Uneven extruder tension

[dramsey]

Finally getting back to printing-- the demand for my HP-themed fobs was high so I need to print some more! And the new hot end arrived, so all should be well.

Actually...no. The problem remains. I think this picture illustrates it well:



The filament extrudes fine, then it stops extruding, then it starts again...with a very even periodicity! The tension screw is adjusted correctly, but the tension seems to vary.

I removed the fans so I could see the extrusion gear turning. It's easy to keep track of by noting the position of the D-shaped shaft the screw is secured to. When the flat part of the shaft hits about 4:30, extrusion drops off dramatically, stopping completely in most cases, for about 1/4 total turn of the screw.

Looking at a segment of filament pulled from the extruder, you can see the depth of the tooth marks lessen. So, somehow, the extruder gear seems to be moving a fraction of a millimeter back from the filament.

Anyone seen anything like this before?

[Jules]

Check to see if the wheel that guides the filament into the teeth of the gear is loose, or if the set screw on the toothed gear is loose allowing it to shift. Check the teeth on the gear, make sure they are not worn or clogged.

[dramsey]

Check to see if the wheel that guides the filament into the teeth of the gear is loose, or if the set screw on the toothed gear is loose allowing it to shift. Check the teeth on the gear, make sure they are not worn or clogged.

And we have a winner! The key was removing the fans and watching more closely, with a flashlight, as a model was printed.

The problem was that the extruder gear was loose, and had backed up on its shaft a few mm so that it wasn't completely overlapping the idler gear. The filament feed hole in the printed guide is much larger than the filament, allowing it enough room to "walk" a mm or two back and forth. During a print, the filament could migrate to an area where the extruder gear and the idler gear didn't overlap, so the extrusion would slow dramatically or stop.

In fact, any time you see a "periodic" extrusion issue like this, it pretty much has to be something other than the hot end or nozzle, which leaves the extruder.

[Jules]

Hmmmn....second time we've seen that recently.

[dramsey]

Of course it just stopped extruding in the middle of a print again, sigh. The gear is in the correct position. More examination. Hm. I'm going to guess the filament feed hole in the top of the extruder should be a simple round hole, and not this:



It looks as if the filament can still "walk" off the edge of the idler wheel...

[Jules]

Yeah ....that one looks wonky.

[dramsey]

Guess I'll see if Makergear will sell me just that part, without having to buy the $110 kit.

I'd print another, but, you know, can't print.

[Jules]

They should be able to - just contact Customer Service.

[dramsey]

Customer service responded promptly and is sending me a new extruder housing gratis. The new housings have a socket on top for the filament guide tube so this type of problem should not recur.

As best I can tell the sequence was this:

1. Extruder gear comes loose on shaft. It still rotates but can slide.
2. Gear slides towards the extruder motor, leaving a few mm of idler wheel "uncovered" by the gear.
3. Filament is pushed towards the end of the extruder gear. This causes it to rub on the extruder housing, enlarging the original hole to a "slot" as shown in my image.
4. The slot eventually becomes large enough to let the filament walk completely off the end of the extruder gear, whereupon extrusion stops.

[dramsey]

The new extruder housing arrived. Instead of a small, barely-larger-than-filament-diameter hole to feed the filament into, the hole is large enough to accept the filament guide tube: thus, there's no friction between the filament and the extruder housing, so presumably no ongoing wear.

This seems to have completely cured my printing problems. I think now I've experienced every possible type of extruder failure other than having the stepper motor crap out! But at least this means I should now be able to instantly diagnose and fix any future problems.

One complaint: Makergear has not updated their online STL parts files for the new extruder housing, so I can't download a file and print a spare.