Fig. 1723.
The edges at the flat end of the drill, as at a, a in [Fig. 1723], should have a little clearance back from the cutting edge though they may be left the full circle as, at a, a, but in any event they should not have clearance sufficient to form them as at b, b, [Fig. 1723], because in that case the side edges c, c would cut the sides of the hole. In large drills, especially, it is necessary that the edges have but little clearance, and that the form of the clearance be as shown in [Fig. 1044], with reference to twist drills. When no edge clearance whatever is given the edges act to a certain extent as guides to the drill, but if the drill is not ground quite true this induces a great deal of friction between the edges of the drill and the side of the hole.
In any case of improper grinding the power required to drive the drill will be increased, because of the improper friction induced between the sides of the drill and the walls of the hole.
Fig. 1724.
Fig. 1725.