3M coated 546D Type 29 flap disc uses alumina zirconia as the abrasive material and has a diameter of 7 in. 3M incorporates this alumina zirconia material into the flap disc forming a grit of 40. All abrasive products that rotate or spin, such as the ones in this category, have a maximum speed at which you can run them. This 3M flap disc can be run at a maximum speed of 8600 RPMs. The 3M flap disc uses fiberglass for the backing plate.