Page:Practical Treatise on Milling and Milling Machines.djvu/69

Rh 56 teeth; 1st gear on stud, 40 teeth; 2nd gear on stud, 44 teeth; and gear on screw, 100 teeth. Putting the 56 tooth gear on the spindle instead of on the worm, gives a lead of $6.160⁄40$ = .154".

By this method, very short leads may be obtained without excessively straining the mechanism, but the regular means of indexing the work cannot be employed. A method that can be used for indexing when using the differential centre is to have the number of teeth in the gear on the spindle some multiple of the number required to

Fig. 13

be indexed. Swing the gears out of mesh and advance the gear on spindle the number of teeth required to index the work one division at each indexing. Thus, if 9 divisions are required with a lead of .261 ", we select a lead from the table equal to about .261" X 40 = 10.440", when the gear on worm (which will now be the gear on spindle) is some multiple of 9, as 72. The nearest lead is 10.467", which gives $10.467⁄10$ = .2617" lead, giving an error of .0007". To index the work, the gear on spindle is advanced $72⁄9$ = 8 teeth at each indexing.

The short lead attachment described in the next chapter can also be used to cut short leads, an index plate being provided for use in cutting multiple threads.