Raspberry Pi 4 with Mesa 7c81

Back in this forum with an unusual issue that has me puzzled...

There is an issue with my router and circular cutting. When cutting arcs the machine is adding a small linear cut somewhere mid arc (see attached photos) In the photo marked A you can see that it was cutting a very nice arc then all of a sudden moves in a direction that is not part of the arc! I wrote very simple gcode program to test the arc in the part of the photo labeled A in an effort to eliminate the problem being a kCAD/CAM issue as this problem was originally discovered cutting an ellipse using CAD/CAM (label B in photo).

The axis are moving in the same direction throughout the arc which has eliminated backlash as the culprit.

Additionally I have been V carving with the machine and the results have been perfect (including curves of letters, picture shapes etc.)

This seems to be a problem with simple arcs

Totally stumped. Not sure if it’s settings or an issue with the 7c81. Any help would be greatly appreciated.

Is this repeatable?

Is position lost when this happens?

The machine does not lose position.

It is not exactly repeatable in that it does not do the same thing each time but it does do something at the same location of the pass if you are making multiple passes to say cut a part out. I’ll try to get an example picture

My first thoughts are that the 7C81 is too "dumb" to do this.

I assume that the backplot does not show this?

There are many possibilities, including mechanical. Check that all your couplings are tight. Though it doesn't feel like a max-force point or a max-speed (backlash) point.

Definitely not mechanical. It is obvious when watching it that it is commanded motion...

You can see in this photo what it has done after multiple passes. Not exactly in the same spot but same type of error.

When you say it does not lose position, are you saying
the tool returns to the correct path after the deviation?

Maybe a simpler test code would help diagnose the issue

What are your min_ferror and ferror settings?
large values of these can mask steggen setup errors


Posting your hal and ini files would also help