A fine-grinding machine composed of a rotating steel cylinder that may be from 15 to 50 ft long and 6 to 8 ft in diameter, within which are steel balls from 1 to 5 inches in diameter. Depending on its construction it may be either batch or continuous. Some types have several compartments, each containing balls of different sizes. Its function is finish-grinding of particulates; it is usually fed with 20-mesh material, which it reduces to 325-mesh.See Ball Mill.