IMPORTANT: THERE WAS A GEAR MISSING! AXLE GEAR IS NOW AVAILABLE FOR DOWNLOAD AMONG THE OTHER PARTS, IT'S ESSENCIAL TO GET THE DESIGN WORKING!Although not as simple as other solutions, this feeder is not overcomplicated and is really reliable after you get the parts working right. The principle is simple: you increase the contact surface with the filament, using rubber wheels to pinch the filament without crushing or bitting it, and alongside the use of gears guarantees that the parts always move in tandem. As the name implies, you got to print two rubber-zigzag wheels on ninjaflex or similar rubberlike material, as they grip just right and fit tightly over the main gears.The 2 main gears get 2 bearings each, and the 3rd gear gets one bearing. I've tried with less bearings but the gears tilted and didn't work so well. I've included cheap sources for the bearings.The only mod you have to do to get this working is to set the steps per E setting of the UM2 to 237 using Pronterface or your software of choice (M92 E237 on Pronterface). You could skip drilling the holes for the two uppermost screws, but I'd strongly recommend you to drill because that extra contact point improves the reliability quite a bit.I've also included the flipped body and cover, which is what you see in the pictures, but that involves cutting part of the left side body acrylic of the printer and also uploading new firmware to invert the extruder stepper motor direction (#define INVERT_E1_DIR true on Marlin's Configuration.h file)I can remodel the body to use the stock bowden adapter if someone shows interest.It is based in this design: https://0x7d.com/2014/07/improved-ultimaker-2-material-extruder/.09-01-2015 - UPDATE: Added 3_body_bowden.stl model, so if you don't want to use the 1/8" fitting print this one. Haven't got a chance to test it, so please let me know thru the comments how it works.