On your last paragraph there gr5. Filament from spool -> extrusion test fails at 4mm3/s. Same filament straightened to reduce friction inside bowden and extruder -> extrusion test fails at 8-10mm3/s. The only change is the curvature of the filament which makes it kinda hard not to suspect the extruder...
It's pretty easy to disassemble - only 4 screws - but then the motor wants to fall so I guess there is a 5th screw to remove the motor cover access.
The spring is too strong so everyone uses the loosest setting. I think UM will come out with a weaker spring so there is more control but right now the loosest setting for the extruder is about right.
The coolest thing about the UM2 extruder is that the firmware (and gcodes) can control the current. So you can set the current to a threshold below where the extruder grinds the filament. So what happens is if you have too high a pressure, the extruder skips steps backwards instead of grinding the filament.
On the downside - it's weaker than the UM extruder. And some people are blaming it for underextrusion issues (completely convinced!) but to me it's not clear where these people's underextrusion issues are coming from so the jury is still out. Also it is made of ABS and gets worn slowly over weeks of use and very tiny pieces of ABS can get into the nozzle which can also cause problems. Maybe. No one knows.
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