Each grain mill has some mechanism for crushing, beating or grinding grain into meal, usually in a range of textures from coarse to fine.
Some mechanisms are more versatile than others: They grind hard, soft, oily or wet items. Aside from the increasingly popular “oat roller, ” two milling mechanisms dominate the home mill market: burr and impact.
Burr mills are the most common. They have two grinding plates, one fixed and the other rotated by a power source. The grain is fed into a gap between the burrs, which are grooved to aid the shearing and crushing of the grain. Composite stone burrs are constructed by pressing natural or artificial stones(and sometimes metal cutting blades) in a bed of cement. Metal burrs/plates, some of which are flat and some cone-shaped, are constructed of hardened cast steel or other metal. Basically, stone burrs tend to crush the grain, and metal burrs tend to break and shear it.
Impact mills employ two flat stainless steel heads with concentric rows of “teeth” that spin within each other at high speeds. Grain drops into the mechanism and is hammered, rather than ground, into flour. These very fast, efficient electric-only mills handle most dry, non-oily grains, but can only produce fine, not coarse, flours.