Abstract: Apparatus for reducing particulate materials such as paper-making pulp, including a housing defining a working chamber having an inlet and an outlet for flow of particulate material therethrough. There is a rotary shaft supported in said housing, and rotors and stators are mounted on the shaft within said working chamber. Radially extending annular rigid refining surfaces are mounted on the outer portions of the rotor plates and stator plates and the rotor plates and stator plates provide complementary rigid refining surfaces mounted in the chamber and cooperating in confronting relation with the refining surfaces so that in the relative rotation of the refining surfaces particulate material flowing through the working chamber and between the refining surfaces is refined. The refining surfaces are capable of having a wobbling motion with respect to the portions of the plates mounted on the shaft.