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 Zirconia alumina is self-sharpening and durable, allowing for a faster cut rate than aluminum oxide Cloth backing is stiffer, tougher, and thicker than a paper or film backing The dense placement of the abrasive grain in a closed-coat disc for a higher-quality finish than an open-coat disc Used with a molded rubber backing pad (sold separately)