Page:Practical Treatise on Milling and Milling Machines.djvu/63

Rh one idler D. The idler D serves to rotate the index plate in the same direction as the crank, thus in making 280 turns of the crank, nine divisions are lost, giving the correct number of divisions, 271. The sector should be set to indicate $1⁄7$ turns, or 3 holes in the 21 hole circle, and the head is ready for 271 divisions, the indexing being made the same as for plain indexing. Head Geared for 319 Divisions. Fig. 10 shows the spiral head geared, compound gearing, for 319 divisions. Referring to the table on page 217, the gears called for are: C, 48 teeth; F, 64 teeth; G, 24 teeth; E, 72 teeth and one idler D, 24 teeth. The sector should be set to $4⁄29$ turns, or 4 holes in the 29 circle; the head is then ready for 319 divisions.

Spacing for Quarter Degrees.

Example 3.


 * Required H, n and x for spacing $1⁄4$ degrees, or 1440 divisions.
 * Assume H = 33, n = l.
 * Then $$\frac{\left(1440\times1\right)-\left(33\times40\right)}{33}=\frac{120}{33}\text{or}\frac{64\times100}{40\times44}$$
 * One idler is required.

The following table gives data required for spacing $1⁄4$° and $1⁄6$°. For fractional parts of degrees obtainable by plain indexing see page 54.