A ball mill, a type of crusher, is a cylindrical device used to grind (or mix) materials like ores, chemicals, ceramics and paints. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding medium, mostly stainless steel balls. An internal cascading effect reduces the material to a fine powder. Industrial ball mills can operate continuously, fed at one end and discharged at the other. Large to medium ball mills are mechanically rotated on their axis, but small ones normally consist of a cylindrical capped container that sits on two drive shafts (pulleys and belts are used to transmit rotary motion). A rock tumbler is a similar device.