Reducing the RPM's generally produces a lower percentage of fines, although the time required to grind is increased. The number and condition of the hammers will affect fineness. Moisture content of the grain determines particle size. Low moisture grain will shatter more creating more fines than normal moisture content of 10 - 12 percent. Increasing the flow rate of grain through the hammer mill will tend to increase particle size, but may result in more variation of particle size.
Roller mills have the advantage of creating a more uniform particle size than a hammer mill.
Roller mills normally have a higher initial purchase cost but are less expensive to operate. Roller mills are limited to non-fibrous products. Most roller mills will use a saw tooth or Dawson configuration. To get a particle size of 800 microns, it is necessary to have rollers with 10 to 12 grooves per inch. Most manufacturers will also recommend a differential drive of 10-25 percent with rolls turned so that the sharp edge of each roll meets the grain.