G29 Auto bed leveling extreme squish

With a newly installed z induction probe, I can use auto-bed leveling (ABL), yay!

BUT, I have an extreme case of z-layer squish because of it. In the first 10mm wihere ABL is compensating, my 10mm hight is coming out as 6.56mm. That is 3.44mm of squish!

Here is a photo

That 3.44mm of lost height is a number suspiciouly close to what is in the bed leveling matrix below.

Loaded /Users/k/Desktop/PI3MK3M_xyzCalibration_cube.gcode, 12834 lines
1.33m of filament used in this print
The print goes:
- from 92.70 mm to 112.30 mm in X and is 19.60 mm wide
- from 107.70 mm to 127.30 mm in Y and is 19.60 mm deep
- from 0.00 mm to 20.00 mm in Z and is 20.00 mm high
Estimated duration: 100 layers, 0:16:02
Print started at: 20:54:48
Selected home phase 896 too close to endstop trigger phase 952. Pick a different phase for 89
Preheating
Wait for bed heating...
Wait for bed heating...
Wait for bed heating...
Bilinear Leveling Grid:
      0      1      2
 0 +3.705 +3.425 +3.560
 1 +3.307 +3.450 +3.557
 2 +3.305 +3.397 +3.425
start

This cube is done with the latest bugfix-2.0.x branch.

Should I be adding the smallest number below to my configuration with a M206 Z-3.305?

And/or should I be adjusting my Z-probe offset height (currenly below)

echo:; Z-Probe Offset (mm):
echo:  M851 X55.00 Y19.00 Z-0.60

In my configuration, related to zprobe offset, I already have

#define NOZZLE_TO_PROBE_OFFSET { 55, 19, -1.11 }

FYI: For this thing to even print like this, I had to reduce the filament flow rate to 50%!
PS: The cube in the photo isn't curved, curves are thanks to my janky-cam macro lenz.

Edited 1 time(s). Last edit at 02/14/2021 08:46AM by NovaHuta.

Ah, so, I'd just tried the above, using M206 command as stated. That didn't solve it. That just made the printer print in the air above the bed by that much (3.305mm).

This is a custom printer build, so, I then guessed that my full build height is off. I'd adjusted

Configuration.h:

#define Z_MAX_POS 255 // was 259

down by 4mm and now I get the following.

>>> g29
SENDING:G29
Preheating hotend (120) and bed (50)
Wait for bed heating...
Bilinear Leveling Grid:
      0      1      2
 0 +0.267 +0.077 +0.217
 1 -0.108 +0.102 +0.237
 2 -0.100 +0.060 +0.132
echo:busy: paused for user

Which is good!

In the end, I guess it was trying to squish a logical 259mm of height into an actual available height of 255mm in the first 10 mm.

Now I get a nice calibration cube measured at x20.04 y20.03 z20.73 with the flow rate at 100%



Note to all: If your bed leveling matrix is not averaging over zero, adjust your total machine z-height!

And yes, I'm gonna go back and adjust that Z_MAX height to remove that 0.73 of extra height!

I'd also just add to this that this likely only applies to machines that home at Z_MAX.