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michaelvk

UM2 calibration causing first layer issue

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Hi all.

Had my UM2 a week now, very happy and impressed, and a weeks worth of printing little greeblies trouble free-ish (still getting the hang of making models for printing, instead of pictures..).

However..

I decided this morning to recalibrate the bed for some reason. Followed the same process as I did before, using a stack of card (and the second time a piece of 1mm plastic, measured on digital calipers). Now here's the weirdness. Despite using as accurate a measure as I could, the first layer, instead of 0.3mm, was squished paper thin on the bed, with the resultant following few layers ending up mushing around a pool of plastic as it filled in the base. I then recalibrated with a piece of plastic measuring 1.25mm. Same thing. I then resorted to dialling the bed back on the screws about 3/4 turn.

The head and bed were cold (enough), there may have been a microscopic grain on the head, but if anything that should push it away from the bed? I also made sure there was no pressure on the bed, i.e. when inserting the 1mm piece the bed wasn't pushed down on the springs. I used a model sliced in Cura this morning, as well as a model that I previously printed without issue. Both were sliced using the quickprint fine settings.

The subsequent layers are printing fine.. So far (4mm up an 8mm ring)..

Any thoughts chaps?

 

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You don't need to use a 1mm spacer at all. The first, 1mm pass is only approximate. The one that matters is the second pass. Where you want to be 0.1mm from the bed - or less.

The first thing to do is to make sure that all of your springs are reasonably tight. If they aren't, it's impossible to find the right height with a single sheet of paper, because the tension on the springs is minimal until they're compressed a bit. So, look through the bed from front to back, and adjust the back thumbscrew until the terminal block in the back left corner is about 1mm from touching the lower plate. Tighten the front screws about the same amount, to keep the bed roughly level as a starting point.

Then heat the nozzle and make sure it is clean at the tip.

Now run the leveling wizard again. When adjusting the rear height, just use the dial on the front of the printer. When adjusting the front corners, use the thumbscrews. I recommend not using the 1mm-then-a-paper-thickness approach. Instead, on both passes level the bed to the point where the nozzle just touches the glass. This is easy to see if you look along the surface of the glass; you can see the nozzle touch its own reflection.

When setting each point, move the bed up until it just touches the nozzle tip, then back it off and allow it to settle untouched, and then gently close the gap again.

If you find that you cannot compress the front springs enough to get the bed down to where it needs to be, then simply raise the back of the bed a few turns of the thumbscrew, and restart the leveling wizard. You want to end up with all the springs in a middle position, with a gap of about 10-13mm between the two plates of the bed assembly. The springs should be neither totally compressed, nor so loose that they aren't applying any meaningful upward force on the bed.

By doing two passes at the same height you should get fewer surprises; the second pass around should only require very minor adjustments. And aiming for the point where the nozzle touches the glass is a much easier target than trying to interpret the feel of nozzle on paper tension.

 

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Ah right.. So set zero to the glass itself. Won't that have a knock on effect for first layer thickness? I must say that the paper thickness does seem a little unscientific. But doesn't the machine then take that into account when it prints, by adding the thickness of the paper? Or does it keep that distance as a safety to keep it from hitting the glass? The thing I found weird is that it was like it was adding more than the thickness of said sheet of paper to compensate for that offset. The first layer and skirt went down transparently thin, despite a 0.3mm first layer height. There's also plenty of tension in the springs, albeit not the 13-odd mm gap like you've mentioned.

Currently it's printing fine, but I'll give this levelling method a go. I might take the thing home with me over the weekend and have a crack at it there. I'll let you know what happens.

Thanks very much for the very informative answer!

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Make sure you have the latest firmware that ships with Cura 14.07. Previous versions had some issues with bed leveling that could cause the first layer height to be wrong. Check your printer's firmware version in Maintenance -> Advanced -> Version.

Yes, the firmware assumes an offset of 0.1mm when leveling is completed using the wizard. So if you level until the nozzle is visually touching the bed, you will be fractionally too close. But that's not really a bad thing most of the time; you generally want the first layer to be a little bit squashed onto the glass to get good adhesion, so being fractionally close helps with that. Just set a first layer height of 0.25 or 0.3mm, so that there is still plenty of clearance above the glass, and everything will be good.

 

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Firmware version is 14.03 on the machine, Cura is 14.07..

The thing was printing fine this morning.. If a little high on the first layer (I could see the head was separated from the filament, not touching it at all. So I think a switching off and on again may have cleared its head a bit, ridding itself of whatever caused it to go superclose to the head for the first layers yesterday). Either way, I'll take the thing home with me this evening and have another go there, were I can focus on it a bit more.

Basically: I shouldn't have screwed around with it. But it's all a learning experience, eh?

Thanks again for the guidance!

 

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There are several leveling bugs that are fixed in Cura 14.07 (not sure the firmware version). One is that after you level you get one calibration but then after every power cycle you get a second calibration value.

Another bug has to do with the bed being all the way down when you start the procedure or when you home. Either bug could cause you headaches so definitely update firmware.

Once you get leveling close, it's best to not use the procedure ever again in my opinion. Just use the screws to get the final .01mm accuracy (if needed).

 

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