Rotary axis backplot — centre of rotation not following WCS offset

Hi,

I have a 4-axis MPCNC with a rotary B axis mounted at the far end of the Y travel (machine Y=700mm). The rotary axis is fixed — it does not move along Y.

G-code is generated by CAM with program zero at the chuck face, all moves go in Y-.

The problem: the backplotter does not display the toolpath correctly. Instead of showing the wrapped toolpath around the tube near the chuck, it draws a large circle. I believe this happens because the backplotter uses machine zero as the centre of rotation rather than the actual chuck position.

Setup:
- LinuxCNC 2.9.8 stable
- trivkins coordinates=XXYYZB, 6 joints
- qtdragon_hd interface
- GEOMETRY = !-BXYZ

Is there any way to tell the backplotter where the centre of rotation is?

Thanks

I am aware of the ! prefix in GEOMETRY — it is already set in my config (GEOMETRY = !-BXYZ).

However, as the documentation states, this provision applies for liveplots only. I am using qtdragon_hd, not AXIS. Does the ! prefix in GEOMETRY have any effect on the qtdragon_hd backplotter, or is it AXIS-specific only?

A "-" character preceding any letter inverts the direction of the operation.