DLP Z axis/uneven part height issue

[Geometrica432]

Good Afternoon all.
I hope all is well, wherever you are in the world.

I have an Anycubic M3 Max at my job, which actually printed "ok" when we first bought it and set it up.
At home, I have a Photon Mono X, which I have had a lot of success with, and barley any fails, furthering my worry for the M3 Max.
After a lot of reading, I have made some improvements and now want to properly calibrate the machine for dimensional perfection, and have been struggling excessively trying to get a basic calibration frame printed!!!!!!

I don't know how to upload a picture on this? so I hope this makes sense.

I have printed a 280mm x 150mm frame, with a 2mm wall thickness, and 6mm high. (design size)
Resin - 3D Materials Superfast Grey. Heated chamber at 28DegC
Printed size - 278.4mm x 149.2mm (adding 0.58% and 0.54% in lychee slicer)
Wall thickness averages 2.03 - 2.2mm.
The height at corners... Back left=5.62mm, Front left=5.46mm, Back right=6.31mm, Front right=6.18mm

I have no idea why the part height is so bad.
I am more than confident I have levelled the bed, and set the home location correctly.
I checked for build plate warp last month with a flat steel ruler, it was fine.
Any advice for what to inspect next, or how to solve this would be greatly appreciated.

Thank you
Geo

[Geometrica432]

I believe I have found the issue!!
When you calibrate the machine, you use the supplied piece of paper as a "spacer" - which acts as the FEP film at the bottom of the resin tank.
For some reason... My FEP sheet sits roughly 0.2-0.3mm higher than the piece of paper/screen!!.

I put the empty tank back on the printer, screwed in space, then lowered the build plate to home.
I noticed (only very, VERY slightly)… the build plate was hitting the FEP sheet. (Spotted by a reflection moving on sheet)
I took the build plate off and pressed the tanks sheet with my finger. I could feel slight movement.

So to adjust - I put the build plate back on, and returned the plate to the "paper calibrated" home position.
Raised the build plate by 0.1mm at a time, until the reflection stopped changing/moving.
Then set this as the home location....

This gave me 3 good quality, successful prints in a row, without the random build plate detachments.
(about 95% better... some minor support detachments, however could be settings related instead of plate position)

I assume we bought one of the poorly manufactured M3 Maxes!
But hey ho.. it is much better now, and I should be able to get by with lower quality work than expected.

I do not recommend the Anycubic M3 Max based on this issue.
Including other problems I have read across other forums with this particular model.

Geo.

[CrazyIvan]

Would this thread be best moved to a specific sub-forum for Anycubic SLA printers?