Abstract: The width of the clearance between a rotor with orbiting beaters and a pivotable impact plate of a machine for comminuting coal, rock, and like materials is adjusted by a computer which controls a reversible motor serving to pivot the impact plate relative to the path of orbital movement of the beaters. The computer causes the plate to pivot toward the path of the beaters when it receives a signal that the admission of material into the housing of the comminuting machine is interrupted and a converted acoustic signal denoting that the machine generates sounds which are indicative of completed evacuation of material from the housing. The direction of pivotal movement of the plate is reversed when the computer receives a series of converted acoustic signals or a single converted acoustic signal denoting that the width of the clearance has been reduced to zero, i.e., that the beaters actually strike the adjacent portion of the plate.