Problems With Auto Bed Levelling

Hi Guys

I have successfully implemented a servo activated probe on my hot end carriage and it happily probes the bed in nine places. The auto level system seems to be working as the Z motor is turning back and forth as the head moves around the bed during a print. My problem is that the nozzle is closer to the bed in the centre than at the edges. If I adjust the Z offset to make the centre print o.k then the first layer won't stick at the edge. If I adjust for correct height at the edge then the gap in the middle is so small that the extruder stalls due to not being able to push plastic through so close to the bed. Any ideas? The machine is a core xy with a 400 x 200 bed.

Andy

Does the switch that you are using for the z axis have a lever (metal arm) if so, remove it because that bending before it hits the switch might be the reason. But I will say that using servos and switches is already old technology, now days proximity sensors are the way to go. I use capacitive proximity sensor for my z calibration. If the arm of the servo doesn't extend properly or the servo suffer any glitch, your calibration will be less than desired.

Quote
ggherbaz
Does the switch that you are using for the z axis have a lever (metal arm) if so, remove it because that bending before it hits the switch might be the reason. But I will say that using servos and switches is already old technology, now days proximity sensors are the way to go. I use capacitive proximity sensor for my z calibration. If the arm of the servo doesn't extend properly or the servo suffer any glitch, your calibration will be less than desired.

Thanks for the reply. Yes I had already removed the lever. I have now found the issue, it's that my bed was not flat. The 4mm glass was bowing slightly. I thought bed levelling took care of that but, apparently not. It seems to just fix a tilt in a perfectly flat bed not discrepancies like a higher centre with lower edges, as in my case. The fix was to make the bed flat which, after some minor rebuilding, I've now done. Out of interest, which capacitive sensor do you use?

Just wondering, what did you do to fix the problem? I have the same issue with mine.

My heated bed is a 'sandwich' of 6mm MDF base, 3mm cork, 2 x MK2 A PCB, 1mm aluminium sheet and 4mm glass and is 430mm x 215mm in area. The MDF was a little bowed but I assumed the bulldog clips that hold the glass on would pull it straight - they don't, the glass just bends slightly. I just substituted 3mm aluminium sheet for the 1mm and it's now fine. All the layers, apart from the glass, are stuck together with heat proof silicone sealant with appropriate kapton tape insulation on the heat bed pcbs.