Got a Printrbot Simple but it needs a knob to turn the z-axis up and down for zeroing. It had to be something small, because there's not a lot of clearance around the screw on such a small machine. So here's a little knob that screws on the top! Should also fit any other 8mm threaded rods.
The hole in the center is 6.75mm, suitable for tapping to M8x1.25, but you want the knob to bind in place around the shaft -- so just hold the shaft with vise-grips and carefully thread the knob on with moderate force. I went about a quarter inch down and haven't had any slippage.
While the original version fits on a stock, properly-assembled Simple, the clearance between the edge of the knob and the rear vertical rod may be a little close if you have significant Z-wobble at the top. In this case, download the slightly smaller version instead for a little extra clearance.
I've printed and tested both knobs, and the smaller one works better with no clearance or handling issues, so I've removed the larger one.
Printed on the Simple at 0.3mm layer height, 0.3 rectilinear infill, 3 shells, PLA. 3 shells are important to ensure you don't crack through into the infill when threading the rod into place.