Bearing for Printrbot Metal Plus Filament Spool Holder

Taking inspiration from BiZaRrO_John's bushings, I decided to add real rolling bearings to my Printrbot Metal Plus spool holder (http://www.thingiverse.com/thing:675439) to make the filament feed really smooth.

Starting with unitconversion's parametric ball bearing design, I tweaked it, optimized it for 3/16-inch balls, and added the cap piece that turns it in to a wheel for the PVC tubes.