Bed leveling at the extreme edges...

#1
Hello!

I'm having a weird issue (unfortunately no pics to provide at this time) involving leveling the bed. Even with my glass bed, it seems to only be really level at the center. At the extreme edges its always incredibly off, causing any printing in those areas to fail miserably (right now I'm trying to print front bezels and rear covers for 17" laptop screens being re purposed for a different project, the bezels stretch almost edge to edge on the x axis, save a couple of mm on each side).

I have tried putting on a tick layer of glue stick (one thin layer at a time) as well as blue painters tape layered up to create a cushion the hot end could then push into slightly to overcome the issue, but its never enough. It seems even if its good on the first loop around, the subsequent loops are off again.

I have tried getting a level test dial indicator and printing a small attachment for the extruder assembly for it and leveling by numbers instead of by paper. Still no luck. Can get all the level points set up at an arbitrary number, but the second pass is once again really off. Get them back to the number, and the 3rd pass is still off. That process continues, over and over. I decided to try and keep the dial in between 2 numbers, thinking maybe I was trying to be too precise, but still, every pass continues to remain crazy.

I did notice during this whole process that it seems the aluminum angles are almost like a pair of scissors; the opposite corners seem to line up. The back left and front right wing nuts are tightened way down while the back right and front left wing nuts are really loose. I checked the aluminum angles and it seems they are bent slightly. I'm not sure if this is an issue with the weight of the glass bed, as if the bed was cause I would think all angles would be bent.

I'm planning on redesigning the whole undercarriage for the bed, but would like some suggestions on how to do so. I do know that I want to move the leveling screws to the corners as opposed to the center 3rd of the bed though, I feel like that may allow a more consistent level across the entire bed. I'm also thinking of taking inspiration from the z axis and using captive nuts riding up and down a threaded rod instead of having the bed float on springs.

Anyway, does anyone have any suggestions on how to proceed?

Thank you very much!
 
#2
Here's a set of ideas, all of which you may already have thought about.

It seems that if the first layer is printing OK then the bed is fairly level at that time. If I read correctly, some of spring loaded leveling bolts are fairly loose. Could it be that they are so loose that the vibration of the printer is loosening them even more during the first layer print? This would screw up the level for all subsequent prints.

If the L brackets holding the glass are bent in the way you suggest it would seem that the bed would teeter back and forth along the axis where the bolts are tight and that you would never get the bed level. Is the print along this axis OK, i.e. are the print failure points occurring where the leveling springs are loosest? I have a glass bed and it is lighter than the original acrylic bed so I don't think the glass is heavy enough to bend the aluminum. Have you tried swapping the L brackets around to see if it makes a difference? If you took them off you could lay them against a flat surface to see if they were bent. If they were, you could place a small shim (thin washer, tape, thin plastic) between the L bracket and whatever is holding the glass up to temporarily fix the problem. You can get a new 4' bracket a Home Depot or Lowes and probably at other hardware stores for about $9. You would have to cut the bracket to size and drill the holes.

In my setup, all of the leveling bolts are fairly tight. The fact that yours are loose on opposite corners implies that something is amiss with the printer fame alignment. Is one of the ends of the bed support assembly sticking up higher than the rest or is the frame itself out of square? If the printer itself is not level that might make it hard to level the bed. You could loosen the assembly bolts and adjust the frame a bit to see if this helps.

Lastly, are the Z axis rods touching the ground and thereby supporting the printer instead of the feet? If so, they could throw your level off when they turn to move up to the next layer.

These are just some ideas. A close up picture or two might help with the diagnoses.

As for the bed redesign, having the bed rest on springs helps prevent print crashes. Even on finely tuned prints, little blobs of plastic form on the print which may cause parts of the print to stick up higher than the rest. The print head needs a way to get around these so it doesn't damage the print or knock it off of the bed. The springs allow the print head to push the bed down to overcome these small obstacles. Affixing the bed to rigid rods might not work but others out there know far more than me and so please chime in if I've got it wrong.

I printed a 17" x 6" plate once and had a heck of time, mainly due to warping, not bed level. Best of luck with the effort.

Chris