an offset profile of the grooves or ridges 9 a the working layer 10 is relatively lightweight typically weighing about 15 25 pounds when the outer peripheral surface 15 of the wheel 9 contains ridges 9 a the inner peripheral surface 17 of the working layer 10 will contain grooves 10 a when the outer peripheral surface 15 of the wheel 9 contains grooves 9 a the inner peripheral surface 17 of the working layer 10 will contain ridges 10 a the grooves or ridges 9 a and the grooves or ridges 10 a permit the working layer 10 to be rotated onto screwed onto the outer peripheral surface 15 of the wheel 9 an optional stop rim or ring 8 b acting as a stop or seat for a back edge of the working layer 10 the grooves or ridges 9 a 10 a can be one or more continuous spirals or a plurality of continuous spirals the wheel 9 can have an optional stop surface extending outward from the outer peripheral surface of the wheel on the side of the wheel closest to a drive for the chopper the purpose of the stop surface is to prevent the working layer 10 from rotating too far so that its leading side goes past the chopper drive side edge of the surface 15 of the wheel 9 the stop surface can be continuous around the periphery of the wheel.