Fig. 1718.
Suppose, for example, the drill to be ground as in [Fig. 1718], the cutting edge f being the longest and at the least angle, then the point g of the drill, when clear of the work, will naturally revolve in a circle around the axial line h of the drill’s rotation. But when the drilling begins, the point of the drill meets the metal first and naturally endeavours to become the centre of rotation, drilling a straight conical recess, the work moving around with the point of the drill. If the work is prevented from moving, either the drill will spring or bend, the point of the drill remaining (at first) the centre of rotation at that end of the drill, or else the recess cut by the drill will be as in the figure, and the hole will be larger in diameter than the drill.
Fig. 1719.
Fig. 1720.