Significant z Offset of bed mesh

Tryptamine

I am using a BLTouch to probe the bed.

How can this behavior occur?
In my thinking, the nozzle offset should not change the z-offset of the height map.
The offset is calculated by every point as the distance to z=0 (from homing)

homeall.g

; homeall.g
; called to home all axes
;
; generated by RepRapFirmware Configuration Tool v3.3.12 on Tue Aug 09 2022 15:55:52 GMT+0200 (Mitteleuropäische Sommerzeit)
M402


G91                      ; relative positioning
G1 H2 Z3 F18000          ; lift Z relative to current position
G1 H1 X-350 Y-310 F12000 ; move quickly to X or Y endstop and stop there (first pass)
G1 H1 X-350              ; home X axis
G1 H1 Y-310              ; home Y axis
G1 X5 Y5 F18000          ; go back a few mm
G1 H1 X-350 F1800        ; move slowly to X axis endstop once more (second pass)
G1 H1 Y-310              ; then move slowly to Y axis endstop
G90                      ; absolute positioning
G1 X162 Y152  F18000        ; go to first bed probe point and home Z
G30                      ; home Z by probing the bed

; Uncomment the following lines to lift Z after probing
;G91                     ; relative positioning
;G1 Z3 F300              ; lift Z relative to current position
;G90                     ; absolute positioning

config.g

; Configuration file for Duet WiFi (firmware version 3.3)
; executed by the firmware on start-up
;
; generated by RepRapFirmware Configuration Tool v3.3.12 on Tue Aug 09 2022 15:55:52 GMT+0200 (Mitteleuropäische Sommerzeit)

; General preferences
M575 P1 S1 B57600                                  ; enable support for PanelDue
G90                                                ; send absolute coordinates...
M83                                                ; ...but relative extruder moves
M550 P"HCEvo"                                      ; set printer name
M669 K1                                            ; select CoreXY mode


; Drives
M569 P0 S0                                         ; physical drive 0 goes forwards
M569 P1 S0                                         ; physical drive 1 goes forwards
M569 P2 S1                                         ; physical drive 2 goes forwards
M569 P3 S0                                         ; physical drive 3 goes forwards
M584 X0 Y1 Z2:4 E3                                   ; set drive mapping
M350 X16 Y16 Z16 E16 I1                            ; configure microstepping with interpolation
M92 X160.00 Y160.00 Z800.00 E420.00                ; set steps per mm
M566 X900.00 Y900.00 Z60.00 E1200.00               ; set maximum instantaneous speed changes (mm/min)
M203 X18000.00 Y18000.00 Z1800.00 E8000.00          ; set maximum speeds (mm/min)
M201 X5000.00 Y5000.00 Z20.00 E4000.00             ; set accelerations (mm/s^2)
M906 X1000 Y1000 Z800 E1000 I30                    ; set motor currents (mA) and motor idle factor in per cent
M84 S30                                            ; Set idle timeout

; Axis Limits
M208 X0 Y0 Z0 S1                                   ; set axis minima
M208 X345 Y305 Z290 S0                             ; set axis maxima

; Endstops
M574 X1 S1 P"xstop"                                ; configure switch-type (e.g. microswitch) endstop for low end on X via pin xstop
M574 Y1 S1 P"ystop"                                ; configure switch-type (e.g. microswitch) endstop for low end on Y via pin ystop
M574 Z1 S2                                         ; configure Z-probe endstop for low end on Z

; Z-Probe
M950 S0 C"exp.heater3"                             ; create servo pin 0 for BLTouch
M558 P9 C"^zprobe.in" H3 F180 T18000               ; set Z probe type to bltouch and the dive height + speeds
G31 P500 X28 Y9 Z0.5                                 ; set Z probe trigger value, offset and trigger height
M557 X28:328 Y9:309 P2                           ; define mesh grid

; Heaters
M308 S0 P"e1temp" Y"thermistor" T105526 B4138      ; configure sensor 0 as thermistor on pin e1temp (bedtemp BROKEN)
M950 H0 C"bedheat" T0                              ; create bed heater output on bedheat and map it to sensor 0
M140 H0                                            ; map heated bed to heater 0
M143 H0 S120                                       ; set temperature limit for heater 0 to 120C

M308 S1 P"e0temp" Y"thermistor" T103901 B4138      ; configure sensor 1 as thermistor on pin e0temp
M950 H1 C"e0heat" T1                               ; create nozzle heater output on e0heat and map it to sensor 1
M143 H1 S280                                       ; set temperature limit for heater 1 to 280C
M307 H1 R10.976 K0.652:0.000 D2.65 E1.35 S0.30 B0 V24.0

; Fans
M950 F0 C"fan0" Q500                               ; create fan 0 on pin fan0 and set its frequency
M106 P0 S0 H-1                                     ; set fan 0 value. Thermostatic control is turned off
M950 F1 C"fan1" Q500                               ; create fan 1 on pin fan1 and set its frequency
M106 P1 S1 H1 T120                                 ; set fan 1 value. Thermostatic control is turned on

; Tools
M563 P0 D0 H1 F0                                   ; define tool 0
G10 P0 X0 Y0 Z0                                    ; set tool 0 axis offsets
G10 P0 R0 S0                                       ; set initial tool 0 active and standby temperatures to 0C

; Custom settings are not defined

; Miscellaneous
M911 S10 R11 P"M913 X0 Y0 G91 M83 G1 Z3 E-5 F1000" ; set voltage thresholds and actions to run on power loss

M572 D0 S0.38 ;0.64 fuer 0.4mm Nozzle

jay_s_uk

@Tryptamine your 0.5 z offset for the bltouch goes seems odd as the bltouch docs recommend an offset of 2mm.
Whats your process to create the heightmap? Home and then G29? or is this part of loading the heightmap that you're seeing the issue?

Tryptamine

@jay_s_uk I know my offset is too little. I will redesign a new carriage anyways once my Flex3D extruder arrives.

My process is homing, then G29.

jay_s_uk

@Tryptamine you don't have a mesh.g file do you?

Tryptamine

@jay_s_uk no.

I also just noticed that when my nozzle is touching the bed at z=0, once i turn on Mesh bed compensation and load the heightmap (G29 S1) my z Position jumps to 0.6, leading 0 to be inside the buildplate.

Phaedrux

How are you measuring the Z probe trigger height?

Your mesh also only has 4 points, so it could be hiding substantial warping since the Z0 point is set at the center of the bed, but you're only probing the corners. Try creating a more detailed mesh. Maybe 4x4

Tryptamine

@Phaedrux said in Significant z Offset of bed mesh:

How are you measuring the Z probe trigger height?

Your mesh also only has 4 points, so it could be hiding substantial warping since the Z0 point is set at the center of the bed, but you're only probing the corners. Try creating a more detailed mesh. Maybe 4x4

I measure it by homing (G28) which probes the middle of the bed once, then I drive the z axis u til the nozzle almost touches the bed.

The story is the same with 3x3 which uses the middle of the bed.
The reason why I only use 2x2 grid is complicated:
(For cable management I have a steel flex tube as for hot water, inside i am running the cables. This pushes back on my carriage when the printer is at Y-max and X1/2. (back middle) so my carriage bends forward, causing the sensor to register the bed closer than it actually is. Took me much too long to find out, even got another plain milled aluminum plate. But the plate is perfectly flat...)

So everything I do now is just a temporary fix until I get my Flex3Drive extruder and redesign everything, but with the exact same setup it was working already.

And I do not want to not be able to print until then.

Phaedrux

@Tryptamine said in Significant z Offset of bed mesh:

I measure it by homing (G28) which probes the middle of the bed once, then I drive the z axis u til the nozzle almost touches the bed.

You should be using this procedure to set your G31 Z value.

https://docs.duet3d.com/en/User_manual/Connecting_hardware/Z_probe_testing