I thought of another approach to this measurement, so this morning I decided to repeat my tests.
I installed the original 1/2" MDF wasteboard on the base plate.
I used a corner probe cycle using the V2 probe, with a top surface 13.1mm above the MDF.
The probe cycle then parked the probe tip at (10, 10, 19), directly above where it just completed the Z part of the probe cycle. Now without changing anything else, I manually jog the probe down in 1mm steps to Z=14, and then in 0.025mm steps to the point where the contact LED turns on.
This SHOULD be at 13.1mm. Instead the contact LED turns on at 13.000mm, off again at 13.050mm. I can cycle between the two heights turning the LED on and off, so there is a little hysteresis there. The problem is that it should be at 13.1mm, not 13mm.
But the really interesting thing is that this time the virtual zero surface is now above the actual surface, not below as I was seeing before. I can check this. I take the probe out of the way and manually jog to (20,20,15) clear of the mounting hole in the SW corner. Then I jog down to (20,20,0).
This should be touching the surface of the MDF, but I can easily slip the 0.05mm paper under the probe tip. I jog down in 0.025mm step until the paper starts to grab at -0.175mm. That says the surface is 0.225mm below zero. This measurement is less accurate than the previous one because I’m looking at a local patch of MDF.
If I do the same thing starting at (30,30,15), the paper touch is at -0.275mm. I go back to (20,20,15) and paper grab is still at -0.175mm, back to (30,30,15), still at -0.275mm. (I’m doing the microjogging blind BTW, using the keyboard with my right hand but looking at the probe and wiggling the paper with my left. I only look at the Z position readout after I’ve decided where the paper is grabbing).
So, the MDF surface height can have at least 0.1mm ripple on it, based on a two spot test. This is not a great surprise, but it means that probing heights straight onto that surface is not going to be super accurate.
The probe flat body should kind of smooth this out, but in a biased way. It doesn’t sit on the average height but rather on the high points of the MDF. The test using the probe LED showed the probe top sitting 0.1mm below the expected height, but the MDF spot checks were 0.175 or 0.275 below the expected position. I believe the probe is sitting on the high spots of the edge of the MDF, where it has slightly swollen due to humidity.
I suspect that my initial results (when I started this thread) were biased the other way because my MDF was more distorted than I thought it was.
Result One: don’t try precision measurements based on MDF
Next I removed the MDF and put the probe directly onto the corner of the base plate and probed the corner. Repeating the height measure of the probe I still see two click (0.05mm) hysteresis centred on 13.00mm (12.975 to 13.025). Doing the paper test at (20,20) it grabs at -0.05mm, so the plate surface is at Z=-0.1mm. This is consistent with the bias on the LED probe top surface.
Now I think that internally, the CM probing process assumes that the V2 probe is 13mm thick, where as it is actually 13.1mm thick. This puts the reference zero surface 0.1mm above the actual surface the probe was sitting on. When we then manually jog to the probe surface we touch it 13.0 mm above the reference surface. Or if we manually jog to the base plate surface we find it 0.1mm below that reference. ie
------ top of probe
------ virtual zero
------ actual base plate surface
I hope this shines some light on the whole Z probing issue that seems to appear in different forms in different threads. I’d be interested to see if others can repeat the V2 probe to base plate measurements and verify Result Two.
The Z-only probing assumes the probe is 15.5 +/- 0.025 high. It’s actually 15.6mm, so there is still a 0.1mm bias using Z only probing.