Page:Practical Treatise on Milling and Milling Machines.djvu/58

52 when setting for taper work. It consists of a bushing that fits over the centre in the spiral head and a blade, the bottom edge of which is the same distance above the centre as the top of the foot-stock centre.

Fig. 8 INDEXING

The first office of the spiral head is to index or divide the periphery of a piece of work into a number of definite or given parts. This is accomplished by means of the index crank and the index plates furnished with the head; or, in the case of some of the more common coarse division, by means of the rapid index plate fastened to the nose of the spindle.

There are two practical and accurate methods of indexing, known as Plain and Differential. A third method, known as the Compound, was used extensively in the past, and is still employed by some shops having machines that are not fitted for Differential indexing. The chances for errors in making the complicated indexing moves, adn the fact that even when the moves are made correctly, exact results cannot be obtained, causes the Compound method to be of little practical value where accurate spacing is required. It has, as a aresult, been largely superseded by the Differential method, by which the same numbers can be indexed accurately, and with little liability of errors in making the indexing moves.

Most spiral heads that are not fitted for Differential indexing can be at a nominal cost, and the unusual simplicity and convenience of this method in themselves are sufficient to warrant doing this.

By the Plain method of indexing, which includes rapid indexing, using the plate on the spindle nose, all divisions up to 50, even numbers up to 100, except 96, and many numbers that are multiples of 5 up to 380, besides many others, can be indexed with the three index plates furnished. With the addition of the change gears furnished, divisions obtained by Plain indexing, together with those that cannot be obtained by that method, from 1 to 382, and many others beyond, can be indexed by the Differential method.