The 3513 Chopper Wheel has 42/30 slots. It is an additional wheel for the Model 3502 Optical Chopper. This high quality chopper wheel is made of a beryllium copper alloy, which offer superior performance compared to lower cost steel alternatives. This material choice allows for better thermal conductivity, higher strength, tighter machine tolerances, and are non-magnetic, non-sparking, and corrosion resistant.