Z probe offset always prints in the air.

dc42

@andrewe said in Z probe offset always prints in the air.:

G31 P500 X0 Y51 Z-3.71 ; Set Z probe trigger value, offset and trigger height

The Z parameter in the G31 command is the nozzle height at which the Z probe triggers, so for most types of Z probe it should be positive. If you are coming from Marlin, this is the opposite of the sign of the M851 Z parameter in recent versions of Marlin.

Phaedrux

It looks like you aren't using G30 in your homing files to actually probe the bed. You are using a G1 Z move to seek the endstop and then a G92 to set the Z position. You'll need to remove those and replace them with G30 to use the Z probe.

andrewe

Thanks, I'm trying again now. So when I Send G30 S-1 to probe the bed without resetting the Z=0 position where am I taking the positive number from the top right in Machine status or from the gcode console.

Thanks
Andy

Phaedrux

@andrewe The G30 S-1 response will be in the console. Your process for measuring the trigger height is correct, but your homing files aren't making use of the results.

andrewe

I've got this in my home files now.

; homeall.g
; called to home all axes
;
; generated by RepRapFirmware Configuration Tool v2 on Sun Nov 18 2018 15:41:33 GMT+0000 (Greenwich Mean Time)
G91 ; relative positioning
G1 Z5 F6000 S2 ; lift Z relative to current position
G1 S1 X-235 Y-215 F1800 ; move quickly to X and Y axis endstops and stop there (first pass)
G1 X5 Y5 F6000 ; go back a few mm
G1 S1 X-225 Y-215 F360 ; move slowly to X and Y axis endstops once more (second pass)
G1 S1 Z-230 F1800 ; move Z down stopping at the endstop
G90 ; absolute positioning
G30
; Uncomment the following lines to lift Z after probing
;G91 ; relative positioning
;G1 S2 Z0 F100 ; lift Z relative to current position
;G90 ; absolute positioning

; homez.g
; called to home the Z axis
;
; generated by RepRapFirmware Configuration Tool v2 on Sun Nov 18 2018 15:41:33 GMT+0000 (Greenwich Mean Time)
G91 ; relative positioning
G1 Z5 F6000 S2 ; lift Z relative to current position
G1 S1 Z3.57 F1800 ; move Z down until the endstop is triggered
G92 Z3.8 ; set Z position to axis minimum (you may want to adjust this)

; Uncomment the following lines to lift Z after probing
;G91 ; relative positioning
;G1 S2 Z5 F100 ; lift Z relative to current position
;G90 ; absolute positioning

Many thanks Andy

Phaedrux

You should remove this from homeall:
G1 S1 Z-230 F1800 ; move Z down stopping at the endstop

And add an X Y move to position the probe where you want the bed probed before you call G30

And remove this from homez:

G1 S1 Z3.57 F1800 ; move Z down until the endstop is triggered
G92 Z3.8 ; set Z position to axis minimum (you may want to adjust this)

And add the same X Y move and a G30.

andrewe

@phaedrux said in Z probe offset always prints in the air.:

add an X Y move to position the probe where you want the bed probed before you call G30

Thanks for your help I've got a lot further and am actually printing on the bed now I don't understand how to do the X Y move to position the probe where you want the bed probed. If use the baby stepping and added 0.2mm do I need to add or subtract that from the hight I set in the config.g?

I've got soo much to learn about RepRap.

Thanks Andy

Phaedrux

Usually you want to probe the center of the bed. So you would move the print head to an x and y position that places the probe in the center.

You would need to add a line before the G30 that looks like

G90 ; absolute positioning
G1 X150 Y150 ; move probe to center
G30 ; probe the bed
G1 X0 Y0 ; move back to home position

You'll have to adjust the x and y values to get your probe to the center and whatever your home position is.

Phaedrux

I'm not exactly sure what you are asking about baby stepping. Do you mean to say that you still need to make a baby stepping adjustment after measuring the trigger height?

If your measured value is 2mm, and you need to lower the nozzle by -0.2 then you would subtract that from your 2mm in config.g if you wanted to make that a permanent change. Is that what you mean?

andrewe

Yes, I was 0.2mm too close to the bed so used the baby stepping to raise the nozzle.

Phaedrux

@andrewe Before you change the trigger height, check to make sure that your e steps are calibrated. Over extrusion on the first layer can be mistaken for printing too close. The trigger height should be accurate, especially if you've measured it several times while the bed and nozzle are heated up and the result is consistent.

Phaedrux

It's also possible that your bed isn't perfectly flat (few are) and so Z=0 is only really true at the position where the bed was probed. You would have to use mesh grid compensation to probe many points across the bed to create a heightmap, which is then compensated for in real time by adjusting the z height to keep the nozzle at a consistent distance from the bed.

https://duet3d.dozuki.com/Wiki/Using_mesh_bed_compensation

andrewe

To be fair the bed is pretty shabby It was an Anet A8 and came bent from new. The bed, 1x end stop and the motors are all that's left now. I have a feeling its too out of true even for mesh compensation but I can see that working watching the z screws constantly moving.

I use a Prusa spring steel sheet with PEI on it, all held in place with magnets and bulldog clips.

I've got a new bed on its way in the post next week but I know how to set the probe up now thanks to yourselves. I think I'll look at different probes and try and get it as close to the nozzle as I can.

Thanks for all your help
All the best Andy