Auto-leveling presses down bed corners

Sorry if this has been answered already, I wasn’t having any luck with searches.

We have a TAZ 6, completely stock with a single extruder head. It has been working great for the past few years but recently problems with the prints have been showing up. I’m going to start with the auto-leveling issue because that will likely solve many other problems…

When a print begins and the head goes around touching the metal pads on the corners, the bed is physically pushed down on three of the four, with the worst being pushed down around 2-3mm. I’m not sure exactly how the sensor works but making the assumption that there is some voltage passed between the metal surfaces I tried wiping down the nozzle while it was hot to make sure there was no excess filament. While I have seen the ABL process fail due to a dirty head (touching the first pad then going back and wiping the nozzle again), it is NOT doing that now. Since the nozzle pushes the bed down quite a bit I’m pretty sure something in the bed leveling process is failing – how would I troubleshoot this?

My own experience is with manually leveled beds on other printers, so I tried a 5-point leveling test print to see where I’m at. The results made it clear the nozzle is too close to the bed, even scraping up the filament on the corner that gets pushed down the worst. I attempted to adjust the Z-offset with M851. The original value was at 1.20, and it looked a little better as I reduced the value, but by 0.90 the filament wouldn’t stick to the bed (not surprising). Even at 1.00 the surface of the test print was very rough as the nozzle offset previous strands of filament, but results were a bit better. Obviously since I’m still not getting a smooth top surface my attempts to change the Z offset did not help the problem, so I’ll revert the value back to 1.20 and try correcting the auto-leveling process instead.

Any help is appreciated.

Yes, the autolevelling is done with a voltage between the washer and the nozzle.

Usually, any visible depression of the corner means that there was some sort of plastic residue or such that had to be compressed or pushed out of the way before sufficient force could cause the washer and nozzle to touch – cleaning both the nozzle and the washer is the normal cure for that. DO NOT USE A WIRE BRUSH (you will short out the control board, which is an expensive repair). Use a scotch-brite (non-metallic) pad. Check both the nozzle and the washers.

It’s also worth checking that your washers are snugly screwed down (don’t screw too tight, you’ll strip the aluminum threads!). They should be just tight enough that they are level (not tilted up on one side), and should no longer rotate when you try to turn them by hand.

If all that is good, then there’s a possibility that you’ve a wiring problem. Check the bed wiring (and connectors) to see if the constant motion of the bed has caused a loose connection or a wire failure due to wire fatigue (the latter seems more common on the mini, though). Same thing on the tool-head connector and wiring.

I’ve been thinking about this problem from the perspective of an electrical issue and realized I could see a light grey film of corrosion on the corner pads. I don’t have much to work with here, but took a pencil eraser to the metal surfaces to give them a bit of a cleaning. Then I homed the head and issued a G29 to check the results.

Absolutely perfect!!!

The nozzle comes down on each pad and barely touches it now, as it should. I did another leveling test print and all five pads printed nearly identical. I am now printing another test object, and it appears one of the other issues I was seeing (a huge elephant-foot) is completely gone. I’ll check the print closer in a few minutes when it finishes, but from all appearances everything is working properly again. I’ll just leave this here in case anyone else runs into the same problem.

So what should I use to ‘properly’ clean those metal sensors on the corners? A light buffing of steel wool seems like the obvious answer, but are there better suggestions?

@mwester – Thanks for the reply! OK, scotchbrite pad sounds good, I’ll grab one to bring in tomorrow. I did actually check all four ‘washers’ (I assume that’s the metal discs on the corners?) and they are solid so I didn’t try tightening any further.

The test print finished, and the quality looks as good as what we were seeing earlier this year, so I think we’re all set now.