Haha it works, it works!
Can't wait to mount this onto my UM2. The only "special" parts used are 2x E3D springs for the latch. Once tested and debugged this will be released as CC0 ofc.
First manual tests are promising: the huge knurled nuts have an iron grip but leave almost no markings on the filament. The whole thing has a little high inertia because of the combined weight of all the 608 bearings i used. So to the feeder-modding-pros in this thread: What do you think, what are the chances that this assembly will not destroy my printer?
Am right that i can divide filament diameter by sqrt(2) to achieve double feeding?
Like, instead of 2.85mm i dial in a filament diameter of 2.02mm and then the feeder will double it's action? Or are there (apart from the retraction setting) other magic things the cura->firmware toolchain does that totally depend on 2.85mm (wiping, coasting, blabla) that might ruin my prints?
Update: Noooo! Seems the "feeder knurled wheel" (part 1179-B2P-A) and it's small set screw seem to be glued onto the motor shaft? tried to unscrew the set screw with wd40, heatgun, inch-unit-allen key (which is a bit too big and therefore fits extremely tight), it didn't move at all. I thought i tried this before i started but now it seems i didn't. So it looks like i have to replace the feeder motor if i want to try my feeder :-(
The UM2 docs do not say anything about the type of motor used as feeder motor. Only the Z motor is specified in detail. Can i use a regular 1.8° nema17 or is some special motor needed? I think i heard the board is reading the motor feedback to decide for skipbacks. (I have a bunch of ACT 17HS5425/max 2.5A here)