Potentiometer Knob to Drill Hex Adapter
thingiverse
Standard 18-tooth spline to hex adapter for common potentiometer knobs, such as those on guitars or cheap speaker amplifiers. I used this to polish my Blank Potentiometer Knob (https://www.thingiverse.com/thing:4549816) - basically I mounted it on a drill and polished it like you would on a small lathe. I printed mine really, really slowly; otherwise the splines will bulge out and won't fit. If it fits loose you can scale it up, or you can wrap a piece of thin tape on it.
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