Nut Embiggenator (M6 to 5/16"-18)

This is a hack part that let me use a simple M6 metric threaded rod (1mm pitch is beautiful for stepper motors), with M6 metric brass nuts, and without changing the main Printrbot components.

All of the physical dimensions of the Printrbot line is in metric. STL files are in millimeters. People discuss layer heights in microns, the filament comes in two metric standard sizes, and so on.

But the Z axis of the Printrbot is implemented with a threaded rod that has 18 turns to the inch, whic