Fig. 2149. Fig. 2150. Fig. 2151.
The Chisel.—The machinist’s cold chisel is made from the two forms of steel shown in [Figs. 2149], [2150], and [2151], and of these the former is preferable because it has two broad flats diametrally opposite and these form a guide to the eye in holding the chisel on the grindstone, and aid in grinding the facets that form the cutting edge true. Furthermore, as the cutting edge is in the same plane as these flats they serve as a guide to denote when the chisel edge lies parallel to the work surface, which is necessary to produce true and smooth chipping.
Fig. 2152. Fig. 2153.
The width of the chisel may be made greater, as in [Figs. 2152] and [2153], for brass or cast-iron work than for wrought iron or steel for the following reasons. On account of the toughness and hardness of wrought iron and steel the full force of a 13⁄4 lb. hammer, having a handle 13 inches long, may be used on a chisel about 7⁄8 inch wide without danger of causing the metal to break out below the chipping line, but if such a chisel be used with full force blows upon cast iron or brass the metal is apt to break out in front of the chisel, the line of fracture often passing below the level it is intended to chip down to. Hence if a narrow chisel is used lighter blows must be delivered. But by using a broader chisel the force of the blow is distributed over a longer length of cutting edge, and full force blows may be used without danger of breaking out the metal.