I bought various thicknesses of HDPE recently and have been milling that. It looked to me that I was on the verge of melting the plastic instead of cutting it and I wanted to reduce the speed of the Dremel, but I was already on the slowest setting. I'm using a Dremel 300 which gives speeds from 10,000 to 33,000 rpm. To get some slower speeds I bought a Fan Speed Controller for less than £10. At very slow speeds the Dremel's speed gets a bit unsteady but away from that bottom end it all works just fine.

I'm now wondering what cutting speed I am actually getting from the Dremel and how could I measure it. Some optical sensor connected to an Arduino with a 7-segment display perhaps. A home-made stroboscope perhaps.