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Mendel90 - 3 point leveling system?

Posted by Arakon 
Mendel90 - 3 point leveling system?
August 09, 2020 01:21PM
I may need a new heatbed, as mine is so badly warped in the center, that I can print well in the center OR at the edges, but never the same (using a steel ruler, there's a noticable 0.3-0.5 mm dip in the center.. and that's with a 2mm glass plate and another 2mm build plate on top of that).
While replacing the bed, has anyone changed to a 3 point leveling system and could point me to some instructions/part lists? I noticed i.e. the MK3 heating beds have a center hole already for 3 point leveling.
Re: Mendel90 - 3 point leveling system?
August 09, 2020 06:28PM
Not sure I understand. If the PCB had a 0.5mm dip in the middle why wouldn't the glass still be flat with a gap underneath it? The glass should be stiff enough to remain flat when it is only fastened at the edges.

Re: Mendel90 - 3 point leveling system?
August 09, 2020 06:33PM
That's what I thought too.. maybe because it's still the original glass plate from 6 years ago and only 2mm thick? When I put the ruler across the plate, there's a clear gap in the center.. and printing/leveling also reflects that. Nothing will stick to the center if the corners are leveled properly, but anything printed outside of the center is fine. If I adjust it to be correct around the center, the nozzle is scraping on the build plate around the corners.
Re: Mendel90 - 3 point leveling system?
August 10, 2020 04:50PM
So what happens if you turn the glass upside down?

If you remove the bulldogs clips is is still bent?

I have never known glass to be permanently bent, although it can be bent slightly with enough force, it always springs flat. My last sheet was probably 5 years old but I recently broke it and replaced it. It was still flat before it broke.

Are you measuring it when cold or hot? Hot sheets can curve when one side is hotter than the other.

Re: Mendel90 - 3 point leveling system?
August 10, 2020 05:34PM
The glass by itself is flat. The heatbed is not and is pulling it out of shape.
I checked both with the bed cold and at 60°, same bend in the center.
With the clips removed, the bend is even more pronounced.
Worst part is, it loses leveling over night. Last night, I printed a case and it was beautiful, perfect skirt and first layer, all layers super smooth. Printed the same case with a small adjustment to a hole position this morning, skirt is squished to hell, nozzle is suddenly too close.

Edited 1 time(s). Last edit at 08/11/2020 02:38AM by Arakon.
Re: Mendel90 - 3 point leveling system?
August 11, 2020 06:04AM
I have switched to picture clips instead of bulldog clips. They only grip at the edge, so should not be able to bend the glass to conform to the PCB. However I never had a problem with the bulldog clips, they just covered more of the bed.

I also have a 1mm aluminium sheet between the PCB and the glass to spread the heat but it does slow the heat up process. I don't think it affects flatness because it is more flexible than the glass and probably the PCB.

There is no gap under a straight edge on my bed. The PCB does curve away from it during warmup and then becomes more flat again giving a wave in the temperature curve but I do warm it to 148C with a 17V PSU for first layer ABS!

I have wondered about adding a fifth pillar in the center to stop the PCB bowing downwards but it would be difficult to adjust its height and would get hotter than the ones in the corner.

The only time my Z calibration changes is with a big change in room temperature, which causes the lead screws to expand a bit I think. I have the machine in an unheated internal garage, so it can get quite cold in the winter.

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