Bowden Direct Extruder for 1.75mm filament

This is a direct extruder for bowden and 1.75 mm filament (NEMA 17 stepper motor)

Idea based on this extruder: http://www.thingiverse.com/thing:275593

Bill of materials:

- **Bowden tube**: ID = 2mm; OD = 4mm (e.g: http://reprapworld.com/?products_details&products_id=167&cPath=1616_1653)
- **Pneufit** for 4 mm tube (e.g: http://reprapworld.com/?products_details&products_id=169&cPath=1616_1653)
- **Drive gear** (e.g: http://www.ebay.es/itm/151122877988?ssPageName=STRK:MEWAX:IT&_trks