It doesn't hurt anything. Dremel sells a speed controller to do exactly
this. It rectifies the waveform and chops it to make pseudo-DC of varying
magnitude. So you can run your motor anywhere from 0 to half its speed on AC.

Grant

Cuezilla wrote:

I was at the bird shop to get my birds nails trimmed. The lady had a
Dremal tool plugged into a sewing machine restate (sp?). It let he
control the speed without having to adjust the speed control switch on
the tool. Would this in any way be damaging to the tool? I have a
single speed tool that I was thinking of doing this with. Seemed like
a good idea.