A printable knob and locking nut for the Printrbot Simple v2 with 1/4" Acme threaded rod on the Z axis.
The .stl file includes both the knob and a locking nut, since Acme nuts are hard to come by. Print them both, thread the nut onto the top of the Z-axis rod, then thread the knob on after it. Tighten the nut against the bottom of the knob - finger tight, no wrenches needed - to hold it in place.
If you would like to tweak the parts, both OpenSCAD files are included. You'll need the Screw Library as well: http://www.thingiverse.com/thing:8793