Nozzle-to-PINDA Z-Distance Calibration  

Nozzle-to-PINDA Z-Distance Calibration

After almost a thousand hours of reliable printing, I started getting a poor first layer, pretty much suddenly. Running first layer calibration again cause me to lower the Z re-calibrated the first layer height and wound up lowering the Z-calibration from -0.725 mm to -0.850 mm, then to -0.925 mm, then to -0.950 mm over several prints. In total, I moved the extruder a total of 225 microns lower / closer to the bed.

Not sure why I had to do this, but I suspected that somehow the Z-Distance between the Nozzle and the PINDA had changed.

I had initially measured the Nozzle-to-PINDA Z-Distance to be 0.82 mm when the Mk3 arrived on 4/2/2018. This was done with paper feeler gauges made of 24lb Staples printer paper. On-line sources ( https://printing.ucr.edu/paper_thickness.html ) quoted 24lb / 60lb offset text (uncoated) paper and 80lb gloss text coated paper at 0.004″ = 4 mil = 0.01016 cm = 0.1016 mm per sheet. I separately measured a stack of sheets with a micrometer and came up with 0.1023 mm per sheet of 24lb paper, so it seemed like a reliable thickness.

However, using paper feeler gauges to measure nozzle height seemed error-prone, so I started using a digital height gauge – specifically, an iGaging Digital Electronic Height Gauge with Magnetic Base, 6 Inch.

This seems to be an ideal tool for measuring Nozzle-to-PINDA Z-Distance, with some caveats:

Note that I am using the top of the horizontal measuring arm of the height gauge rather than the typical use of the bottom of the arm.
 
Here is a close-up of the horizontal arm raised to the level of the PINDA probe:

Re-measuring the Nozzle-to-PINDA Z-Distance yesterday gave 0.70 mm – a difference of 120 microns. This would account for the need to lower the nozzle by 120 microns, where I actually needed an adjustment of 225 microns.

How / why the PINDA got lower (or the nozzle got higher) is a mystery.