Where it is essential that gages retain their exact size for a considerable length of time, the gaging surfaces must be lapped to size after grinding. The surface left by the emery wheel, even when the utmost care is used, consists of a series of small ridges or irregularities which wear away as the gage is used and leave the opening too large. Lapping the gaging surfaces with oil and emery grinds these minute particles away and produces a perfectly flat surface, thereby increasing the durability of the tool.
A convenient form of lap to use on snap gages is illustrated in Fig. 433. It consists of a piece of copper or brass wire, bent as shown; the surface A is filed or hammered flat, and is charged with some abrasive material, as emery. Extreme care must be used in lapping the surfaces, that they may remain perfectly flat and parallel. Unless the operator has had considerable experience in this particular work, he will be likely to cut the edges away more than the center. To avoid doing this, pieces of hardened steel may be clamped to each side of the gage before grinding, Fig. 434. As the tendency when lapping is to make the outer edges round, the portions rounded will be the edges of the pieces clamped to the gage. After the gage has been lapped to size these pieces may be removed.'
Fig. 433. Lap for Gaging Surfaces.
Fig. 434. Method of Clamping Pieces on Gage to Prevent Rounding Edges.