Hammer mills: hammermills Feed Mill Machinery . For a rotor speed of about 3,600 rpm, hammers should be 15 to 20 cm (~ 68 inches long, 5 cm (~ 2 inches) wide, and 64 mm (025 inches) thick The number of hammers used for a hammer mill of 1,800 rpm, should be 1 for every 25 to 35 horsepower, and for 3,600 rpm, one for every 1 to 2 horsepower