Fig. 1048.
Fig. 1049.
Referring again to the angle of clearance of the end faces, it can be shown that in the usual manner of grinding twist drills the conditions compel the amount of clearance to be made suitable for the point of the drill, and therefore unsuitable for the corner c, giving to it too much clearance in order to obtain sufficient clearance for the remainder of the cutting edge. Suppose, for example, that we have in [Fig. 1048] a spiral representing the path of corner c during one revolution, the rate of feed being shown magnified by the distance p, and the spiral will represent the inclination of that part of the bottom of the hole that is cut by corner c, and the angle of the end face of the drill to the drill axis will be angle r. The actual clearance will be represented by the angle between the end face s of the drill and the spiral beneath it, as denoted by t. But if we take the path of the point f, [Fig. 1045], during the same revolution, which is represented by the spiral in [Fig. 1049], we find that, in order to clear the end of the hole, it must have more angle to the centre line of the drill, as is clearly shown, in order to have the clearance necessary to enable the point f to cut, because of the increased spiral. It follows that, if the same degree of clearance is given throughout the full length of the cutting edge, it must be made suitable for the point of the drill, and will therefore be excessive for the corner c.
Fig. 1050.
