Page:Practical Treatise on Milling and Milling Machines.djvu/80

74 Fig. 23

of holes in the back of the worm wheel rim, and an index pin at the top of the head-stock provide for indexing when this is done.

These centres are built in sizes to accommodate work up to 10 inches and 12 inches diameter respectively. The nose of the spindle is threaded to receive a face plate or chuck. They are fitted with index sectors similar to those of the spiral head, and the index crank is adjustable so that it can be brought to the nearest hole without disturbing the setting. The index plates furnished divide all numbers to SO and all even numbers to 100, except 96.

Fig. 24 shows a pair of Universal Index Centres. The resemblance between them and the spiral head is marked; in fact, the foot-stock is identical with that furnished with the latter mechanism. All operations upon centres that do not require other than plain indexing and where there is no spiral to be cut, can be performed with these centres equally as well as with a spiral head.

These universal index centres are built in six sizes, to accommodate work up to 6, 10, 12, $12 1/2$, 14 and 15 inches diameter. Divisions are indexed by means of the index crank and plates, the same

Fig. 24