Dimmer switch

I saw this suggested somewhere.
Would it be possible to use a cheap dimmer switch to vary the speed on a belt grinder? Or would that mess up the motor?

The answer is: it depends.
I use a dimmer switch on a fan for controlling the airflow of my forge. That works ok-ish because the fan does not draw a lot of power.
A belt grinder otoh will draw more power if you start to press something against it. The dimmer will simply chop the sinewave, so the motor will have a limited amount of power. At low speeds, it will have very little power. If you press something against it, you risk slowing down the belt significantly. When that happens, you risk burning the motor.

It will work, as long as you don't let the belt get too slow, because the amount of power is related to the speed (due to the way the dimmer works). As long as the load is light and the speed is high enough, you're fine.

It is far from an ideal solution however. the best solution is a frequency drive. If you don't have that, use stepped pulley, or alternatively, have 2 or 3 different drive wheels so that you can change the speed by changing the drive wheel.

It also depends on the type of motor he has. You can burn up a capacitive start motor by lowering the voltage. It must be a universal motor.

Variable Frequency Drives only work on 3 phase motors.