I’ve found the same with about 50% of G32 and G30 S-1 commands. Been doing a lot of bed levelling lately.

DWC 2 and the latest firmware.

@dc42:

From your description, I think you are probably homing Z using the Z probe to probe with the head at point (15, 15). it that right?

Yes that is correct

@dc42:

When you say "The home position and the first probe point are at the same location so I would have expected the heightmap.csv to show 0 for the first point.", have you allowed for the fact that when probing, the firmware allows for the declared X and Y offsets of the probe? The first G29 probe point will put the inductive sensor at (15, 15) which means putting the head at (-20, 15). Whereas if your homeall.g files puts the head at (15, 15) and then executes G30, it will probe where it is, which would have the inductive sensor at (15, 50).

My homez.g and homeall.g have the following for the position

[[language]]
G1 X-20 Y15 F6000      ; go to first bed probe point and home Z
G30 
```From my understanding this should, allowing for the probe offset from the nozzle have the probe using 15,15 the same as the start position for the mesh.

My reasoning for homing at the corner was that then I could compare the z position from homing with the position at the start of mesh generation. I had thought that they would be the same.

@dc42:

> When using an inductive sensor to do Z homing, I recommend you probe near the centre of the bed. If the main problem is that the bed isn't flat, probe with the sensor at bed centre. If the main problem is that the gantry sags when the head is near the middle, probe with the nozzle at bed centre.

I'll try using the centre of the bed tomorrow. I think it's the bed that's off as the x gantry is 2020 profile with a 12mm linear rail bolted to the top.

Anyway thanks for your help.

I've just finished rebuilding an old i3 style cartesian printer with a new aluminium extrusion frame, Bondtech extruder bowden setup and a Duet Wifi controller running version 1.20 of the firmware and web control.

I'm having issues getting mesh bed levelling to work repeatably which may just be a mis understanding on my part.

I'm using an inductive z-probe onto a 3mm aluminium bed levelled with 4 screws. I've levelled it manually as best I can.

I've calculated the z-offset using the paper method and get an 1.54mm which I've put in my config.g file.

[[language]]
G31 P500 X35 Y0 Z1.54 

When I home all axes Z is set to 1.54 as expected.
Running a single layer print with this setting allows me to use microstepping to finely tune the offset to get a good layer (-0.03mm). I update config.g with this new offset of 1.57.

[[language]]
G31 P500 X35 Y0 Z1.57

```Printing larger single layer test show that the bed is not flat across the whole area so I tried mesh levelling using the following procedure.

[[language]]
G31 P500 X35 Y0 Z1.57
M557 X15:195 Y15:195 S20

Home all (Z = 1.57)
Run Mesh Grid Compensation.

Visulaization shows a slightly lumpy bed but nothing too far out.

I tried printing the single layer test again and it ground the head into the bed.

Checking the homing again it shows a Z offset of 1.80 rather than 1.57 so as expected the head drops 1.6 mm before extruding the test layer (1.80 - 0.2 layer height) and hits the bed which is at 1.57.

Turning off mesh compensation and it homes again at 1.57

The home position and the first probe point are at the same location so I would have expected the heightmap.csv to show 0 for the first point.

[[language]]
RepRapFirmware height map file v2 generated at 2018-02-19 12:33, mean error -0.303, deviation 0.108
xmin,xmax,ymin,ymax,radius,xspacing,yspacing,xnum,ynum
15.00,195.00,15.00,195.00,-1.00,20.00,20.00,10,10
-0.065, -0.083, -0.095, -0.110, -0.115, -0.113, -0.090, -0.075, -0.055, -0.013
-0.093, -0.120, -0.158, -0.175, -0.200, -0.213, -0.203, -0.198, -0.180, -0.150
-0.133, -0.178, -0.223, -0.248, -0.268, -0.283, -0.285, -0.280, -0.265, -0.243
-0.225, -0.255, -0.298, -0.310, -0.323, -0.345, -0.333, -0.323, -0.300, -0.270
-0.253, -0.288, -0.317, -0.345, -0.368, -0.380, -0.378, -0.373, -0.360, -0.325
-0.305, -0.340, -0.378, -0.393, -0.415, -0.410, -0.405, -0.405, -0.375, -0.343
-0.315, -0.353, -0.383, -0.405, -0.425, -0.433, -0.420, -0.423, -0.383, -0.370
-0.333, -0.368, -0.393, -0.415, -0.438, -0.442, -0.433, -0.425, -0.403, -0.375
-0.315, -0.348, -0.385, -0.398, -0.420, -0.428, -0.420, -0.413, -0.398, -0.375
-0.270, -0.303, -0.335, -0.355, -0.368, -0.370, -0.370, -0.368, -0.348, -0.325

Could anybody give me some pointers to what's going on.
For the moment I've abandoned using mesh levelling and just live with the imperfect first layer but I'd like to get it working.

Thanks

Richard