A grate is a rectangular piece of steel that contains a pattern of holes which helps size feed material and allows it to pass from the grinding chamber to the discharge belt. After the holes are cut into the steel, the rectangle is bent in a radius along the horizontal axis. In a Peterson grinder, four grates are installed in the grinding chamber, one grate is installed on the anvil housing and the other three rest on grate supports.

Other manufacturers may refer to grates as screens, not to be confused with trommel and star screens.