A cutter hub and blade assembly supported and driven for rotational and axial movement in relation to the die face of a die plate in an underwater pelletizer including a positive control of such axial movement to obtain and maintain optimal axial position of the cutter hub and blades during the pelletizing operation and to minimize wear of the mechanical components involved. Positive control of the axial movement of the cutter hub and blades is obtained by a hydraulic/pneumatic actuation system controlling an elongated motion rod that extends through a driven hollow motor shaft. One end of the motion rod is connected with a hydraulic/pneumatic control and the other end of the rod is connected to a cutter hub holder that supports the cutter hub and blades for axial movement toward or away from the die face.