Page:Practical Treatise on Milling and Milling Machines.djvu/81

Rh as on the spiral head. The two smaller sizes are arranged for rapid indexing of coarser divisions by disengaging the worm, and indexing with the plate fastened directly to the nose of the spindle, as on the spiral head. The index crank is adjustable and index sectors are employed. The index plates furnished with the 6 inch, 10 inch, 12 inch, 14 inch and 15 inch centres divide all numbers to 50, and all even numbers to 100, except 96; those furnished with the $12 1⁄2$ inch centres divide all numbers to 100 and all even numbers to 134.

Index centres designed for manufacturing purposes where economy and rapidity of production are important factors, often have more than one spindle. Fig. 25 shows triple centres of this type. All three spindles of these centres are indexed simultaneously, and one thumbscrew firmly clamps them all, consequently three pieces of work can

Fig. 25

be finished in practically the same time it takes to machine one on single centres.

The spindles are rotated by a ratchet operated by the lever shown at the left of the head-stock. Indexing is accomplished by an index plate which divides all numbers as follows: 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 20 and 24. The index stop pin is shown at the left of the head-stock.

Using all three spindles, work up to $2 1⁄2$ inches diameter can be taken; when only the two outside spindles are employed, work up to 5 inches diameter will swing.

Triple index centres of the design that has the index plate at the side of the head-stock similar to the spiral head are shown in Fig. 26. Centres of this same general design, but arranged for rapid indexing only, are also built.

The index plates furnished with these centres divide all numbers to 50, even numbers to 100, except 96. When rapid indexing is desired, the worm of the index crank is disengaged and the centres are turned by means of a pinion actuated by the crank at the left of the headstock; an index plate and stop pin provide for the divisions.