I installed a BLTouch on my printer (Wanhao Duplicator i3) with Marlin and enabled AUTO_BED_LEVELING_BILINEAR.

However, the results are disastrous when I print. The first few layers do not adhere to the build plate at all. It is much worse with the probe than when I was leveling by hand.

In my Marlin configuration, I left the probe's Z offset from the nozzle at 0. Then I went through the Z offset calibration process to set Z=0 to barely touching the build plate. Then I ran the bed leveling process.

How can I diagnose what's going wrong?

When I run G30 X100 Y100 to probe the center of the bed, I get a measurement of Z=1.18. I'm assuming that represents the true Z offset between the probe and the nozzle tip?

Here's another thing I tried. I probed with G30 X100 Y100 which output a measurement like this:

Recv: Bed X: 100.00 Y: 100.00 Z: 0.06

I tweaked the Z offset with M851 Z... repeatedly until a probe yielded Z: 0.00 (the offset was -1.07). Then I ran G29 to run bed leveling, which gave me this grid:

Bilinear Leveling Grid:
      0      1      2      3      4      5      6      7      8
 0 -0.403 -0.313 -0.258 -0.210 -0.178 -0.138 -0.128 -0.085 -0.058
 1 -0.285 -0.223 -0.178 -0.145 -0.103 -0.060 -0.050 -0.023 -0.045
 2 -0.178 -0.128 -0.080 -0.083 -0.078 -0.053 -0.045 -0.048 -0.038
 3 -0.103 -0.058 -0.028 -0.030 -0.018 -0.015 -0.018 -0.000 -0.030
 4 -0.020 +0.015 +0.015 +0.002 -0.000 -0.013 -0.035 -0.060 -0.088
 5 +0.030 +0.055 +0.037 +0.020 +0.035 -0.015 -0.023 -0.073 -0.140
 6 +0.060 +0.082 +0.087 +0.020 -0.000 -0.055 -0.103 -0.160 -0.210
 7 +0.097 +0.102 +0.082 +0.055 +0.002 -0.035 -0.105 -0.138 -0.265
 8 +0.145 +0.130 +0.102 +0.077 -0.045 -0.108 -0.183 -0.193 -0.263

You can see at (4,4), the center of the bed is 0.0.

Then I saved the mesh and Z-offset with M500. I confirmed repeatedly that G28 puts the nozzle just touching the build plate and not pushing on it.

This strategy seems to be work alright. I'm not sure if it's the correct way to do it. I'd welcome any feedback. If anything, I think I should probably raise the nozzle a fraction of a millimeter higher off the bed.

  • $\begingroup$ Did or didn't you redefine the trigger to nozzle offset by using M851 Zxxx (where xxx denotes the offset)? $\endgroup$ – 0scar Jun 14 '20 at 22:18
  • $\begingroup$ Well, according to this guide yes I used that command, but computed offset was 0.05. $\endgroup$ – rgov Jun 14 '20 at 22:36
  • $\begingroup$ What is a computed offset? You need to use the one that has been determined using the guide you referenced. It should be a value of about 1 to 2 mm, if it is 0.05, the trigger point is too close to the bed, you should then lower your sensor ( I guess you mean -0.05, because if it is a positive value, the trigger point is at the wrong side, and cannot be reached unless the carriage flexes). $\endgroup$ – 0scar Jun 15 '20 at 6:07
  • $\begingroup$ "I tweaked the Z offset with M851 Z... repeatedly...", you only need to do that once. $\endgroup$ – 0scar Jun 15 '20 at 6:13
  • 1
    $\begingroup$ Your leveling grid tells me: LEvel your bed! All the numbers should be within about 0 +- 0.1 $\endgroup$ – Trish Jun 15 '20 at 6:30

Even with Auto Mesh Bed Leveling, you need to level the bed as good as you can. Your leveling grid show it varies between -0.403 and +0.145, that is about 0.55 mm or about 2-3 layers of filament (0.2 or 0.3 mm per layer). That can't be compensated for reasonably.

  • Re-level your bed so that the corners only have an absolute error of at max 0.1 mm around their average 0 - that's half what I consider the standard layer 1 height of 0.2 mm, regardless of other settings.
  • Make sure the 0 of the extended probe is a sliver under the nozzle, just to prevent piercing your bed on probing
  • G28 should bring the nozzle a hair sliver above the bed - if you move the head in X or Y direction (by hand after disabling the steppers, or move the carriage using the LCD) it should not make contact.
  • $\begingroup$ If the probe is correctly configured it can do even worse oriented beds, but the question is if you want a printed part with a skew underside. But practically, or reasonably as you state it, it is unwanted. $\endgroup$ – 0scar Jun 15 '20 at 9:42
  • $\begingroup$ Thanks. I'm working on this still. I leveled the bed better, however, on printing the nozzle crashed into the bed. Is the right order: disable auto leveling, move nozzle to Z=0, level bed, home all, probe new auto level grid, save? If so, how does it determine when it reaches Z=0, if G28 doesn't deploy the probe? $\endgroup$ – rgov Jun 22 '20 at 0:01

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