The grain mill is used for crushing the grains to produce grist. This exposes the starches and allows for a much higher efficiency as well as a quicker mash time. Grain mills are anywhere from 2-roll to 8-rolls and this affect the breakdown and separation of the grain from the husk. The object is to rush the grains, but not completely destroy the husks. The husks end up as an important filter bed in the lauter tun, and help provide clarity in the beer. Moreover, if the grain is not milled effectively enough the fragments will stay packed together and will not allow the water to fully permeate. The fineness of the grist needed is in the end dependent on the equipment being used, and this is generally based on the depth of the filter bed.
Today's mills are automated with motors to help drive the rollers. Some mills introduce water or steam at this stage in order to help eliminate the dust that is created. Grains are also sometimes soaked ahead of time for this purpose and to also raise the moisture content prior to milling. The dust that is released is bad news for the brewery as it can spread over distances and can also be very explosive.
The number of rollers vary based on the amount of fine tuning that is needed with the milling. The higher roller count offers more flexibility for dealing with variable consistency in grain. The rollers tension can be adjusted, and this allows for customization based on the job or brewery. Moreover the higher roller count allows for seperation of the different grist size from flour, to course grist, to husks.
The grain mills are normally situated just above the mash tun, and feed directly into it. The flow of grist into the mash tun is important as it needs to properly mix with the water that is being introduced at the same time. For small breweries, 50 lbs bags of grain is simply dumped in. When grain silos are in effect, convery belts can be used to transport the grain to the mill.