Ultra-minimal plastic-spring extruder

Inspired by http://www.thingiverse.com/thing:411462, but designed from scratch in OpenSCAD, using http://www.thingiverse.com/thing:46749. Added spring for tensioning without any additional hardware. For Mk8 drive gear, 3mm filament and 624ZZ bearing (but fully parameterised so should be easy to alter for other motors, filament and bearings). Currently untested, so dimensions and spring constant likely wrong.
To do:
- test spring - it works!
- add finger pull - done
- add filament guides