First off, kudos to cmidgley for this exceptionally informative post! I ran the tests and found my tolerance differences to be .542 max between the largest and smallest reading. I printed off the shim leveling corner.stl files with a layer height of .05 and added them where needed. Two in the right corners and one in the back left. I then ran multiple 10 tests varying times over several days. My average tolerance is .130 And now after the new firmware release v1.1.9.34 I’m getting readings within .101 and .107 (only ran 2 tests so far with the new firmware).
These are the commands I run:
M420 S0 ; shut off auto-leveling
G28 X Y ; home the X & Y axis
G28 Z ; home the Z axis
G29 V4 ; Perform Auto Leveling Test, verbose output
… didn’t know about having to do the ‘z’ offset afterwards support helped out with that
I don’t understand why you would have to re-adjust your z-offset?
If you placed shims under the supports that hold the glass bed and then secured it back with the washers, you effectively raised the bed and your z-offset the exact same height in parallel, thus which means your z-offset was raised 0 height and remained exactly the same as it was before.
z-offset is measured from the top of the washer to the bed. So the top of the washer is the 0 reference point, move down in a negative direction towards the bed, the height between your bed and the top of the washer is the z-offset.
https://www.lulzbot.com/learn/tutorials/Z-axis-offset
Secondly the screw should not go in crooked or slightly cocked, that would’ve been your fault not screwing it in correctly and started cross threading the screw. I experienced the same problem, screw was going in a bit crookedly, I backed it off, re-adjusted and screwed it back in. It went in straight and flush with the top of the washer.