does your first layer hight grow over time? it’s like the bed is ‘settling’, moving further away from the head. that doesn’t seem normal?
Its thermal expansion. Let the bed sit for about 15 minutes at temp before starting the print.
Even with auto-leveling, the bed may still have some distortion. To say it another way, software can largely compensate for, but not correct geometric imperfection. My MK3 has some distortion in the platform due to a slight warp in the Y carriage. Not enough to merit correction, unless I want a perfect first layer appearance covering the entire bed.
My Z axis was not perpendicular to the Y axis by a bit more than 0.5 mm (at a height of 200 mm), so I printed new top brackets for the Z axis to correct the geometry. Not that I anticipate needing better perpendicularity than that. I was just thinking about how cool the geometry compensation of the MK3 is, and realized there are some aspects of the machine it can’t address. So I looked into it, and found I could make the machine a little better with just a little effort. Now, if I ever print parts 200 mm tall that need to be perpendicular to their base within about 0.15, then I can do that. Knowing me, I’d probably want to do it in ABS…
None of this should be seen as a shortcoming of the printers, of course. Geometric “perfection” is not even slightly practical at the price point, which is why the software compensation Prusa has developed is so cool. The best way to make the situation better would be firmware parameters allowing software compensation for all the perpendicularity and parallelism errors (I don’t know if ATmega is fast enough). Unfortunately, a print bed that isn’t perfectly flat can’t be fully compensated for.
I have the opposite effect, the bed seems to rise over time. It is due to one Z axis getting slightly out of alignment. It could as well go the other way. If that’s the cause, the bed will be at different levels on the two sides, the middle in between.
I wonder if a recalibration would sure everything up…
Hmm, the recalibration would help if something disturbed your X/Y axis alignment, but not this.
Also if you change your bed temp during a print especially if it’s a aluminium base