SMT 626 - Alumina Zirconia Convex Shape. These FLAP DISCS, or SMT, have a slight 6 degree convex form so that grinding on a variety of welds, castings, and other fabrication work can be completed with ease. The special convex surface with its ring of overlapping coated abrasive sheets, adapts to special workpiece configurations with flat or slightly contoured surfaces. A built-in fiber-reinforced consumable backing plate eliminates the need for a separate backing pad. No special center nut is needed with the SMT 626, use your standard mounting nut.