Camel Grinding Wheels' Z-Thru flap discs have laser cut flaps precisely layered to create a transparent grinding zone, enabling users to view the work piece while grinding. Z-Thru flap discs are made from zirconia alumina grain, and a heat-treated poly/cotton backing material. The flap discs are available in 4-1/2" and 5" diameters, 40-, 60-, and 80-grit sizes and 7/8" or 5/8"-11 arbor holes.