Fig. 1161.
Or the ball may be held in a universal joint, and thus work more frictionless. Thus, in [Fig. 1161] it is held by the conical points of two screws diametrically opposite in a ring which is held by the conical points of two screws threading through an outer ring, these latter screws being at a right angle to those in the inner ring. The outer ring is held to the holder by the conical points of two screws, all the conical points seating in conical recesses.
It is obvious that the contact of the point of the needle and the work may be more delicately made when there is some elasticity provided, as is the case with the spiral spring in [Fig. 1159].
Fig. 1162.
Indicators of this class may be used to test the truth of cylindrical work: thus, in [Fig. 1162] is an application to a piece of work between the lathe centres, there being fitted to one end of the needle a fork a that may be removed at pleasure.
One of the difficulties in turning up a lathe centre to run true arises from the difference in cutting speed at the point and at the full diameter of the cone, the speed necessary to produce true smooth work at the point being too fast for the full diameter. This may be remedied on centres for small work, as, say, three inches and less in diameter, by cutting away the back part of the cone, leaving but a short part to be turned up to true the centre.
To permit the cutting off or squaring tool to pass close up to the centre, and thus prevent leaving a burr or projection on the work end, the centre may be thus relieved at the back and have a small parallel relief, as in [Fig. 1164] at a, the coned point being left as large as possible, but still small enough to pass within the countersink.