Grate position refers to the order in which the grates are installed in the grinding mill. On a Peterson machine, the first grate is attached to the anvil housing and is referred to as the #1 position. The #2 position is just below the anvil housing followed by the #3 and #4 positions. The three grates that follow the #1 position all rest on a set of grate supports in the grinding mill.
With four different grate positions, grates can be mixed and matched to create different combinations depending on feedstock and finished product. With so many variables to grinding (everything from moisture content, feedstock, and even season) there are no set rules and lots of experimentation. However, when using a grate combination, generally, smaller opening grates in the top two positions for sizing followed by progressively larger opening grates to use as exhaust grates. Hole shapes are generally the same but this too can vary based on preferences.