Grain mills are used to crack the husk of malted barley and other grains used in brewing so the brewing water can gain access to the interior. Most brewing shops have mills to crack grains as they are purchased because they stay fresher prior to being cracked. Brewers who use the all grain brewing process, and brew frequently, tend to purchase their own mills to allow purchase of bulk sacks of grain for cost savings. Roller mills are generally regarded as providing an optimal crush for all grain brewing due to the fact that they crush while leaving the husks intact. Typical cost is $100-150 USD depending on options. These mills can be cranked by hand or turned with a hand drill or even dedicated mounted motors.

External Links

http://www.homebrewtalk.com/showthread.php?t=12858

http://www.zymico.com/hhhmill.htm