Really bad layer shift - though really more like belt hop....  

Really bad layer shift - though really more like belt hop....

Hi,

I recently got my Prusa XL 5T ("fully assembled") ... ermm... assembled. The assembly went pretty easily but when I went to print the first layer test I got some massive shifting:

Didn't completely dawn on me what was happening at this point so I re-ran the calibrations and tried the batarang print to test a small print and it turned out great:

Ran the first layer print again and it had a massive shift but this time I was watching just after it shifted and saw the carriage crash into the side and undock the toolhead so I knew something was really off. 

So doing more digging I ran into the article on belt tensioning and checked the belts with the tuning tool and both sides registered as "too tight". I adjusted them and verified the alignment (I think - more on that below). Printed the 1t and 2t first layer tests and they came out fine. Yay, right?

So- getting ahead of myself and full of hubris 😉 I setup to print the MK4 upgrade parts that I needed to upgrade and revive my MK3S printer. Get them all laid out in slicer (why aren't there files out there for printing these on the XL already, I can't be the only one in this boat) - slice them and start printing the first batch (all the black PETG parts). Does great through the first 14% or so of the ~12 hour print. A couple of small strings but nothing I'm too worried about.

Come back a couple of minutes later to look, and another shift like the first couple of first layer tests above. I can see the outline of the LCD cover several cm diagonally being printed over different parts. So frustrated I forgot to grab a picture before I aborted it. The parts I all seemed to have good first layers when I peeled them off. I don't think this is (just) a belt issue - or rather not a purely belt tensioning issue. It's like the whole belt is slipping and throwing off the positioning when the print is spanning a wide enough range of the print bed.

Anyone have pointers to things to try? I've thought about printing the homing tower but it doesn't seem like that's going to help with this.

Also, does anyone have a pointer to a video or pictures with arrows or something so I can visually see what this paragraph in the tension and alignment adjustment guide means? I should have built this one from kit like I did the MK3 so I'd know the part names and pieces better but I didn't wanted to save some time with pre-assembled but I cannot follow which parts they are talking about in this description:

Test the alignment by squeezing the CoreXY Front Left and the X-axis on either side: there should be no play between the parts. If there is play, tighten the screw on the same side slightly, move the X-axis assembly back and all the way to the front, and test the alignment again on both sides.

I can't believe there is a whole video on how to pluck the belts and just a static, unannotated picture for this part.