Fig. 1200.
If it be attempted to countersink a piece of work whose end face is not square, the countersinking will not be true with the centre hole, and furthermore the causes producing this want of truth will continue to operate to throw the work out of true while it is being turned. Thus, in [Fig. 1199], a represents a piece of work and b the dead centre; if the side c is higher than side d of the work end, the increased bearing area at c will cause the most wear to occur at d, and the countersink in the work will move over towards d, and it follows that the face of a rough piece of work should be faced before being countersunk. Professor Sweet designed the centre-drilling device shown in [Fig. 1200], which consists of a stock fitting the holes for the lathe centres, and carrying what may be called a turret head, in which are the centre drills, facing tools, and countersinks. The turret has 6 holes corresponding to the number of tools it carries, and each tool is held in position by a pin, upon a spring, which projects into the necessary hole, the construction being obvious. The facing tool is placed next to the drill and is followed by a countersink, in whatever direction the turret is rotated to bring the next tool into operation. The work should, on account of the power necessary for the facing, be driven in a chuck.
Fig. 1201.
A similar tool, which may, however, be used for other work besides centring and countersinking, is shown in [Fig. 1201]. It consists of a stem fitting into the hole of the tail spindle, and carrying a base having a pin d, on which fits a cap having holes b, and set-screws c for fastening drills, countersinks, or cutting tools. The cap is pierced with six taper holes, and a pin projects through the base into these holes to lock the cap in position, this pin being operated by the spring lever shown.
Fig. 1202.