A mixing apparatus for pulverulent materials includes guide rods carrying a pair of opposed pressure plates having centers aligned along a common axis of rotation. A tubular frame encloses a three-dimensional volume disposed about said guide rods and pressure plates to shield the guide rods and pressure plates from inadvertent contact, and to mount an electric motor. The pressure plates are locked in position by a transmission shaft which is geared to a lead screw which engages the pressure plates.