Fig. 2108.
The handle should also stand at a right angle to the plane of the length of the hammer head, viewed from the side elevation, as shown in [Fig. 2108], in which the dotted line is the plane of the hammer’s length, while b represents a line at a right angle to a, and should, therefore, represent the axial line of the hammer handle. But suppose the handle stood as denoted by the dotted line c, then the face of the hammer would wear to one side, as shown in the figure at d.
In the operation of straightening iron or steel plates by hammer blows, the process when correctly carried out is one of liberating the strains (whose existence throws the plate out of a true plane) by stretching those parts that are unduly contracted. Every hammer blow should, therefore, be directed towards this end, for one misdirected blow entails the delivery of many others to correct its evil influence; hence, if several of such misdirected blows are given, the plate will have upon it a great many more hammer marks, or “hammer sinks” or chops, as they are sometimes termed, than are necessary. As a result, not only will the painter (in fine work) be given extra trouble in stopping the hollows to make a smooth surface, but the following evil will result: Every blow struck by the hammer compresses and proportionately stiffens the small surface upon which it is delivered, and creates a local tension upon the surrounding metal. The misdirected blows then cause a tension acting in opposition to the effect of the properly delivered ones; and though the whole plate may be stiffened by the gross amount of blows, yet there will be created local tensions in various parts of the plate, rendering it very likely to spring or buckle out of truth again. If, for example, we take a plate of iron and hammer it indiscriminately all over its surface, we shall find it very difficult to straighten it afterwards, not only on account of the foregoing reasons, but for the additional and most important one that the effect of the straightening blows will be less, on account of the hammered surface of the plate offering increased resistance to the effects of each blow; and after the plate is straightened, there will exist in it conflicting strains, an equilibrium of which holds the plate straight, but the weakening of any of which will cause the preponderance of the others to throw the plate out of straight; for the effects of the blows cannot be permanent unless the whole body of the iron is acted upon to an equal extent by the hammer. Suppose, for example, that we take a flat plate, and deliver upon it a series of blows round about its centre. The effect will be to make it hollow on one side and rounding on the other, the effect of the blows being, not only to indent the plate in the spots where they fell, but to carry the whole body of the middle out of true; because, the area of the iron being increased by the stretching effect of the blows, the centre leaves the straight line to accommodate the increased area. Thus, if we mark off a circle of, say, a foot in diameter, in the middle of a plate, and hammer it so as to stretch it and increase its area 1⁄8 inch each way, the form of the plate must alter to suit this added area, and the form of a dish or curve is the only one it can assume.
The skilful workman, so soon as he has ascertained where the plate is out of true, sets to work to stretch it, so as to draw the crooked place straight, taking care that the shape and weight of the hammer and the weight of the blows delivered shall bear a proper relation to the thickness of the plate and the material of which it is composed. If it is of consequence that the finished work shall bear no marks of the hammering (as in the case of engravers’ plates), an almost flat-faced hammer is employed; but for other work the shapes, as well as the weights, of the hammers vary.
Fig. 2109.
Fig. 2110.