I use the same type of speed controller on my 1/4" capacity electric die grinder, which also serves as a light-duty tool post grinder for my lathe. It's sort of like a Dremel tool on steroids and it uses 1/4" shank grinding wheels and carbide burrs. It's a lot more versatile than the air die grinders I've had back when I was doing a lot of porting and polishing of race engine cylinder heads. Some of the speed controllers are more efficient than others. In general, "You get what you pay for!"
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!