The hammer mill is used to reduce the sample size, previously crushed to 5 mm size with the Laboratory crusher, in order to perform various tests as, for instance, the chemical properties of aggregates. The grinding operation is obtained by  the combination of three efforts: impact, shear and rebound. Once entered into the grinding chamber through the hopper, the material reaches the required fineness and through the filtering hoses arrives to the collector. The machine is supplied complete with screens of 3, 2 and 1 mm opening. The material passing to the 1 mm screen goes down practically to powder.