Huge layer shifts twice and last bit disappeared

[fallen lance]

On my new v-core 3.1 300 build tried a fairly lengthy overnight print. Have been successful with a few other long prints, but this one did something really odd. About 10% in, it seems that the print head just shifted 1 to 2 cm in X and Y. The same happened again another 10% into the print. And the last 10-20% just seemed to not print at all. I did have a timelapse being filmed (sorry about my poor camera angle, gotta fix that) but even looking at it frame-by-frame where things went wrong cannot figure it out. I uploaded the timelapse to my google drive here : https://drive.google.com/file/d/1hTUkjHNCfb5uChPPo7AK1ACwfJcuaHTD/view?usp=sharing.

No errors were reported on the console. After the failed print, I tested toolhead movement and it seemed completely normal. But when I tested extrusion, it could not feed filament, so that explains why the last 10-20% failed to print, presumably because of the jam. Turns out the spool of PLA somehow got crossed up (see last picture)--not sure how that happened as it was a new roll of filament. After unloading, untangling the spool, and reloading, extrusion worked perfectly.

But the filament jam does not explain the huge layer shifts. I have observed no problems with the toolhead movement system during weeks of operation. I have been converting from PrusaSlicer to SuperSlicer. Could a "too fast" toolhead move cause this? Or could it have hit the printed object and jammed momentarily (seems unlikely, would think it would have just knocked it off the bed)?

I used the G-code viewer built-into RatOs to view the gcode to make sure it was not a slicer problem and the gcode looks fine (snapshot of this attached).

I got a snapshot from the timelapse of 3 frames around the first incident--one right before the first layer shift, then the one where it shifted, and one a few layers later to show it more clearly.

Thanks in advance for any help!

[vague zodiac]

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[graceful ember]

If the filament jammed hard enough, it might have tried to move but the filament didn't let it

[charred obsidian]

I agree that given the amount of slack in the umbillical cord it is unlikely that the tangled filament caused the layer shift.. There are really two many issues for layer shifts, something is loose in the motion system or electrical issues with the motors (bad wiring, not enough current, overheating due to too much current, etc). Are you able to post some belt tension and input sharper graphs so that we can check for problems with the motion system?

edit: on second thought I am not quite sure how the filament being constrained in the PTFE tube would affect the toolhead it if it tangled, but if you can share the graphs we can check for other issues anway.

[fallen lance]

I don't think the filament jam caused the first 2 shifts -- once it jammed the spool did not move, so it could not possibly have kept printing. Also, you can see the filament spool in the timelapse and it is rotating normally until the last 20% when it clearly jams and stops unspooling. I will redo input shaping and tension tests to ensure nothing has crept in.

[charred obsidian]

Sometimes with tangled filament if there is enough of a pull on it, the filament will be moved back into a position where it can still continue printing even if it crossed over itself. It only really stops moving when it gets trapped on a side like in your photo. It is possible that where the layer shifts happened it just needed a hard enough tug on the spool that it caused the motors to skip a few steps, but with the information we have we can only speculate.

[fallen lance]

I just re-ran the input shaper tests. I know they are not perfect, but don't think they indicate a serious problem. The X and Y frequencies shifted from the previous values of XY 65Hz,41Hz to 62Hz,40Hz and shapes look similar to previous runs. Looking at timelapse I do not see any sign of umbilical causing problems early in the print.

[charred obsidian]

As far as I can tell those don´t show any major issues with your printer. I think what happened is that because the length of filament in the PTFE tube always has to be the same that means that the tangled spool can tug on your printhead even though it looks like there should still be slack in the umbillical cord. I am fairly confident that if you retry the print with a non-tangled spool of filament it should complete without any issues.

[fallen lance]

I see what you are saying, but playing around with yanking on the filament, I cannot get it to even twitch the toolhead at all even when steppers are unpowered. And the way it tangled, it was not that the spool started out tangled. It was perfectly wound and unspooled fine until the obvious jam and even when it jammed the toolhead was aligned with the print below it. I examined the print very closely and saw some suspicious black marks right at the level where the first shift happened (see photo). It was right where the first aggressive overhang was. I am thinking that some filament did not bond with the previous layer or some other issue and cooled sticking up in a spot and jammed the print nozzle. I had been printing black PETG before and the nozzle has black residue on it from that and it looks like it rubbed off on the print where it hit. It has a big base in contact with the print bed, so maybe it was bonded strong enough to bind the toolhead. Might try printing again with different overhang settings.

[charred obsidian]

I don´t know if you have ever gotten in the way of your printer moving, ideally you shouldn´t have, but I can tell you that it takes quite a lot of force to stall the motors when the belts are correctly tensioned. And the layer shifts on your print aren´t exactly small where the motor might have just skipped a step or two, so for that to happen some part of your print would have had to have held up against a nozzle that is hot enough to melt through it for at least a second or so. I don´t really see any part of your print that looks deformed enough to have put up that much resistance.

It is of course possible that there was some other issue, but before you waste a bunch of time going down a rabbit hole searching for an issue that may or may not exist, I would just try printing it again, as currently the tangled spool is the most likely cause for the layer shift.

[graceful ember]

Something went wildly wrong but it somehow recovered

[fallen lance]

I did try moving the print head when the steppers were engaged and agree that it would take a lot of force, so yeah you are probably right that the plastic part could not block it enough. But by the same token it seems that the filament being jammed could also not do that. I will try again and hopefully will not see this problem again.

[fallen lance]

So I reprinted it with slightly different settings in my slicer (used brim and put fan on more aggressively on overhangs). I printed fine this time. May never know what happened. Thanks for you advice.