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, which may be cheap in the US but makes for some bad math on the slicer. For instance, the number of stepper motor steps per millimeter is not an integer, which can cause ribbing issues as some layers are rounded down and some layers are rounded up to the nearest microstep.
Without this part, the M6 nut would just slip and fall through the Printrbot carriage. I could have made a more intricate part, but this was the last thing I wanted to print on the old threaded rod, so I kept it simple.
Slice these for moderate-to-coarse quality. I would review the slicer output on the screen before printing, though. These are basically single-wall cups if you're using a 0.5mm nozzle. If the slicer ends up making poor corners this part would fall apart.
Before installing the new threaded rod, print one or two of these. Actually, print a few at once, as they print so fast at 0.254mm layers that you won't begrudge the time.
- Shove a standard M6 nut into the hole. Brass preferred, if you can find one; it's almost self-lubricating on a steel rod. Voila, you now have a 5/16"-18 nut that fits on an M6 threaded rod.
If you got your rod and nuts from McMaster-Carr, a bit of cleanup is in order; they apparently keep the steel rods in a filthy but dry anti-rust stuff. Use soapy hot water on the rod, then dry well. A little PTFE lube like TriFlow will seal it up. The nut should fly up and down the rod with no resistance.