I made this video to show how I salvaged the motor and speed control from a used treadmill. With a small modification, I was able to simplify the electronics for later use as a variable speed conversion for a fixed speed motor power tool.
I removed the bulky control interface and replaced it with a 10K ohm potentiometer. (I chose a wire wound potentiometer, but a regular carbon film one would work just fine.)
I also show how you can reverse the direction of the motor and also stop and start the motor without having to constantly adjust the potentiometer.