Problem with cutting a thread on my lathe

I have a Taiwanese Jet 1024 lathe. When everything goes right, I've managed to cut nice threads. But on two occasions, I've had a strange problem that I
can't understand.
I'm cutting an external thread, towards the headstock. I make a pass, back out the compound, run the motor in reverse a good 1/4 inch past the beginning of the thread, turn in the compound and make another cut.
Twice, about halfway through, the halfnut lever popped out, probably when I reverse at the end of the cut. But that's not my problem. If I run the spindle until #1 comes up on the halfnut dial, (where I set it when I began), engage the halfnut lever, and try to get back into the thread, the cutting tool is nowhere near the vee of the thread. How can this be? I'm cutting 20 tpi, nothing tricky there, I just can't figure out why I can't get back into the thread.
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I think your problem is running the motor in reverse. When you reach the end of the thread, diesengage the half nut and manually run the carriage back to the start and then reingage the half nut at the proper number.
Bob Swinney

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    O.K. If you have half nuts and a threading dial, I'm finding myself wondering why you have to run the lathe backwards to reset for the next pass. Normally (e.g. for metric threads with a metric leadscrew or inch threads with an inch leadscrew), you simply disengage the half nuts, crank the cross slide out enough to clear, and hand crank the carriage back to the proper starting point. Once there (with the spindle still spinning) you wait for the proper number to come up on the threading dial and re-engage the halfnuts.
    However -- your experience suggests that this is not the case with your machine.
    My *guess* is that the leadscrew is a metric pitch, and you're using transposing gears to cut inch threads. If that is so, you *do* need to run the spindle backwards, and the threading dial alone will be of no help. You will need to run the spindle backwards as many turns as you ran it forward, leaving the half-nuts engaged.
    Or, there is one person in another forum who says that you can get away with disengaging the half nuts, cranking the carriage back, and running the spindle backwards until the starting number returns on your threading dial. I've never tried this, and I would need to set my Clausing up for metric threading to test it. I figure that it would work as long as the thread was not long enough to accomplish a full rotation of the threading dial while traversing it by hand crank.
    So -- what it looks like is that you need to hold the half-nut lever down when you reverse the spindle to keep it from popping up -- or find out whether you can replace the detent spring with a stronger one.
    Good Luck,         DoN.
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DoN. Nichols wrote:

all of the above but you have to run the carriage back to a hard stop so the start position is the same each time. Same as a CNC threading cycle.
John
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DoN. Nichols wrote:

Thanks for the advise. I went out and measured the pitch of the leadscrew and it was exactly 8 tpi. So it's not a metric issue there. I thought about this problem more and I remembered that the reason I was leaving the halfnut engaged and backing the motor was because I had had this same issue with not being able to pick up the thread after I closed the halfnut.
My present thinking is that the halfnut can be disengaged and reengaged as long as I return the carriage to the same place each time. If I cause the halfnut dial to go a full turn past where I started, I get out of sync. I will try some tests tonight to see.
Then the next question is why. The threads seem perfect when it works. I'm thinking that somehow the metric/inch change gears between the spindle and the quickchange gearbox are off by some small number of teeth. I don't know why this would be, but it's the only thing I have to go on.
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I would check and see if you have the metric change gears installed. Get the manual out and see if the right gears are installed from the spindle to the input shaft on the quick change box. I had some one leave the metric set of gears in a lathe at work and I had the same problem you are having. Once I put the correct set of gears in, it worked fine.
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    [ ... ]

    [ ... ]

    [ ... ]

    Bingo! This is the problem. Metric transposing gears on an inch machine, or inch transposing gears on a metric machine means that you *cannot* disengage the half nuts until the thread is finished.
    Since yours is an inch leadscrew, and you said (IIRC) that you were wanting to cut a 20 TPI thread, you should not even *have* the metric transposing gears installed. They will make you cut a metric thread of some sort.
    The ideal set of metric transposing gears (on machines which have room for them) are a pair of gears of 127 and 100 teeth on a common shaft. This makes the conversion to make metric threads possible, but throws off all work with the half nuts and threading dial.
    So -- get rid of the metric gears for your 20 TPI threads, and your half nuts should work as expected.
    Good Luck,         DoN.
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It may be that your thread indicating dial's pinion wheel is not fully engaged with the leadscrew. Usually (certainly on my Colchester) the pinion on the opposite end of the dial's spindle can be disengaged from the leadscrew to reduce wear when you are not cutting threads. If it is not properly engaged (perhaps it has been "engaged" with one of its teeth sitting on top (i.e. the Outside Diameter) of the leadscrew's thread, then the dial will rotate erratically. Hope this helps Regards Ian
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Ian wrote:

Another possibility is that the pinion of the thread dial doesn't have an exact multiple of 8 teeth (16, 32, etc). If it doesn't, then it will never work right. You can't trust ANYTHING on these imports until you've checked it yourself.
Jon
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I think your problem is in the next paragraph.....>

After engaging the half-nuts, running the thread the length you need, you then dis-engage the half-nuts.... back out the CROSS-SLIDE, hand crank the carriage back to before thread start, turn in CROSS- SLIDE to exactly the same reading, then slightly advance the COMPOUND, and engage the half-nuts on the same number as before. HTH Ken.

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Well, did we come up with a solution to the problem??? Ken.
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