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Non-constant Pitch when Threading on Lathe
- Joined
- Nov 16, 2012
- Messages
- 5,596
Here's the manual for it in-case you don't have one. There's only so many things that can go wrong here and there's not much to it. With the power off, inspect all the gears. Are the gears too tight or loosely meshed? When it runs, is everything spinning in a good circle or does something look out-of-round? Check the belts/pullies for anything that is out of round. Look at parts 101 and 117 in the diagram. It's what attaches the leadscrew to the sidegears. Is that a clutch by any chance? Make sure there are no missing roll-pins in any of the fittings that connect to the various shafts. Is the leadscrew bent?
Ray
Ray
- Joined
- Jan 28, 2011
- Messages
- 3,582
Ok, keep looking.
Perhaps the half nut mount system is loose, but I would believe you would have seen that as you had that apart.
Next I would suspect the lead screw is moving left to right or the carriage is not square to the ways.
There is likely a thrust bearing for the leadscrew, find it and make sure it is lubed with no debris and adjusted correctly.
For the carriage the leadscrew is not on the centerline and thus the forces are not as well. As the leadscrew pushes and pulls on it depending on the direction of feed it twists the carriage and any of these could cause feed inaccuracies if the carriage is too loose. Check the gibs, remove them and make sure they are nice and smooth on the rubbing surfaces and reinstall, lube, and adjust for minimal play with still smooth motion. Surface finish of the gibs and their running surface on the bed are known to be rough on some lathes. A fine stone or fine grit wet and dry paper with oil or mineral spirits will help there.
Perhaps the half nut mount system is loose, but I would believe you would have seen that as you had that apart.
Next I would suspect the lead screw is moving left to right or the carriage is not square to the ways.
There is likely a thrust bearing for the leadscrew, find it and make sure it is lubed with no debris and adjusted correctly.
For the carriage the leadscrew is not on the centerline and thus the forces are not as well. As the leadscrew pushes and pulls on it depending on the direction of feed it twists the carriage and any of these could cause feed inaccuracies if the carriage is too loose. Check the gibs, remove them and make sure they are nice and smooth on the rubbing surfaces and reinstall, lube, and adjust for minimal play with still smooth motion. Surface finish of the gibs and their running surface on the bed are known to be rough on some lathes. A fine stone or fine grit wet and dry paper with oil or mineral spirits will help there.
- Joined
- Jan 28, 2011
- Messages
- 3,582
Had a thought as I was rereading your responses. Are the threads repeating the error randomly or every turn of the leadscrew or the spindle? Is your tool sharp and on centerline? How much of a cut are you taking? Any chance of picture?
Thank you all for your help and suggestions. I think pdentrem is on the right track with the leadscrew moving left and right. I took off the one and only “thrust bearing” surface (or “adjusting disc” per Tom’s comment) and indicated the thrusting face on the leadscrew to find...0.0045” of variation (see attached video). That’s not to say that face itself is responsible for all the variation, as the opposite face may have some variation as well (since I was pressing the leadscrew left while rotating it). It also could be interference from the key at the far left end of the leadscrew pressing against a washer unevenly. I took the gears, key, washer, and bushing off the leadscrew to take the measurement just to give a bit more control. Note that both the indicated surface and the possible key-washer interference would be surfaces under force when cutting threads right-to-left (the leadscrew being “pushed” to the right by the carriage drag), as I’ve been doing for all the tests.
So, I think the next step is to take the leadscrew to a more capable (longer) lathe, set it between centers, indicate it square, and clean up the thrust surfaces. I’ll clean up the “adjusting disc” as well (it’s a pretty rough cast iron piece) and put in a bronze washer on both left and right thrusting faces to get smoother motion and reduced wear. I’ll post an update as soon as I know if all that works, but will surely be listening in for more great advice.
So, I think the next step is to take the leadscrew to a more capable (longer) lathe, set it between centers, indicate it square, and clean up the thrust surfaces. I’ll clean up the “adjusting disc” as well (it’s a pretty rough cast iron piece) and put in a bronze washer on both left and right thrusting faces to get smoother motion and reduced wear. I’ll post an update as soon as I know if all that works, but will surely be listening in for more great advice.
Out of curiosity, I took the same measurement with the full geartrain installed so I could push the leadscrew to the right (like while it’s actually in use). It showed about 0.0084” of variation, but on closer inspection, I don’t think the washer by the gears has to be a thrusting surface at any time. There’s may 0.025” of clearance available with the leadscrew seated all the way to the left, and maybe the adjustment disc is supposed to set this distance by design, but in practice, there’s still some wiggle room left with the adjustment disc fully seated. Easy fix, though.
My lathe had the shear pin, well, sheared when I started using it to learn how to do threading. The leadscrew would rotate normally most of the time, but when I applied the brake too hard. the leadscrew would rotate (relative to the geartrain) or shift sideways a bit, and then the pass would wipe out the threads. Normally this would happen when the threads were almost complete. It took me 6 or so tries (to create threads) before I figured out the problem...
So I took make leadscrew to a larger lathe and made a skim pass on the thrusting surfaces to get them true to the axis, then made a proper bronze bushing for said surface to ride against, and I’m happy to say that all is well. At the end of the day, I think only one of the two steps above is really necessary to fix the problem if you’re in a pinch and can’t work on the leadscrew. Thanks to all again for all the help and suggestions!