Hey everyone,
I always had problems with underextrusion, sometimes it was caused by the filament, sometimes by the bowden tube and/or the nozzle and sometimes the extruder, where grinding and slipping occurred again and again.
I tried a lot of selfmade extruders like the H3, Robert's and other UM2 clones. I bought a 0.9° high torque stepper motor, an UM2 drivegear, the MK8 and so on.... Some worked for a while, some was crap from the beginning on...
Finally I decided to buy a new commercial, stronger Extruder which can simply brute-force the filament through the old bowdentube and the nozzl