# Offset understanding

• Hi again,

the piezo is working in general.

But after homing Z, and going to Z0 I still got a gap between nozzle and bed.
Should there be no gap when I'm at Z0?

Is the gap defined by G31 Z-0.14?

I thought G31 defines, that the trigger point is higher than the real touch point, that the nozzle does not bang the bed.

My config.g:

;Piezo
M558 P8 I1 R0.4 F300 Z1
; Define Mesh Grid for G29
; M557 X5:300 Y5:300 S40 ; Define mesh grid
M557 X10:340 Y10:340 S110
G31 X0 Y0 Z-0.14 P100

My homez.g
; Homez.g
;G30 ; Do a single probe to home our Z axis
;G90 ; Make sure we are in absolute mode
;G1 Z10 F6000 ; Rapidly move the Z axis to Z=10.
M564 H0 ; allow move without homing
G91 ; relative mode
G1 Z4 F400 ; lowers bed 4mm to avoid dragging nozzle over the bed
G90 ; back to absolute mode
G1 X200 Y200 F4000 ; Move probe to middle of bed
G30 ; probe
G91 ; relative mode
G1 Z4 F150 ; move slowly 4mm in the +Z direction
G90 ; back to absolute mode
G30 ; probe
G1 Z2.5 F150 ; Move Z to 2.5 above the bed
G90 ;back to absolute
; G1 X0 F4000 ;move to X0
M564 H1 ; allow move without homing

• I did it and the result is Z-0,14

but what does G31?

• If you do that test several times is the result consistently -0.14?

G31 is the probe offset in XYZ

• You would help me more if you explain what the offset really mean, what it does. I read it in the wiki, but I dont understand.

Is my question so unclear?

• G31 tells the system where the probe is in relation to the nozzle tip.
So in your case the nozzle tip is the probe so x and y are 0. But because the nozzle tip presses into the bed slightly before the probe registers the z value is -0.14. meaning the probe is slightly below the surface of the bed when it triggers.

• That brings me to 2 questions:

1. My sensetive is so high, when I watch while G30, the nozzle tips just a very little bit the bed.
2. Is my offset is right, the should be no gap between nozzle and bed?, right?

I did it now 3 times in a row in the center of the bed:

-0,04 and -0,04 and 0,013.

Why does it differs so much from my first value -0,14, which was in one corner of the bed?

• I'm not very familiar with tuning the precision piezo, but I suggest you follow the documentation as closely as possible.

To get a good average value for G31 Z you should measure it with G30 S-1 ten times and take the average. Center of the bed would be the best place to probe. It would also be a good ideal to try and mechanically level your bed as good as you can first. It would also help to use a hot nozzle and hot bed.

• My piezo doesn't work reliable in some way.

I will open another thread for it, it is annoying....

• How do you do the Z-offset calibration?

With a piece of paper between nozzle and bed or nozzle and bed direct in contact.

My thought is, to have the nozzle really have on the bed with Z0 and then put the needed (normally paper) gap in the G92 command to have it perfect.

Which is the best way?

• I touch the nozzle to the bed and don't use paper at all. I don't use a G92 either. That's really what a z probe gains for you. You don't have to fiddle with paper or other tricks anymore.

Z0 is touching the bed and the first layer height is exactly what the slicer asks for.

• I got a JGAuroa A5 (no autobed level, prusa design) beside my HEVO (piezo).

On the A5 I do a mechanical bed level with 0,15mm metall sheet under the nozzle between the bed. Exactly shown here https://www.youtube.com/watch?v=7G2OftpUxow&t=32s

If I bring the nozzle to the bed with no gap, no filament comes out.

• @hevilp the gap should be from your first layer height, no?

• Yep,

If I level my bed with the 0,15 mm metall sheet, my first nozzle height, if I print 0,2mm should be 0,35mm or?
0,15mm is the distance I gave with the metall sheet, where the printer thinks it is Z0
and
0,2mm for the layer height.
It is working fine. In the link is the code of the lcd where the leveling is coded.

I wonder where the gap of 0,15mm would be with my HEVO?

https://www.dropbox.com/s/ccreium22rki3cq/JGAurora A5 Custom LCD firmware - 20180111.zip?dl=0&file_subpath=%2FJGAurora+A5+Custom+LCD+firmware+-+20180111%2Fmks_config.txt

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