Fig. 1044.
The land of the drill—or, in other words, the circumference between the flutes—is backed off to give clearance, as is shown in [Fig. 1044], a true circle being marked with a dotted line, and the drill being of full diameter from a to b only. The object of this clearance is to prevent the drill from seizing or grinding against the walls of the hole, as it would otherwise be apt to do when the outer corner wore off, as is likely to be the case.
Twist drills having three and more flutes have been devised and made, but the increased cost and the weakness induced by the extra flutes have been found to more than counterbalance the gain due to an increase in the number of cutting edges, Further, the increase in the number of flutes renders the grinding of the drill a more delicate and complicated operation.
Fig. 1045.
The keenness and durability of the cutting edge of a twist drill are governed by the amount of clearance given by the grinding to the cutting edge, by the angle of one cutting edge to the other, and by the degree of twist of the flute. Beginning with the angle of the front face, we shall find that it varies at every point in the diameter of the drill, being greatest at the outer corner and least at the centre of the drill, whatever degree of spirality the groove or flute may possess. In [Fig. 1045], for example, we may consider the angle at the corner c and at the point f in the length of the cutting edge. The angle or front rake of the corner c is obviously that of the outer edge of the spiral c d, while that of the point f is denoted by the line f f, more nearly parallel to the drill axis, and it is seen that the front rake increases in proportion as the corner c is approached, and diminishes as the drill centre or point is approached.
