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Lead screw or thread dial not right?
- Thread starter jareese
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- Oct 11, 2016
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A quick check: chuck up some bar stock. Choose a very coarse thread, say 10tpi. Make a clear scratch pass (about .005") in the forward direction more than 2". Don't change anything, except dial in another .002" or so. put lathe in reverse, and scratch (presumably over top) of your previous scratch pass.
These passes should be identical.
Before you go "WTF" over this - This is like trying to solve a computer problem, and the first question is "Is the computer turned on?"
When you replaced the damaged parts on the carriage, did you swap the carriage drive gears from the original carriage to the new one, or did it come with all those parts already installed? I’m wondering if something in there is slipping when going forward, but is secure when going in reverse.
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- Dec 28, 2022
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all my carriage drive gears are on the apron, which are original.
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- Dec 28, 2022
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I have different tools for threading in reverse and forward - they won't match up...
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This is true, but for this test one is trying to see if the tool tracks the same way in reverse as forward. So use a RH (normal right hand) tool to thread forward, then reverse. Save for backlash, the track followed by reversing shouldn't cut much metal, but simply should be following the cut. This is true if and only if the half nuts stay engaged for the test!
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Gotcha - I’ll try it this weekend.
something is going on there and I don't think it's backlash or bad parts. Even if the lead screw were sloppy, that backlash gets taken up by motion just like any other backlash. Getting a quarter of that thread shouldn't be possible. If you were doing something like starting the threads going towards the chuck, and changed your mind half way and decided to finish them going away from the chuck, then backlash would be an issue.... But you'd make that up picking up the thread again when you rearranged the tool to cut the other way.... If you had two feet of backlash in the leadscrew, the leadscrew was the wrong pitch, the cross slide screw was backwards, and the compound was at the wrong angle, you'd have a hard time making threads, but the results would be consistent....
I have one thought. I don't know if this is the answer, but it's gonna take me longer to type than it would for you to check. I believe that you have a 16 tooth gear on your threading dial, so this SHOULD be real easy to spot. But there are lines between your lines. It is incremental. If you don't even start the lathe, just set it up for threading, and manually turn the chuck. Drop your half nuts anywhere they are dead nutz on a line. Manually advance the chuck (and thereby the whole geartrain and leadscrew) until the carriage just moves. Release the half nuts, and SLOWLY advance the chuck until the half nuts WON'T engage again. Holding light pressure on the half nut lever, slowly advance the chuck some more. Slowly, just unitl the half nuts drop in again. You'll see the dial move and pick a new home that is not on a line..... Now, look at that position on the dial is at. And the question, is it plausible that your engagement was off by this much during the errant threads?
I have one thought. I don't know if this is the answer, but it's gonna take me longer to type than it would for you to check. I believe that you have a 16 tooth gear on your threading dial, so this SHOULD be real easy to spot. But there are lines between your lines. It is incremental. If you don't even start the lathe, just set it up for threading, and manually turn the chuck. Drop your half nuts anywhere they are dead nutz on a line. Manually advance the chuck (and thereby the whole geartrain and leadscrew) until the carriage just moves. Release the half nuts, and SLOWLY advance the chuck until the half nuts WON'T engage again. Holding light pressure on the half nut lever, slowly advance the chuck some more. Slowly, just unitl the half nuts drop in again. You'll see the dial move and pick a new home that is not on a line..... Now, look at that position on the dial is at. And the question, is it plausible that your engagement was off by this much during the errant threads?
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- Dec 28, 2022
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I don’t believe so, because it happens every time. For a 24 TPI thread I should be able to engage anywhere with the 1/2 nut, not even looking at the dial. As I’m just starting, I pick a number, typically 2 or 4. If I stay with that number, everything is fine. If I choose 4 or 2 respectively, meaning the opposite of what I started at, I get what appears to be a quarter thread.
Threading in reverse, away from the chuck, I don’t get this. The threads are dead on, every time, with 16, 24, and 28. (the only threads I’ve tried so far.)
I’m wondering if maybe one side of the 1/2 nut isn’t messed up from the fall….???
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In a message to @Dabbler - I sent this earlier:
I'm actually wondering if the 1/2 nut was damaged on one side when it fell on it's face. The leadscrew was bent pretty badly as well. The lathe was laying on the controls for the Apron and the cross slide. When it fell over, it hit the apron handles first, then the cross slide, then came to rest on both, and sat that way for almost a week.
I'm actually wondering if the 1/2 nut was damaged on one side when it fell on it's face. The leadscrew was bent pretty badly as well. The lathe was laying on the controls for the Apron and the cross slide. When it fell over, it hit the apron handles first, then the cross slide, then came to rest on both, and sat that way for almost a week.