If, however, the drill is ground as shown in [Fig. 1719], the edge e being nearest to a right angle to the axial line h of the drill, the drilling will be performed as shown in the figure, the edge e cutting the cone l, the edge f serving simply to enlarge the hole drilled by e. Here, again, if the work is held so that it cannot move, the point of the drill will revolve in a circle, and in either case, so soon as the point of the drill emerges the diameter of the hole drilled will decrease, the finished hole being conical as shown in [Fig. 1720] at a.
It may be remarked that the eye of the workman is (for rough work, such as tapping or clearing holes) sufficient guide to enable the grinding of the drill true enough to partly avoid the conditions shown in these two figures (in which the errors are magnified for clearness of illustration), because when the want of truth is less in amount than the thickness of the drill point, the centre of motion of the drill point when the drill has entered the work to its full diameter becomes neither at the point of the drill nor in the centre of its diameter, but intermediate between the two.
Fig. 1721.
Fig. 1722.
Thus, in [Fig. 1721], a is the centre of the diameter of the drill, but the cutting edge c being shorter than d throws the point of the drill towards e, hence the extra pressure of d on the incline of the recess it cuts, over the like pressure exerted by c tends to throw the centre of rotation towards e, the natural endeavor of the drill point to press into the centre of the recess acting in the same direction. This is in part resisted by the strength of the drill, hence the centre of rotation is intermediate as at b in figure. The dotted circle is drawn from the axial line of the drill as a centre, while the full circle is drawn from b as a centre. The result of this would be that the point of the drill would perform more duty than is due to its thickness, and the recess cut would have a flat place at the bottom, as shown in [Fig. 1722] at o. This, from the want of keenness of the cutting edge running diagonally across the drill point, would cause the drill to cut badly and require more power to drive and feed.