Thread cutting on a reversible lathe

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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.)
Enjoy, DoN.
Reply to
DoN. Nichols
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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 reaction time.
Enjoy, DoN.
Reply to
DoN. Nichols
He *did* say "with the machine turning in the same direction". Here is the quoted text which you left below and which I have moved up here to make it clear what refers to what.
Enjoy, DoN.
Reply to
DoN. Nichols
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?
Reply to
Jim Wilkins
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.
Stealth Pilot
Reply to
Stealth Pilot
yep, yep, yep
(i'm not a yes man :-) ) Stealth Pilot
Reply to
Stealth Pilot

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