Unable to cut correct thread with G76

Patrik T wrote: I did not change the G76 algorithm. I have a separate code for moving and syncing the tool in my Xbox pendant implementation.
This code uses the same spindle encoder signal, but jog counts to sync the tool to the spindle absolute position.

That really shouldn't be better. In fact it ought to be far worse as the position commands are passing through non-realtime layers and USB!

Does your lathe have a spindle-at-speed signal? Are you threading in CSS mode?
Is this a servo or stepper machine?

I am not using the very slow halui. I am using the 1000 Hz servo thread, which apparently is fast enough to make fine threads. An RT thread would maybe be a further advantage, but I did not need it. The proof is in the pudding.

The USB is not involved in the actual syncing, so that is not an issue.

I have no spindle at speed and I did not use CSS.
It is a stepper so it is possible that I am losing steps, but why would I only lose steps when using G76.
Edit: Also, the thread appearance is not consistent with what I would expect if it was a losing steps issue.

Patrik T wrote: It is a stepper so it is possible I am losing steps, but why would I only lose steps when using G76.

I was wondering if, perhaps, trying to synch to the spindle is causing more-than-usual accceleration.

Out of interest, what is your acceleration setting for the Z-axis (and the max velocity).

The thing is that for most people threading works perfectly, so I am wondering what is different about your setup.

Yeah, that was what we concluded the last time we discussed this, that the fast variations of my light weight spindle meant the z-axis acceleration can not keep up. I accept that that is the cause of my problem. Therefore I expect to not have the same problem on the lathe I am building now. Still, the same acceleration settings work with the other algorithm.

So, to cadcam, I kind of resolved the issue by accepting the cause to be as described above.