Abstract: Apparatus is disclosed for reducing the particle size of various materials (e.g., grains, forage materials, fibrous materials, bulk materials, disposed materials, etc.). The apparatus includes a housing including a circular interior wall, a rotor member which is rotatably mounted in the housing, a plurality of blades or hammers attached at one end to the periphery of the rotor, drive means for driving the rotor within the housing, and feed means for feeding the material to the rotor in substantially full face feed. An annular channel or chamber is defined between the rotor and the interior wall of the housing. The blade members extend into the channel or chamber. After the material has been reduced in particle size by the blades or hammers it passes into a cavity adjacent to the annular channel or chamber and the rotor. The cavity includes an unrestricted discharge opening for exit of the processed material.