Crazy Threads...

I agree half nut engagement. find a place where the halfnut engages solidly. it does not matter where it is. then shut the lathe off and with a felt pen make your own mark use this mark to machine these threads. if it messes up again then repair the halfnut if it work then repair your halfnut or make a new mark each time you thread, bill
 
This is probably unlikely but,

Does the threading dial look like original equipment for the lathe?

Different pitch lead screws have threading dials with different number of teath on the meshing gear. Possibly fitted an incorect threading dial to a lathe that didn't come with one?

The thread dial should indicate fractions of inches when the carage is moved up and down the bed, off the top of my head i cant remember if each revolution is one whole inch of a part of an inch but it should repeat and come out as a nice round number. You could move the carage back and forth with a steel ruller clamped on the tool post to look for inconsistances in the movement.

Stuart
 
Everything points to the point of entry on the new pass. If the half nuts were worn or the leadscrew was somehow loosing time with the spindle it would seem that it would do it when you left it engaged while threading too. You could do like one of the previous posters said and just thread away from the shoulder to finish the project your currently working on and sort it out later.
 
Being an "Asian" India made lathe, it may have been originally built to run on metric standards. If the leadscrew was switched for an imperial one, the thread dial must be replaced with the matching one. If that was not done you will get the results you have described. Not 100% sure, but I believe if you look at the gear in the thread dial, it should have a number of teeth equal to or a multiple of the teeth per inch of the leadscrew ..probably 4/8/16?.... Plus if not matched, they may not even engage correctly.
 
juiceclone said:
Being an "Asian" India made lathe, it may have been originally built to run on metric standards. If the leadscrew was switched for an imperial one, the thread dial must be replaced with the matching one. If that was not done you will get the results you have described. Not 100% sure, but I believe if you look at the gear in the thread dial, it should have a number of teeth equal to or a multiple of the teeth per inch of the leadscrew ..probably 4/8/16?.... Plus if not matched, they may not even engage correctly.
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I like this comment. the thread dial was what I was thinking the whole time. everything else works. but I never get a start in the same place. Each pass always about 2 thou in front of the previous pass. also, the half nut does not engage properly where it should on the dial marks. on the half marks the half nut hits the top of the threads it seats either just in front of or just past the half mark. I will count the threads on the dial tonight.

It looks original, but who knows. I believe this machine was built in the 80's. that's 36 years ago...

So if the gear on the dial is wrong, the new question will be how to find/make a new gear. how to math all that out. (I'd have to send away to have one made, I don't have an indexer.)
 
If I remember right, the thread dial gear should be 2X the leadscrew. So in your case, it should be 16 tooth gear. When I get home I will check my lathe and get back to you if no one else has beat me to it.
Pierre
 
pdentrem said:
If I remember right, the thread dial gear should be 2X the leadscrew. So in your case, it should be 16 tooth gear. When I get home I will check my lathe and get back to you if no one else has beat me to it.
Pierre
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I can say with a reasonable amount of certainty it is not 16 teeth. it's quite small. maybe only about 3/4" round.
 
Chuck K said:
You need to verify whether the leadscrew is metric or imperial.
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I agree, but that could only cause this sort of problem if the leadscrew and the halfnut don't match.

Also count the teeth on the threading dial gear and verify that it is meshing correctly with the leadscrew.

I'm still suspicious of that floppy leadscrew.
 
When a company makes a lathe for both the metric and the imperial markets, it is set up with different labeling, gearing, lead screw, and threading dial. Do you have a manual for the lathe? If so, check that all the tooth counts are correct. Sometimes a mistake is made at the factory and both metric and imperial components are mixed during assembly. QC would normally catch that, but QC is sometimes poor or nonexistent, and sometimes mistakes are made. I would also carefully check the last gear in the gear train that drives the quick change box. That is a compound gear. Make sure both tooth counts on that gear are correct. Also, you made that one thread that looks pretty good. Could you check that thread with a thread gage or a ruler to see if it is the correct pitch to match the machine setup used when it was cut?
 
pdentrem said:
If I remember right, the thread dial gear should be 2X the leadscrew. So in your case, it should be 16 tooth gear. When I get home I will check my lathe and get back to you if no one else has beat me to it.
Pierre
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The threading dial moves one tooth for each revolution of the leadscrew. Any integer will work but a multiple of two is most convenient. One would work ok.

Suggestion: count the TPI of your leadscrew and then set up to cut that TPI. Then the spindle and the leadscrew should be running at the same speed and you can start on any leadscrew thread.
 
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