UBL probes off the edge of the bed?

I've just fitted a capacitive sensor to my i3-style printer, and the sensor is working OK to home Z. However, when I try to use the UBL wizard to probe a mesh, it tries to probe off the +X and -Y edges.

The printer has a RAMPS 1.4 running Marlin 2.0.8.1. The sensor is a generic LJC18A3-H-Z/BX.

So far, I've set X_MIN_POS and Y_MIN_POS to reflect my home offset (previously I used M206, which is now set to X0 Y0).

G0 X0 Y0 goes to the corner of the bed correctly. The X_BED_SIZE and Y_BED_SIZE are correct (220x220). The printer was previously working fine for some years with a microswitch Z limit switch, so the basic
dimensional stuff is OK.

Other relevant settings:
#define X_MIN_POS -40
#define Y_MIN_POS -10

#define FIX_MOUNTED_PROBE
#define NOZZLE_TO_PROBE_OFFSET { 56, -34, 0 }
#define PROBING_MARGIN 20

#define AUTO_BED_LEVELING_UBL
#define GRID_MAX_POINTS_X 5
#define UBL_HILBERT_CURVE
#define MESH_INSET 40

And somehow it still wants to leave the bed by at least 25mm. Is there some other setting I need?

Edited 2 time(s). Last edit at 06/27/2021 07:08AM by AnotherHowie.

Make sure that the probe offset is actually being used. If you send 'M851' it should show what probe offsets are stored in EEPROM. If they are not correct, then you need to use 'M502' and 'M500' to reinitialize your EEPROM with the values stored in the firmware.
Otherwise everything looks fine with your configuration.

Hmm, no, M851 shows the correct offset. I guess I'll try the absolute latest release, although I can't see anything obvious in the changelog related to this.

Are you actually doing an explicit "M206 X0 Y0"? I wonder if that's affecting X_MIN_POS/Y_MIN_POS. One thing to experiment with is to compile with NO_WORKSPACE_OFFSETS enabled (which completely takes M206 out of the picture). Though as far as I can tell "M206 X0 Y0" should have the same effect as not setting it at all. I'm honestly stumped. You seem to have your configuration right.

So I don't know if it was an actual code change, or just me altering configuration.h from scratch again, but with 2.0.9.1 I have working mesh probing.

I still had to set MESH_INSET up to 25mm to stop the probe going off the edge, so some offset is still not quite right... once I figure out the Z probe offset properly, I'll find out if it's off enough to make a difference, I guess!

EDIT: and it turns out that 3 Point Levelling (G29 J) that is recommended before a print, ignores MESH_INSET and tries to probe off edges.

Edited 1 time(s). Last edit at 07/03/2021 03:07PM by AnotherHowie.