Meaning of UBL mesh values?

What is the exact meaning of the mesh values in UBL? I know they are differences from Z=0 for each point of the bed, but what is the sign? Does a positive value for a mesh point mean that point is closer or further away from the nozzle than 0? I looked at the documentation for UBL and couldn't find any place where it specified how to adjust a mesh point - i.e. if G26 shows that the nozzle is too close to the bed at a mesh point, should I decrease or increase the mesh value for that point?

The mesh point values are what the nozzle has to do to track the bed. If a value is .1mm, then the nozzle needs to be raised by .1mm at that location to track the bed.

You can get some insight by doing:

• G28
• G29 P0
• G29 Q1
• G29 T
• G1 X0 Y100
• G1 Y180

Got it. If the nozzle is too close to the bed, I should increase the value of the mesh point, and if the nozzle is too far away I should decrease the value. It might make sense to add a comment to that respect to the "Fine-tuning of the matrix" section of the documentation. Something like this, after the "Use the encoder wheel to change the value" sentence:

"If the test pattern shows that the nozzle is too close to the bed (the line looks squished) increase the value of the mesh point. If the pattern shows that the nozzle is too far away from the bed, decrease the value."

I think I'll make the doc change and create a PR for this. Thanks for the info!

Quote
MMcLure
Got it. If the nozzle is too close to the bed, I should increase the value of the mesh point, and if the nozzle is too far away I should decrease the value.
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I think I'll make the doc change and create a PR for this. Thanks for the info!

Yes... The mesh point value is what is necessary to get the nozzle in the 'correct' location. A positive value lifts it up. A negative value lowers it.
And... User contributions to Marlin are very welcome! Some of us can code... But we don't do very well writing documentation.

And the prize for biggest idiot goes to... this guy!!! I just spent all day trying to adjust my matrix, and after several hours (!!) figured out that even after writing the documentation for this, I was adjusting my points in the wrong direction!!! No wonder the mesh was getting worse and worse!!!