I used a credit card which my caliper says is 0.8 mm, but when I tighten the screw the sensor goes up a bit, so I figured that was okay!?
Now when I just a calibration, the tip doesn’t get close to the bed during leveling (looks like about 1mm from the bed) and I have to adjust the Z-offset up by 0.30-0.35mm to not “flatten” the first layer.
During Automatic Levelling it doesn’t matter that the tip of the nozzle is higher than you would expect - as long as you set the ‘last’ point manually to the correct height. The points when the machine moves automatically are only to find the relative heights of points around the bed.
And the last point offsets them down to where they are physically located.
I have to say that the leveling I get (even with 5x5) is subpar. The bed is clearly lower in the front on my printer and I get no adhesion, but in the middle filament is being pushed down so it’s totally flat.
How are you running the leveling procedure? I tend to do it ‘hot’. Heat bed to 65C, extruder to 100-110C (not hot enough that I’m burning filament while not extruding). Build in any bed warpage as much as I can. Then run the bed calibration. I usually use 4x4. After I’ve set the last point (I go pretty tight, zero movement of 2 sheets of paper pushing, the paper buckles immediately, but I can still pull forward) the temps have typically drifted back down to say 65-70C extruder, 40-45C bed. But so far so good.
I only have the A250 though. Someone else has posted regarding the whole bed ‘rocking’ the tiniest bit on the central support during fwd/back movement reversals. Maybe that’s part of what you’re seeing too, as the bed hits its most forward Y and stops the front end is dipping away from the nozzle a bit. I guess you could test that with lines along X in different positions so you can tell if it’s consistent or only when it first starts a line.
Yes, the front of the bed on my A350 is lower than at the centre, I put a dial indicator on it whilst heating and it moved 0.4mm. I’ve done a couple of things to try to mitigate the issue, I heated the bed to 60 and left it to soak before tightening the screws attaching it to the frame. I’ve also left off the outer screws all around the bed. There seems to be some improvement but I’ve not figured it all out yet, I’ve just bought a new dial indicator with a data interface so I can capture levels via an Arduino or something in order to experiment properly.
Did you make sure you installed the aluminum frame correctly and not upside down? This seems to be a common assembly error. The nuts need to be at the bottom side of the frame.
