Another way employed to finish small keyways is by the aid of the tools shown in [Figs. 2309] and [2310], which are termed drifts, because they are driven through with a hand hammer. That shown in [Fig. 2309] is intended for holes having but little depth and not requiring to be very true, such, for instance, as those cut in the ends of keyways or bolts to receive cotters; the thickness at a a is made greater than at b c to give the cutting edge clearance.
Fig. 2310.
The form shown in [Fig. 2309] is for use by hand, the teeth being cut diagonally instead of across, as at a a, to preserve the strength. This end may also be attained by making the serrations round at the bottom, as shown in the figure.
The slant of the teeth on one side of the drift should cross the slant of the teeth on the diametrally opposite side, because if the teeth on opposite sides were parallel one to the other the drift would have a tendency to move over to one side, and crowd there during the process of drifting.
In using these drifts the keyway should first be filed out to very nearly the finished size, leaving very little duty for the drift to perform, although the drift may be driven a short distance into the keyway occasionally during the filing, so as to show where filing is requisite. The work must lie flat and level upon a metal block, lead being preferable, and oil freely supplied to the drift. “If the hole is a deep one, and the cuttings clog in the teeth, or if the cut becomes too great (which may be detected by the drift making but little progress, or by the blows sounding solid) the drift must be driven out again, the cuttings removed, and the surplus metal, if any, removed by filing. The drift must again be freely oiled, and driven in as before, and the operation continued until the drift is driven through the keyway. After the drift has passed once through it should be reversed (or, if a square one, turned a quarter revolution) and again driven through, so that each side of the drift will have cut on each side of the hole, which is done to correct any variation in the size of the drift” (“Complete Practical Machinist”).
The great desideratum in using these drifts is to drive them true, and to strike fair blows, otherwise they will break. While the drift is first used, it should be examined for straightness at almost every blow; and if it requires drawing to one side, it should be done by altering the direction in which the hammer travels, and not by tilting the hammer face.
