Non-constant Pitch when Threading on Lathe

T

tasat

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I picked up a bench lathe (BD 812) a little while ago, and I’ve had an ongoing issue with threading that I haven’t been able to solve. When cutting the threads, it becomes immediately noticeable that the pitch isn’t quite constant, making the distance in between the threads look “wavy” or “pulsating” as the part rotates. I’ve verified the lead screw isn’t bent on a surface plate and is dead in line with the half nuts, indicated off the ways from the carriage (in all directions) with a tenths indicator while thread cutting to find no significant movement, and have tried various setups to be sure the part isn’t flexing away from the tool in odd ways. Only thing I have found not quite right was a change gear I almost always use that’s out of concentricity by 0.006”, which could have a slight effect on how quickly the leadscrew rotates. Anyone else encounter such an issue?

I’ve attached a video showing both the start of a thread and the leadscrew. If you zoom in on the thread, you’ll be able to see the problem, and I count ~16 “pulses” of the variable pitch. Interestingly enough, I also count ~16 revolutions of the leadscrew in the same amount of time. Not sure if this means I’ve got a messed up pitch on the leadscrew, but I’ve no way to measure that directly.
 

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I see you are new here. Welcome to Hobby Machinist.

For the pitch to be inconsistant, there has to be a difference in speed of rotation of the part and the speed of rotation of
the lead screw. If something is binding, the lead screw could twist slightly and then release, causing an inconsistancy
in the pitch. I would unplug the machine and turn the gear train manually to locate something binding. A gear that is not
concentric would be very likely to cause this condition. It's all part of the fun to make things and solve problems along the way.

Good luck. Let us know how it goes.
 
I agree with cathead, there must be slippage between the lathe spindle and lead screw. I would look for a sheared pin or key on either of these.
 
Welcome to H-M! Sorry to see that having problems is what brought you here, but I believe that you will like it here after you've had a good look around.

Everything that cathead said is correct and a non-concentric gear could easily cause the problem but I don't think that .006" would be enough to do it.

I saw something similar happen once when some goofball (me) forgot to put a key in the change gear on the end of the lead screw, allowing it to slip a little bit here and there. That made for a pretty wonky thread, but I spotted it right off the bat and was able to resolve the issue and save the part.

You might try running another pass and see if the tool follows the exact same groove. If it does you will know that the problem is very consistent, pointing to a non-concentric gear. If it cuts a new groove in places, it indicates that something is slipping.
 
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TerryWerm said:
Welcome to H-M! Sorry to see that having problems is what brought you here, but I believe that you will like it here after you've had a good look around.

Everything that cathead said is correct and a non-concentric gear could easily cause the problem but I don't think that .006" would be enough to do it.

I saw something similar happen once when some goofball (me) forgot to put a key in the change gear on the end of the lead screw, allowing it to slip a little bit here and there. That made for a pretty wonky thread, but I spotted it right off the bat and was able to resolve the issue and save the part.

You might try running another pass and see if the tool follows the exact same groove. If it does you will know that the problem is very consistent, pointing to a non-concentric gear. If it cuts a new groove in places, it indicates that something is slipping.
Click to expand...



If the gear clearance is less then .006, the teeth would bind. I would try testing the lathe with a different gear to isolate the problem.
 
You are correct again, and I never gave a thought to the gear clearance being too tight and its effect on a non-concentric gear. I think you might just be on to something here...
 
Also slip / stick of the saddle on the ways as it is moved by the leadscrew. Lubrication issue. Another which I had with mine was the Feed handle on the carriage actually moving faster and slower with it engaged with the rack and it’s unbalanced weight affecting the cut. Once I noticed this I keep a light drag on the handle to maintain it loading. This appears on fine threads the most but is likely there on all threads.
 
So here’s the things I’ve tried in consideration of all of your feedback:

- Swapped out for completely different change gears and verified there is no binding between the gears or between the leadscrew and half nuts
- Put some drag on the carriage wheel so it wouldn’t add inconsistent force to the carriage
- Lubricated everything and checked by hand to make sure there’s no sticking points
- Cleaned all gears, leadscrew, and half nuts to be free of gunk and any small chips

With the above changes, I cut threads near the chuck and all the way back by the tailstock, with both exhibiting the same inconsistent pitch. I also indicated the leadscrew along its ground section right next to the bushing by the headstock (0.0008” runout) and about 3.5” down by where the threads start (0.0020” runout). This caught me a little off guard, as slowly rolling the leadscrew along a surface plate revealed no noticeable bends. There’s no garuntee the ground section was made in the same setup as the threads, but as a sanity check, I cut threads with the half nuts engaged at the far end right next to the tailstock end bushing and the middle of the leadscrew, which is where the runout should be the least and most prevalent, respectively. On both cuts, the pitch was inconsistent and the two threads were not discernible from each other.

I also thought it may be an issue with the half nut engagement, so I adjusted it looser and tighter, but to no effect on the cut. There’s also no slip between the spindle and leadscrew, I can easily and consistently follow the same groove to full thread depth. I’m starting to run out of knobs to turn, short of truing the bores of all changes gears or buying new hardware, and I haven’t been able to find other threads dealing with similar issues.
 
Like Pierre mentioned, you might be experiencing stick-slip of the carriage on the ways. What are you using to lube the ways?
 
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