Núm vặn On/Off/Volume cho 3M WorkTunes

To work with the 3M WorkTunes, I had to heat up the knob to 170*F, press it onto the shaft, and let cool.  Heating the knob and allowing it to cool on the shaft is what makes the splines on the shaft bite into the plastic.  I did not try to print it without undersize and not sure how that would work as there are no internal splines on the knob, only the external splines on the ear muffs.