A Corona Mill pushes the grain between two flat plates, one of which rotates. This mill is cheap, and very effective at crushing grains. It can be motorized very easily, by adding a 5/16” carriage bolt with its head cut off and drill, instead of the crank that comes with the mill.  We recommend also purchasing the Extra High Hopper for greater capacity.  David  Weisbergs book “50 Great Homebrewing Tips” has instructions on "supercharging" a Corona Mill.

Update: The new version of the Corona Grain Mill now comes with the Extra High Hopper included. The Extra High Hopper can still be purchased for older models that did not come with the Extra High Hopper.