Voidmonster, thank you. I was just trying (unsuccessfully) to point out that you can use a potientiometer, which is basically what a dimmer switch and the Harbor Freight example above are. All a dimmer switch do is restrict, the voltage, not regulate the voltage. So the voltage that we restrict using a dimmer switch produces heat. Which as other forum members have pointed out, will overheat the grinder motor. A variac, or VFD (variable frequency drive) regulates the voltage and does not produce excess heat as a by-product.
Good call from you!