G33.1 synchronized tapping problem

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.

The other thread:
forum.linuxcnc.org/38-general-linuxcnc-q...s-with-speed?start=0

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.

Do you get the same if you use a program rather then MDI?

Thanks for the troubleshooting suggestion.

Yes, same behavior, but much less Z-distance during the initial 'dive' than when using MDI.

Approximate position the Z-axis rapids to before synchronization while running a program:
G33.1 Z-0.5 K0.05 $0
(Starting at Z0.500")

• 60rpm: 0.440"
  • rapid dist: 0.060"
• 100rpm: 0.390"
  • rapid dist: 0.110"
• 200rpm: 0.370"
  • rapid dist: 0.130"
• 400rpm: 0.300"
  • rapid dist: 0.200"
• 750rpm: 0.140"
  • rapid dist: 0.360"
• 1000rpm: -0.125"
  • rapid dist: 0.625"

So perhaps 1/4 the distance vs MDI.

Still don't think this behavior is right.

Changing the Z-axis accel only slightly affected the rapid distance, if at all.

The earlier tests were done with MAX_VEL = 4, and MAX_ACCEL = 175.

Dropping down to MAX_ACCEL = 2 resulted in approximately the same pre-synch rapid distance at all speeds.

Much less violent rapids, of course.