Variable speed rotary tools are great! But just how are you supposed to know just how fast it's turning? That is the issue I am going to try and address with this project. I have the tachometer kit on order, but I'm puzzling over just how and where to mount the magnet on the rotary tool. Obviously, it will need to be mounted on an area of the tool that rotates as it is running, and that leads me to think that the collet nut would be the most logical place to locate the magnet. However, given that the tool can reach speeds up to 35,000 RPM, I'm doubtful any magnet would be able to remain in place on its own. I'm thinking the use of a 5-minute epoxy will affix the magnet permanently to the collet nut, but then I am wondering if I should use a tiny, N52 neodymium magnet (3mm x 1.5mm) set into a shallow depression I would make in the collet nut with an 1/8" drill bit (about 1 mm deep)?

My ignorance is great! And I appreciate all the feedback (constructive) I can