Fig. 1195.
This labor, however, may be lessened in two ways: first, the faces may be fluted, as in [Fig. 1194], at a and at b, or its diameter may be turned down, as in [Fig. 1195]. In using the square centre it is placed in the position of the live centre and revolved at high speed, all the cutting edges operating simultaneously; the work is fed up by the dead centre and held in the hand.
Fig. 1196.
To prevent the weight of the work from causing the countersinking being out of true with the hole, the work should be occasionally allowed (by relaxing the grip upon it) to make part of a revolution, as explained with reference to centre-drilling without a work guide. Another and simple form of square centre for countersinking is shown in [Fig. 1196]. It consists of a piece of square steel set into a stock or holder.
