I don't have much documentation, but these two precursors have good info:
RichRap's original: http://www.thingiverse.com/thing:20733
Nick's version: http://www.thingiverse.com/thing:249933
Thingiverse has gone funky on me, I can no longer update or edit this thing! We've tried everything, even sending an email to the Thingiverse people...