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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.

Joco wrote:

Don’t make that assumption re feedhold. And tapping. The planner is taking things direct from the Hal pin. Not from the lcnc motmod module.

it needs to be explicitly tested. Unless someone has done so already and can confirm what the behaviour is. 
 

I can confirm that LCNC 2.10 disables feed-hold while synchronized motion is in progress.  FH has no effect during a G33 or G33.1 command until after the synch move is complete.

cmorley wrote:

I would expect speed changes to change the sync start point.
There is another post about non syncing threads with speed changes.
In linuxcnc, you can't change the spindle speed and track the same thread, the sync point moves.

Linuxcnc moves the axis as fast as possible till it is moving at the approximate correct 'pitch' speed. It then works to get the exact pitch and maintain it. This distance to get up to speed is kept track of for use of calculating position and position error.

IIRC linuxcnc allows 10 rotations past target before deciding there is an error.
 

Thanks for giving this some thought Chris.

I understand that LCNC needs a short period/distance of max accel to go from zero speed to synch speed - the other thread has a code snippet which explicitly states this in a comment.

What I don't get is why LCNC is moving so far to get up to speed.  My lathe has pretty aggressive acceleration - it can reach max speed in a very short distance.

But it's moving over an inch at rapid speed to get to the synch point at only 200rpm with a 0.05" thread pitch.  Feedrate at that spindle rpm and pitch is really slow - like 4ipm or so.

I guess what I'm seeing is identical behavior from the other thread... but much worse start point variation.