Fig. 1051.
Fig. 1052.
This fault is inseparable from the method of grinding drills in ordinary drill-grinding machines, which is shown in [Fig. 1050], the line a a representing the axis of the motion given to the drill in these machines. It is obvious that the line a a being parallel to the face of the emery-wheel, the angle of clearance is made equal throughout the whole length of the cutting edge. This is, perhaps, made more clear in [Fig. 1051], in which we have supposed the drill to take a full revolution upon the axis a a, and as a result it would be ground to the cylinder represented by the dotted lines. We may, however, place the axis on which the drill is moved to grind it at an angle to the emery-wheel face, as at b, [Fig. 1052], and by this means we shall obtain two important results: (1) The angle of b may be made such that the clearance will be the same to the actual surface it cuts at every point in the length of the cutting edge, making every point in that length equally keen and equally strong, the clearance being such as it is determined is the most desirable. (2) The clearance may be made to increase as the heels of each end face are approached from the cutting edge. This is an advantage, inasmuch as it affords freer access to the oil or other lubricating or cooling material. If we were to prolong the point of the drill sufficiently, and give it a complete revolution on the axis b, we should grind it to a cone, as shown by the dotted lines in [Fig. 1052].
Fig. 1053. Top View.
