At work, we have a DoAll Lathe and was trying to cut 32 TPI with it.
One thread chart jumped from 30 to 33 TPI.
There was another chart where you had to swap some gears in back of the
machine but the chart said MODULE and DP.
We set it up for 32 on the DP chart and the carriage advanced approximately
.090" per spindle revolution.
I'm familiar with Module and DP for gears, but what is this for on a lathe?
(PS. we ended up cutting the thread with a 0.969 X 32TPI thread chasing
Module [usually metric] and DP [usually inch] is for cutting
worms to fit worm gears. Because pi is a factor, the TPI or m/m
pitch is always a funky number.
Most any of the old machining books and older editions of
machinery's handbook will have instructions on how to calculate
required gear ratios and gears to generate a desired thread with
a given pitch/lead leadscrew.
Assuming for the moment that you have an 8 TPI leadscrew, then to
generate a 32 TPI thread you need to have the leadscrew turn at
1:4 ratio with the spindle. To look at this another way, when
you turn the spindle 32 times you want the slide/threading tool
to move 1 inch.
Each lathe is different, however you can generally set the QC box
to 8 or 16 TPI and then swap the pick-off gears between the QC
box and leadscrew, or between the spindle and QC box for a 1:4 or
1:2 reduction for finer/different threads than the QC box
directly generates. If you do this, be sure to put a sign on the
machine that the pick-off gears were changed [or change them back
as soon as you are done] so the next person won't generate scrap
when they use the QC box plate thread data.
Hope this helps