Sherline lathe or mill speed is set with a knob, but the only indications for the knob are which direction to turn to go faster or slower. I have gotten along with just this for about a year and a half, but when I recently ruined a slitting saw blade, I realized that I needed to be more careful in setting speed.
Industrial speed sensors are available at on-line auctions for much less than industrial prices. I was able to get a Red Lion MP37CA00. This is a magnetic pickup; inside there is a coil and a permanent magnet. It can sense passing objects of steel.
Here is a better picture of the rotor that I made of mild steel. It has 6 teeth so that 100 Hz corresponds to 1000 rpm. The data sheet for the sensor explains proper tooth size. This rotor is 0.72 inch diameter, and was profiled with a 1/8 inch end mill.
No more ruined tools? Better part finish?