Fig. 2021.
Fig. 2022.
[Fig. 2020] represents the jaws j j holding on the left a small, and on the right a large drill. The line of motion of the right-hand jaw in opening and closing to grip the drill is along the line r, while that of the left-hand is along the line p p, the centre upon which the chuck is revolved to grind the drill being denoted by the small circle at s. x′ represents the centre line of the large drill when held in the chuck, and it is seen that the action of the jaws in closing upon small drills is to lift the drill point closer to the centre s upon which the chuck revolves (the cutting edge being ground to be on the line y′ y′). The reason for this peculiar and simple but exceedingly ingenious construction is, as before remarked, to maintain the cutting edge in its proper relation to the thickness of the drill point (which thickness varies in different diameters of drills), and to maintain a proper degree of clearance at every point along the length of the cutting edge. In other drill grinding machines the drill when rotated to grind the clearance is moved on the axis a a in [Fig. 2022] as a centre of motion, and as this line is parallel to the face of the emery wheel it follows that if the drill were given a full revolution its circumference would be ground to a cylinder as shown in [Fig. 2021] by the dotted lines.
Fig. 2023.
