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So this is a bit of an odd problem, but basically trying to describe the situation, the gear on the extruder seems to be grinding and tearing the filament as it passes up into the bowden tube, for lack of better words. I "think" (still very much a newbie, only been printing for about a month) it only seems to occur on large prints where multiple retractions happens it looks like; the first time it occurred it happened on an overnight print, so I didn't catch anything. But tonight I was able to catch it mid way through a print that was failing when I checked it, and the filament was moving up and down as if it were retracting, but wasn't advancing at all since it had almost tore through the filament so nothing was coming out on the hot end.
Looking at my spreadsheet of printer stats, this has only happened on larger prints where I disable combing in Cura (so no retractions occur), as last night I had a long print go through fine with combing on, and thinking about it both times it has failed was on longer prints with combing off so it could retract (due to flat surfaces on the print that I didn't want z scaring to show on due to leaked filament).
So I guess the question is, is this a slicer setting I can change in Cura to prevent this, or is this a mechanical problem with the extruder that I can fix? I've just been using the default Cura retraction settings (6.5mm retraction distance, 25mm/s retraction speed, 0.8mm minimum retraction distance, etc), and on small prints I've never had a problem with combing off, so that's probably why I haven't noticed this before, it seems to be the larger prints that have problems. I guess the obvious solution would be to leave combing on, but again I hate z-scaring, especially on prints I don't plan on painting, so would rather fix the problem and let it be able to retract when I want it to.
Any and all suggestions welcomed. Thanks!