I really liked the knob that owens made for the Z axis on the Printrbot Simple. When I downloaded his part, I discovered the hole didn't go all the way through, and the nut fit was very tight. So I created a new version that is just like his, but that fixes these two problems. I've also uploaded an IGES file for those with 3D CAD programs that can edit and modify such files.
This fits over the Printrbot's 5/16 threaded rod. Use two 5/16 18 nuts (one on the bottom and one on the top) to hold this in place so you can use the knob to turn the threaded rod without slipping.
I recommend using Kisslicer with support material to slice this model. I've had problems with Slic3r on both this model, and also with owens' model.