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So you have a single phase motor. This is one of the places
where three phase wins -- you can "plug" reverse -- just switch into
reverse at the proper time (and quickly crank the cross-feed out during
the moment in which the spindle is stalled). Or -- if you have three
phase and a VFD, you can tune the ramp-down and ramp-up times to more
reasonably match the available power, and have a little more time to
crank out the cross slide -- if you *have* to run it in reverse instead
of disengaging the half nuts and cranking the carriage back. (You do
wind up in this situation when you're doing metric threads on an
imperial lathe, or vice versa.)
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Yes. Ideally you want to move the compound from the position of
29.5 degrees crank towards tailstock to the same angle towards headstock
instead so you can follow the flank of the trailing edge of the cut,
only making a very shallow cut there for each pass while making a deeper
cut on the leading edge.
And you don't do the full depth in a single pass -- unless it is
a very fine and shallow thread -- say something like 80 TPI or finer.
And also -- aside from setting the gearbox (or changing the
gears, depending on the lathe involved), you also want to use the
half-nuts to engage carriage motion -- not the power feed which works
through the handwheel. (Of course, some lathes don't have the power
feed, only the half nuts, so the question becomes moot.)
The main thing for the first threads is to make sure that you've
got plenty of space for reaction time to stop the cut -- don't make your
first threads cut to a shoulder. :-)
Easiest is if you have a reduced diameter both before and after
the area to be threaded. If you don't have that, cut a runout groove to
the depth of the thread at the end of your thread. The needed width
will vary with the coarseness of the thread, the speed of the spindle,
and your reaction time. Before you actually cut the thread, back the
tool out far enough so it will clear the increased diameter of the
workpiece at the end of the thread, and try some practice runs -- using
the dial to determine when to engage the half nuts, and seeing how
consistent you can be as to where you disengage the half nuts. That
will give you an idea how wide the runout groove needs to be for this
thread. Coarser threads move the carriage faster and require faster
reaction times. Really fine threads give you a lot more leeway in
I did that once or twice by drilling at the end of the thread scratch
with a hand drill, since the stock was in a collet.
The point was that my belt-driven lathe doesn't seem to break bits
when stopped during a cut. Could it happen only on geared lathes?
there is one other gotcha.
you need to position the tool so that the body of the tool below the
cutting edge is aligned to the helix angle of the thread that you are
cutting. ...otherwise the (what do you call it??? the base, the foot
of the tool???) bit of the tool below the cutting edge will tear the
thread as it graunches past it.
this is easily achieved by tilting the tool.
the gotcha is that for threads in different directions the tilt is
reversed. the tool is set up the opposite way to a right hand thread
for left hand threads.