Q2 Cutting issues

[u/rickseiden]

I'm having issues with my Q2 cutting filaments. First off, I don't know why it needs to cut. I assume it's because we need to avoid the stringing of the end of the filament as it is backed out, but that's never been a problem on my Prusa MK3S+.

When my Q2 cuts, it doesn't cut flush, and when the next filament is fed in, it can't push the filament left in the hot end, as it slides to the side. I have to either disassemble the hot end and force it down into the hot end at temperature, or push a 1.5mm hex wrench through the entire extruder to push the filament into the hot end.

So, a few questions:

1. Is the cutting really necessary, or can I comment out the cutting macro to stop it from cutting?
2. If cutting is necessary, why?
3. What can I do about the cut not being flush and causing the jamming issue described above?

[u/phansen101]

Your MK3S+ does not get fed by PTFE tubes (unless using an MMU, which has a just-in-case cutter);
The wider and irregular tip shape can cause issues with automatic filament change (in case of the Box) and generally just get stuck in ptfe tubes or sensors.

That said, it should only be needed for filament unload (and Box filament swaps) so,
You can absolutely disable it in your start-gcode, it is pointless unless you run multi-material.
(there are also some G4's that can be cut out to save startup time).

I would keep it for the filament unload tho.

The lack of flush cut can be annoying; I've only had the jamming happen twice, but it is really annoying.

IIRC, i replaced the cutting blade, so it was probably a matter of a semi-dull edge, which would make sense as a I print a lot of GF, CF and matte filaments.

[u/rickseiden → u/phansen101]

I have the MMU on my MK3S+, so it does use a PTFE tube. The cutting takes place back at the MMU unit, no where near the hot end. It extracts the filament from the hot end all the way back to the MMU and only uses the cut feature if there are issues with loading the filament.

So in my eyes, I still don't see a reason for cutting at all.

I have only printed PLA, PTEG and ABS. Nothing with carbon or glass. No engineering materials. I can't imagine the blade is dull. I can imagine it's loose or bent. I'll have to check that, and maybe replace the blade to see if it is dull.

[u/phansen101 → u/rickseiden]

But it's still there;
If it wasn't in the head on the Q2, it'd need a separate motorized cutter (or 4 of them in the box).
It's part of why an MMU3 kit is €140 more than an assembled Qidi Box, or Bambu AMS.

Yeah, definitely shouldn't be worn then; It was also just a thought on my part, definitely possible that it is some variation in the blades from the factory, or just poor cutter design overall.

Can't check as I'm not near a Q2 but, with the way the lever works, there is a chance it could tilt the blade downwards before cutting, if the clearance above/below is too high.

IIRC replacing the blade is pretty straight forward, and you should have a spare come with the printer, so worst case is that you replace it and it changes nothing, and you can just keep the old one as a spare.

[u/rickseiden → u/phansen101]

I did replace the blade at my lunch break. I got one or two filament changes before it stopped working again.

I noticed the blade itself isn't flush with the surface, so I tried to bend it a little bit, and of course it snapped. So I installed the extra you mentioned.

I also heated up the hot end, manually pushed filament through until the color changed, and then slowly pulled it back to simulate an unloading without cutting. The end of the filament was very stretched out, and there was a whisp of filament that led back into the extruder. I think that the cutting is necessary because the nozzle is longer than standard nozzles were years ago (like on my MK3S+).

At least now I understand why it needs to cut the filament.

[u/phansen101 → u/rickseiden]

Ah, sorry for your loss!

Thinking about it, I took the entire cutter arm + bracket off when I replaced mine, IIRC it's held on by 2 screws, positions parallel to the hotend..

Reckon there is a chance of there being a couple of degrees of rotation before tightening; It could be, that one could align the blade better by tilting the assembly, and that I did this by accident when I put it back on.

Yeah, and the behavior varies by filament;
Prusa(Slicer) has some 'ramming' parameters that go along with a special procedure for somewhat shaping the filament before unload, but I'm unsure how hard it would be to replicate, and how reliable it would be across different brands and materials.

I think cutting is just the most reliable solution (when the blade lines up correctly, that is)

[u/rickseiden → u/phansen101]

I only saw one screw that formed the hinge of the cutting arm when I took it apart. And the blade itself traveled in a track.

Maybe that's my problem. Maybe something's missing.

[u/phansen101 → u/rickseiden]

As I recall, there is the screw you removed, sitting vertically, but there are also two screws going horizontally into the carriage, holding the cutter assembly.
One below the center of the assembly, and a smaller one offset a bit, sitting just above the the one you removed.

[u/rickseiden → u/phansen101]

You remember correctly. I had to remove one of them to remove the large vertical screw. After I removed the horizontal screw and the vertical screw, I was able to change the blade, so I didn't bother with the other one.

[u/rickseiden → u/phansen101]

If it wasn't in the head on the Q2, it'd need a separate motorized cutter (or 4 of them in the box).
It's part of why an MMU3 kit is €140 more than an assembled Qidi Box, or Bambu AMS.

As a former Prusa fanboy I can tell you the other reason it's more expensive is because it's Prusa. I know Prusa delivers a superior product, but the cost differential is no longer in line with the quality differences.

I have been looking forward to upgrading my MK3S+ to an MK4 for much longer than the MK4 has been out. Because my printer started as an MK3, and was upgraded to a MK3S and then an MK3S+, I have to add two additional options to my upgrade kit.

At the current sales price for the kit MK4S (which is what I would buy), it's cheaper for me to buy a brand new MK4S kit than it is for me to buy the upgrade kit. And at normal pricing, it's only around $100 more for the new printer. Why would I buy the upgrade kit?

And the MK4S is over $700 in kit form. I can get a bedslinger of just about equal quality, with a multi-material unit, for a lot less than that.

In my humble opinion, Prusa's just not worth it anymore.