Mesh Bed Levelling error or bent sheet?
I have encountered a strange anomaly. Although the Mesh Bed Levelling (MBL) tells me my flexible steel sheet is perfectly straight, the test prints tells me that the left hand side of the steel sheet is dropping off.
On my MK3S I align the smooth sheet using the Octoprint Prusa leveling feature. I have modified the bed using the nylock nut method, so I can easily adjust the height of the bed by turning each screw a few degrees. After preheating for 10 minutes I can reach a stable situation with a max variance of 0,03 mm or better.
I can also print a live-z square on the center of the sheet with a perfect layer and adhesion.
But if I then print a test pattern with 3x3 squares close to the edges, the 3 left-most squares are far off, the rear left square almost printed in the air and with practically no adhesion, like if the bed actually is bent down.
Of course the bed or sheet could be bent, but how come the MBL then tells me it is perfectly straight?
( I understand that the firmware in some way compensates for bed variances, but how this affects this situation is unclear to me)
If I compensate for the error, by actually lifting the bed on the left hand side - and getting a MBL readout that tells me that the left hand side is too high, and the three left hand screws should be turned 90 degrees down - my test print comes out much better in all squares.
By “lifting” the left hand side I have a workaround that allows me to print large models, but I am really curious as to what causes this contradiction between theoretical straightness (MBL) and real life straightness (Test print).
I mostly print smaller footprint models in the center of the sheet and seldom have problems on first layer look or adhesion, but I have occasionally seen adhesion problems on large models, and as I recall it mostly on the left hand side. I have always considered this a warp issue and counteracted by mouse ears or brims, but I realise now that it may well have been caused by this issue.
Anyone having a similar experience, or an explanation?
Thanks,
Ole
RE: Mesh Bed Levelling error or bent sheet?
The steel bed has tolerances as well.
The sensor is registering the steel bed itself. But it is covered by a layer of glue ... and the PEI sheet itself. Depending on the tolerances your sheet has, it might result in some significant differences. The bed leveling mainly covers an angle of the bed, but not so much warping (at least as far as I understand since I had similar issues). That might explain, why you have better results, wenn you actually "bend" the sheet upwards.
In case you use a different sheet (or just the other side) ... is the effect still the same? Personally I made the experience, that power coated sheets work better. You have to go through some pain until the coated sheets get a good adhesion, but after a few ten prints they stick quite nice and also seem to have better tolerances.
- Made by me -
RE: Mesh Bed Levelling error or bent sheet?
Thank you for your answer and sorry for my late reply.
I realize that there are even more parameters in the process, and I just need to go deeper in my testing to get a better understanding.
I have also experienced that the countermeasure mentioned above ( counter-bending the sheet) was a limited success since the automatic bed mesh levelling process registers this unevenness and counteracts it to some extent - so I couldn’t cheat the system.
When I find the time - and this may not be soon - I will do some systematic testing to better understand the bed levelling and all the parameters
Ole