Page:Practical Treatise on Milling and Milling Machines.djvu/190

184 .00625", it will take more than one turn to move the table a distance of .0218". Hence,

$\frac{.02180}{.00625} =3\frac{.00305}{.00625} $

Taking the remainder, .00305", and referring to the tables on pages 318 to 320, we find that it is very near .0030488, which is the distance the table will be moved by using the 41 hole circle in one of the index plates furnished and indexing 20 holes. The error between the actual remainder and the amount given in the table is so small that it can be safely ignored.

Therefore, to graduate a scale with divisions .0218 of an inch apart, an index plate having a 41 hole circle would be used and the crank would have to make three complete turns and then be advanced 20 holes in the 41 hole circle for each division.

It should be remembered in graduating that care must be exercised to prevent backlash between the index crank and table feed screw. To this end, the crank should always be turned in the same direction.

If required, the ratio of gearing between the spiral head spindle and the table feed screw can be changed, but this complicates the operation somewhat and should be resorted to only when it is impossible to get accurate enough results with the method described. Upon referring to the tables on pages 318 to 320 and noting the extreme fineness in divisions that it is possible to obtain, it is apparent that there is little occasion to change the ratio of gearing.

Accurate graduating can also be done by using scales and verniers such as illustrated and described in Chapter V.

Illustrations of cam cutting, and many miscellaneous milling operations will be found on the following pages, and a careful study of the cuts and descriptions may be of value to the reader.