# Autobed leveling doesn't compensate

I finally manage to setup the ABL on my Prusa. I run G28 to home all axes (for z I use the same probe used for ABL). Then I run G29 E ( I use E because otherwise for some reason I have not understood yet, Z does not lift during probings) probing 4 points on the corners of the bed.

finally I get the map of the bed:

Bed Level Correction Matrix:
+0.999999 +0.000000 +0.001233
-0.000005 +0.999992 +0.003905
-0.001233 -0.003905 +0.999992


and after that, the print starts. The first layer looks perfect however I have not seen Z moving a single step along the printing.

Any hint about what to check? Is the obtained map indicating that the bed is already too leveled to act on any compensation?

UPDATE I printed a 180 mm diamater cylinder and the Z axis is not compensating the 1mm difference from edge to edge of the bed.

• If it ain't broke, why fix it? – Tom van der Zanden Nov 9 '16 at 16:42
• @TomvanderZanden, Lol....are you saying that is too leveled to call for Z compensation? – FeliceM Nov 9 '16 at 19:55

I found the issue. As I mentioned in my question UPDATE, trying to print something that cover the full bed, the ABL was not working. The issue was the Gcode I added after the G29. In fact for some reason I added a G28 X0 after the G29 and that basically cancel the data acquired in the G29. I learned something new, do not use G28 after G29!

Now it is working nicely.

I believe that the matrix is shown transposed from how it should be, but that doesn't affect the answer. The compensated Z position is derived from the original (X,Y,Z) position by multiplying the corresponding vector with that matrix. This means that the new Z position would be

Z' = 0.999992Z - 0.001233X - 0.003905Y


If you have a 200 x 200 printbed, a diagonal move from one corner to the other would correspond to a 1mm change in Z-height. This should be noticeable, but if perhaps you're printing something quite small you might not notice it.

Though, if your first layer is perfect, I would see no reason to mess with it.

• Agree with you about "don't fix what is fine" but if I installed the ABL is to have it working. I tried printing a 180mm diameter cylinder 2 mm thickness. Since there is 1 mm change from corner to corner on Z, the ABL should have compensated. Nothing is happening. I do not see the Z working on the same layer from one edge to the opposite. – FeliceM Nov 10 '16 at 8:32