On Sat, 01 Apr 2006 23:02:43 GMT, "SJ" wrote:

It would be relatively easy to single point a 4-start thread on a CNC lathe
with a C-axis, but not having access to one, how would you cut a 4-start
1/4"-20 thread on a manual small lathe (like the mini-lathe)?

Thanks,
Sam

Lotsa ways to do this. Two ways involve the compound. Either set
parallel with the Z axis or set at an angle. With the compound set
parallel you cut the first pass to depth, advance the compound 1/4 of
the lead, thread to depth, repeat until done. Example: Say you want to
cut your thread so that every rev of the screw moves the screw 1 inch.
That's your lead. But you want 4 starts so the thread form can be
smaller. So your pitch would be 4 threads per inch. And 4 TPI= .250"
Look in the book for the thread depth of a 4 TPI thread. Touch the
threading tool to the part and zero the dial for the X axis. Make
several passes until you have moved in on the X axis until the tool
cuts the full depth. Then advance or retard the compound .250". Repeat
the cutting operation. Move the compound. Cut. Etc. You may want the
compound set at 45 degrees to clear the tailstock because you need to
use a center to support the part. Or maybe you want to set the
compound at 29.5 degrees if you are cutting a 60 degree profile
thread. If the compound is set at any angle other than parallel to the
Z axis use trig to figure out how much to advance the compound. E.G.
the multiplier for 45 degrees is 1.4142 so the compound would need to
be advanced (or retarded) .3536. In the above example the Z axis is
the longitudinal axis and the X axis is the cross slide.
ERS