BSDF is a totally different type hammer mill.3-stage means in mill chamber,there are three groups of knives-coarse crushing area, middle-fine grinding area,fine milling area.Material is fed into the mill chamber,pass through the high speed rotation caused by the three groups of knives one by one.Material is crushed three times in one mill chamber,then become fine powder.Between middle fine crush knife group and fine mill knife group,there is a device designed for preventing coarse granule going to fine milling area,so powder fineness is assured.The rotor speed is controlled by inverter,diferent particle size is achieved by adjusting rotation speed.