TR discs have threaded central hubs that require two-and-a-half turns and mount to Norton TR or 3M Roloc TR back-up pads Ceramic aluminum oxide is a self-sharpening grit that is harder, tougher, and lasts longer than aluminum oxide "Supersize" lubricant helps cut faster and cooler for an improved workpiece finish, and extends the life of the abrasive 3-ply laminate disc for an aggressive cut, as the rigid construction provides durability and long life Used with a molded rubber or foam backing pad (sold separately)