Parametric Printrbot Variant

This is an edit of whosawhatsis's excellent design of a printrbot variant, but with the smooth rods on the outside (whosawhatsis did this mod for me on request), and with the distance between the smooth and threaded rod set to 27mm (did by me... very hacky... and the parameter for setting it isn't the real width, but has to be adjusted).

These things are used for http://www.thingiverse.com/thing:15611, which is my variant of the printrbot.