(reprinted from 2009)
It's a Lathe motor (1 HP) I got at a garage sale for $20. It has the advantage of variable speed settings by adjusting the belt. It has plenty of power for this use.
I first noticed that when hooked up to electricity the mill worked so well that it was spitting some wheat and cuttings out the sides making a mess and also throwing some on the floor. I wanted to eliminate the waste and reduce the likelihood of mice - so I built a snug-fitting cover (see pic) that slides into place over the mill end. the cover pretty much eliminates most all of the side spray directing it down to the bin in the bottom.
This thing now works great - we can grind a pound of hard red wheat or corn or whatever, into a fine flour in about 3 minutes. This compares nicely to the 20-25 minutes of hard hand cranking this thing used to require. In a power-off scenario I'll hook it up to my stationary bicycle. I still need to finish that.