaining thick forgings thoroughly sound throughout their substance is very great, and it seems as if we had now reached the limit beyond which the hammer cannot be advantageously applied for their production. It has been found indeed in the case of large shafts that dispens-ing altogether with the central portion, thus making them tubular instead of solid, is accompanied by an increase in their strength, owing to the possibility of thus forging the metal uniformly throughout. But a much more widely applicable remedy, and one which will doubtless come into general use for heavy work, is the substitution of hydraulic nr other pressure for the force of impact, a system which has lately been employed by Sir Joseph Whit worth with wonderfully good results. The reason of the superiority of its effect seems to be mainly this, that a certain amount of time is essential for completing the "flow" of the metal which it is the object of forging to induce. Under continuous pressure this flow can take place uniformly throughout the mass, whereas the instantaneous blow of the hammer, though it acts violently on the surface particles, and to a decreasing extent on the adjacent ones, is entirely expended before the action has had time to reach those at the centre, so that unequal density and consequent weakness is the result. For massive forgings therefore the old saying can be no longer accepted that " there is no machine like a hammer." (c. p. B. S.)