Unable to cut correct thread with G76

Thanks, but that does not prove to me that it is keeping track of the spindle's absolute position.
I am doing a small M5 thread. The lower M5 pitch makes a small pitch error more apparent than it would be on the big thread in the video..
I think the pitch error also only appears if the spindle speed varies rapidly.

No, I have not verified that the encoder does not lose counts.
I don't think it does since it works in another algorithm.

G33 and G76 use spindle position sync, not velocity,
if you have threading issues its most likely a hardware or setup issue

This is what happens when two people used to being right meet
I am not listening to you saying it is hardware because I have made up my mind that it is software.
You are not listening to me saying it is software because you made up your mind that is is hardware.
We can agree that we disagree.

Let's maybe give the thread back to the OP.
Thanks for the chat.

I have not made up my mind either way (and there are some known bugs in G76 in some LinuxCNC versions),
I just know that G76 and G33 use position mode spindle synchronization and that many people cut fine and long threads without issue with LinuxCNCs G76 so it just seems more likely the problem is something specific to his setup

Patrik T wrote: You are not listening to me saying it is software because you made up your mind that is is hardware..

The difference is that we can _all_ look at the software and verify that it uses spindle position from HAL.

One case where things might go wrong with a varying-speed spindle is if the Z-axis acceleration rate is set two low to follow the changes in spindle speed. For threading you do want the Z accel to be pretty close to the physical limit.

Yes, and when the spindle speed varies faster than the Z-axis is allowed to accelerate there will be a minor dent in the thread if the algorithm keeps track of the spindle's absolute position and syncs the tool to that.
If the algorithm does not sync to the spindle absolute position the errors will accumulate and produce a bad thread.