Page:Practical Treatise on Milling and Milling Machines.djvu/60

54 method, and when these are used figuring, such as that above, is uneccesary.

Indexing in Degrees and Parts of Degrees. When it is desired to divide the circumference of a piece in this manner, it can often be done by plain indexing. One complete turn of the index crank produces $1⁄40$ of a turn of the work, or $360°⁄40$ = 9 degrees. Following this method:


 * 2 holes in the 18-hole circle = 1 degree.


 * 2 holes in the 27-hole circle = $2⁄3$ degree.


 * 1 hole in the 18-hole circle = $1⁄2$ degree.


 * 1 hole in the 27-hole circle = $1⁄3$ degree.

Other odd fractional parts of a degree can be easily found by dividing the number of holes in any given circle into 9 degrees. It will be noticed that $1⁄4$ degree spacing cannot be obtained in this way; but with differential indexing, as explained on page 57, it is easy to get $1⁄4$ degree and other fractional spacings.

Differential Indexing. Differential indexing enables a wide range of divisions to be indexed, which cannot be obtained by plain indexing. With the change gears and three index plates furnashed with the spiral head, it is possible to index all numbers, not obtainable by plain indexing, from 1 to 382; in addition, many other divisions beyond 382 can be indexed.

By this method, the index crank is moved in the same circle of holes, and the operation is like that of plain indexing. The spiral head spindle and index plate are connected by a train of gearing, as shown above, and teh stop pin at the back of the plate is thrown out. As the index crank is turned, the spindle is rotated through the worm and wheel, and the plate moves either in the same or opposite direction to that of the crank. The total movement of the crank at every indexing is, therefore, equal to its movement relative to the plate, plus the movement of the plate, when the plate revolves in the same direction as the crank,