Bed Leveling With a BLTouch.

I designed and built a custom CoreXY machine and so far everything works great.

The one thing I noticed was that I had to fine tune the bed levelness manually to get the machine to do a bilinear level job without failing. I had to slightly raise the front right corner of the bed to get the machine to probe that corner successfully. When I did that the back right corner failed, but once I raised that slightly it would level the bed successfully.

It's a 12" x 12" fixed bed. I thought I had it nearly level just by eyeing it while moving the nozzle around from corner to corner, but apparently that was good enough. This is the thing though... why is that happening? That bed was nearly level. Auto-leveling should of been able to handle that. It not like the bed was seriously out of whack, which I have seen guys on YouTube auto-level to show how all that works.

Why was I experiencing fails with such small tuning problems? Does fade height have anything to do with it? How does hade height work? it's set to 10mm now. what happens if I decrease that?

Thanks.

Fade height only tells the printer how to gradually decrease its bed leveling data as the layers increase. In other words, once you have a good, even layer then bed leveling is irrelevant. Even though the bed may not be level, the layers are, as far as the nozzle is concerned.

If you have access to fade height, I'm assuming you are using Marlin? Somewhere you have told it that it is not allowed to go below Z=0 (or some other, limited range), so it cannot correct for the slight tilt of your bed as reported by the BL Touch. You fixed this by slightly raising the bed so that the values reported by the BL Touch are within the allowable Z axis range of the printer. That's a valid fix. You would have accomplished the same thing by physically lowering the BL Touch just a tad, but your fix was just as good, equally valid, and accomplishes the same thing.

Also, it is important to understand that mesh bed leveling (i.e. the Bilinear or UBL in Marlin) is not meant to compensate for a tilted bed plane, but rather for any deviations between the actual bed surface and an ideal plane. It is still important to get the bed as level as possible to the nozzle/gantry so that the leveling is only adjusting for differences in the bed surface. Bed leveling can compensate for a tilted bed, but that can end up with parts that are not quite the shape that they're supposed to be - a cube might end up being trapezoid or parallelogram shaped depending on the fade height you set. In general, the less the mesh needs to compensate the better.