Peening consists in stretching the metal on one side of a piece of work in order to alter its shape. There is a wide difference between peening and bending. For example, suppose the curved or warped piece in Fig. 254 is to be straightened. If it were to be bent until it were straight, it would be placed on the block A with the concave surface down, as shown by the dotted lines. It could then be struck by the hammer and driven down past the line of support, and strained so that it would remain approximately straight. Such a method of straightening could not be applied to a piece of complicated outlines. It would remain wavy. In peening to trueness such a piece as shown in Fig. 254, it is laid on an anvil with the convex surface down. It is then struck with the peen of the hammer on the concave side. The blow must be quick and sharp. The result is that the metal is stretched at the point where the blow is struck. By working successively over the whole surface, the concave side is stretched so that it is equal, in its dimensions, to the convex side. The piece then becomes straight, and will so remain. A skilful use of the hammer will straighten almost any piece of thin metal.
Fij. 253. Block to be Finished.