Trimmable Built-in 5/8-11 arbor Zirconia Z3 grain with Cool grinding Aid. Trimmable backing easily trims down to expose flaps for extended working life. To trim - simply angle the backing edge against a hard metal to expose more flap material. Compacted with 20% more material than regular Z3 flap discs. Active grinding agent provides cool cutting with light pressure. Contaminate-free.