The black RSC hard disk is used to renew an edge that has begun to lose sharpness. It is made of carbide impregnated rubber and when used in conjunction with the diamond disk leaves a nice finished edge on the instruments. Please Note: The RSC disk is available in two different shaft shapes, please look at the shafts on your RX Honing Machine
to determine which one you will need. The "R" Shaft is perfectly round and the "D" shaft has a flat side to the shaft.