I wrote up an article about 4 years ago on fitting one to both a myford (or any other lathe) and a drilling machine. Used a small magnet and a hall effect sensor. Either ME or MEW. Cost very little. Noel.

I bought all the bits and pieces to fit one to my custom Cowells lathe base. A module similar to the one linked above and a small power supply. When they arrived I felt that the sensor was a bit big and bulky and would have made fitting awkward. I didn’t bother in the end. Fitting such a thing to an even small watchmakers lathe might prove tricky. A handheld laser tachometer as suggested above would be ideal.

Though the commercially available unit comes with a sensor what I used was JUST a hall effect device, smaller than a match head. The update speed is unimportant in this application as the speed is unlikely to vary rapidly. In the OPs application simply reading the speed is all that’s required. On the Myford there was no drilling or such, easy to fit or remove without any harm. Noel.

The physics toolbox app has a tone detector which measures frequency. If the drive is an induction motor you can get close enough from the drive frequency and pulley ratios.

These from Amazon are the ones I used when I put  an RPM measurement on my Taig. I 3D printed a little cover that fit them, and clipped onto the headstock. Two little magnets in the back of the spindle pulley. I used an Arduino and an LCD display. There is RPM meter code and instructions available on the internet.

I’ve had good results with a cheap unit from ebay. I keep a spare as the display tends to fail after a year or two..

Jimb

The sensor I used is now obsolete, but identical in size and function to these https://ebay.us/m/FTGgVU

The speed changes quite quickly when you’re setting the right speed. If anyone wants details of my arduino version send me a pm