Abstract: A grinder for reducing particle size of an organic matter, for example herbs, includes a housing assembly supporting a gauge screen therein to define a cutting chamber thereabove and a receiving chamber therebelow. A plurality of fixed blades are mounted in the cutting chamber in fixed relation to the housing assembly and a plurality of rotating blades rotate relative to the fixed blades to reduce size of the organic matter by cutting, shearing and/or grinding. The gauge screen is replaceable with other screens of different gauge independently of the fixed and rotating blades. The blades are bidirectional and include guide surfaces on the fixed blades which guide material downwardly and into the direction of rotation relative to respective cutting edges of the blades as the rotating blades are rotated relative to the fixed blades in either direction.