Improved 5mm to 3/8" z-Axis Coupling for Printrbot Plus

A self-centering coupling modified from Thingiverse user loco (http://www.thingiverse.com/loco). Fits the larger 3/8 inch Acme rods for decreased a-axis wobble compared to the stock couplings.
v1.1 Increased the internal diameter of the bolts to be easier to tighten.