Hi.
I have always had repeatability issues when probing using BL touch (and also with a clone) on my Core XY, to the extent I abandoned ABL and distortion correction. Some recent changes to what I print made it worthwhile for me to look at it again.
After much frustration, I went back to basics, checked all the mechanics and fasteners, removed the spring-loaded bed, and applied a dial indicator to the Z axis carriage. After a few small tweaks, eventually I had a Z axis that I was certain was accurate to better than .01 mm in 25 mm, repeated to much better than .01 mm over 100 cycles, had undetectable backlash, and with a home setting repeatability to .01 mm or better.
Despite this, the probe still would not repeat. Over a series of 30 G30 commands, with 3 probes per G30, at the same machine position, the probe measurement drifted down by 0.116 mm, which gives about .00128 per probe or about .004 per G30. A dial indicator on the Z carriage confirmed that the carriage had moved up this same amount. On re-homing the Z axis, the Z carriage position reverted to that before the series of G30 commands. This seems to say the Z axis position is actually changing, rather than any defect in the probe.
Note: I'm not saying the probe has no errors. The variation between any two G30 measurements can be +.005 to -.010, but the average change is -.004. Suggests the G30 operation itself is accurate to about .007 mm?
As a further test, I programmed a series of 50 Z axis move cycles emulating the probing motion (e.g., Z at 10 mm, G0 Z1 F120, then G0 Z10 F600). The Z carriage finished in the same place it started.
I tried several probe speeds and Z axis acceleration values, with no significant differences.
At this point, I realized the Z axis resolution is 798 steps per mm, or about .00125 mm per step, which is very close to the observed Z change at every probe cycle; probably not just a coincidence. It doesn't feel like a simple rounding artifact, or some timing issue, as that would likely show up as randomness rather than a steady trend. So I suppose I'm suggesting there is a bug that somehow loses (as in miscalculates) one -Z step every time the probe is triggered.
I'd like your comments, please, and any suggestions about how to deal with the issue, or maybe definite identification of the cause. As you will realize, it takes 75 probes to map a 5x5 grid for distortion correction, which means the last correction will be in error by about 0.1 mm, even if the probe starts off perfectly calibrated. This makes the distortion correction unusable. This, in turn, is making my distorted bed unusable for any print that uses a lot of the bed area.
Thanks!
