This printable extruder is intended to work on RepRap Mendel (both standard and Prusa), RepRap Huxley, and virtually any other open-source 3D printer you can find.

It features:

1. 1.75 mm filament
2. Adaptable mounting plate to attach it to virtually any 3D printer
3. Very compact high-torque NEMA 11 motor
4. Active ducted fan cooling for high reliability
5. Wade-style hobbed bolt filament transport
6. Wing-nut drive to spread the torque loading on the plastic gears
7. Push-fit hot-end parts - no thread cutting
8. Easily replaced PTFE liner for the hot end
9. A single M3-threaded rod cut to lengths makes all the fixings
   1. Lightweight: 420g
   2. Compact design (110 mm x 90 mm x 80 mm)

I based this design (loosely) on Jstkatz's extruder (http://www.thingiverse.com/thing:7037), in particular, the idea of using a lever to trap the filament against its drive.

The full Bill of Materials and detailed construction instructions are on the RepRap Wiki here: http://reprap.org/wiki/RepRap_Universal_Mini_Extruder.