Threading Index Varies With Speed

 I'm having an odd problem. I'm running a threading routine using G76. I thought that, while threading, I could change the spindle speed without affecting the operation. Everything seemed to be working fine until I starting threading something a little larger for me - a 1"-8. I started to see bad gouging off and on during the threading. that at times nearly stopped the (7HP) lathe. I was only taking off about 0.002" per pass for most of the operation. I thought something must be moving, and checked all the obvious things. Everything looked OK.

One of the things I tried was making a scratch pass at three different speeds, 100, 150 and 200 RPM. I ran the same program each time without changing the setup. You can see that the cut indexed to a different point for each speed. Conversely, if I ran it three times at the same speed, it would index to the same cut.



This is not good! It means if I cut a thread and then need to recut it, I need to make sure that the spindle speed is exactly the same or I'll get a different thread position. Needing to cut the thread again can be because the piece rotated in the chuck during the cut or I didn't set the tool X position deep enough. I'm also thinking that as the lathe slows down slightly, it might change the Y position of the tool incorrectly and cause the gouging.

Any ideas?

Two indexing signals to LinuxCNC: one pulse per revolution for indexing, and ten pulses per revolution for position
Spindle speed set manually, and not by LinuxCNC
LinuxCNC v2.9.3

It's certainly supposed to be speed independent,
that is, the Z motion is supposed to be "geared" to the
spindle rotation.

(there's a video I can't find at the moment that shows
LinuxCNC threading while hand turning the spindle)

Possibly an encoder/index/Z axis issue?

You are correct, you can do this experiment by hand turning the spindle, and the Z axis will advance. I have not done this against the cuts at various speeds in the picture, so I don't know where the hand turned cut would be.

The behavior is exactly reproducible at the different speeds. That is, if you thread at 100 RPM and then run the program again at 100 RPM, it will cut the exact same thread. If you cut at 100, then at 150, and then back at 100 RPM again, the two 100 RPM cuts will align. I don't think that points towards a encoder/indexer error.

which one is which?

\\\

200\150\100\

?

If index is detected properly (at the identical rotor angle regardless of speed)
The Z Axis position should be in strict relation to spindle angle so this does seem to
suggest an Index issue or perhaps a Z axis problem (What are the following error limits?).

Perhaps plotting the z axis position and spindle angle in halscope
while threading at different speeds would shed some light on whats going on...