Threading Index Varies With Speed

As far as I understand it, ideally there should be no offset.
The offset is there because the algorithm only matches velocity
and does not do the calculations needed to match velocity
_and_ correct position.

After pulling my hair out trying to figure out why G33.1 wasn't working on my lathe while attempting to test the problem raised in this thread, @cmorely pointed out I'm having the same issue.

forum.linuxcnc.org/38-general-linuxcnc-q...pping-problem#319786

So I can confirm spindle synchronization start points vary with spindle speed in G33.1 as well as G76 - and it's way worse on my lathe than the OP reported on his.

In my case a difference of 140rpm (60 vs 200) resulted in appx 0.750" difference between the Z-start point when synchronization kicked in on a 1/20 pitch (0.050").

According the the formula Andy posted earlier, my Z-axis requires the following distance to get up to speed

 

I don't understand why there is such a significant difference between what I'm seeing and what the OP's lathe is doing.

Unless somone has other thoughts, I think this is a bug.

Update here:
forum.linuxcnc.org/38-general-linuxcnc-q...blem?start=10#319865

So my lathe's behavior is closer to the OP's when running a program vs. MDI.

But as I mentioned in the other thread, I don't think this behavior is correct even with the reduced 'dive' towards the synch point.