M12 x 1.75 threads
The pic is black abs sanded smooth in steps starting with 320 grit and I worked my way to 1500 grit. I used a threaded piece and I stuck the other end in my drill. Will upload that tool shortly. You could make an easier job of this with slightly better results by sanding with just 320 grit and using an acetone vapor bath. I will likely try that with another knob.
Link for tool:https://www.thingiverse.com/thing:3525268