PrintrBot Zero Friction Spool Holder

I've seen [another design](http://www.thingiverse.com/thing:355889) by [emmett](http://www.thingiverse.com/emmett/about) for a horizontal spool holder that fits on to the Z-bars of the PrintrBot but I found that the friction on the spool was a little too great and gave artefacts on the print when the filament was being pulled.

I think that mounting the spool horizontally gives a very neat and stable design so I produced my own one that contains a built-in bearing to minimise friction (okay describing it as "zero friction" may be somewhat of an exaggeration). I guess I could have used real ball bearings here but picking up some marbles from a visit to the local toy shop seemed more in the DIY spirit of 3D printing. The result is a super-smooth action.. really, you should try it!

I'm also rather pleased with the threaded spool retainer. Not only is this the first thread I've printed (and it works really well) but the conical design allows me to fit all the spool sizes that I