in order that the work may be rigidly supported. They should also be convenient to operate, so that indexing may be quickly accomplished.
One of the simplest forms of index centres, known as Single Dial Index Centres, is shown in Fig. 22. It consists of a head-stock and foot-stock of solid construction. The spindle of the head-stock is turned by means of the hand-wheel, and the divisions are indicated on the periphery of an index plate fastened to the spindle near the hand-wheel. There are holes in the back of the index plate corresponding to the divisions on its periphery, and a hardened steel taper pin is provided that is forced into the bushings of these holes by a
Fig. 22
spring, efficiently locking the spindle at any one of the divisions. The small lever near the top of the head-stock withdraws the taper pin when it is desired to index the work.
This style of index centres is found convenient whenever rapid indexing is to be done, as in cutting teeth in sprocket wheels and mills, or in milling grooves in taps, reamers and work of a similar kind. They are built in two sizes, one to accommodate work up to 8 inches diameter, and the other for work up to 12 inches diameter. The index plates or dials furnished have 24 divisions, or holes, but special plates having, for 8 inch centres, any number of holes up to 32, and, for 12 inch centres, any number up to 32, are sometimes made to order.
A common style of index centres, known as Plain Index Centres, is shown in Fig. 23. The spindle of the head-stock is revolved by means of a worm and wheel. The handle of the crank fastened to the worm shaft constitutes an index pin, and indexing is accomplished by means of a plate drilled with circles of different numbers of holes into which the spring pin of the crank fits. Thus it will be seen that the principle of indexing with these centres is the same as with the spiral head. For rapid indexing of the coarser divisions, the worm can be thrown out of mesh with the wheel and the spindle turned by hand; a circle
