two piece filament spool holder - rigidbot

This is a "universal" holder that spins on to your filament spool centering it on an 8 mm rod - as found in the Rigidbot. It's a variation on several similar designs on Thingiverse. There's sufficient slop in the threads that it should connect smoothly after printing. It's in two pieces, making it a little easier to put together than a three piece (two ends and a threaded rod) design.

Inspired by http://www.thingiverse.com/thing:940762 who has a three piece design, where at least wi