Depressed-center grinding wheels are used for stock removal, grinding, blending, cut off, and deburring, typically with right angle grinders. Reinforced wheels have a filament added throughout, typically fiberglass, for additional strength. Wheels can have one of three shapes, all with depressed centers: Type 27 wheels are flat, Type 28 wheels are concave, and Type 29 wheels are convex. Common abrasive materials include aluminum oxide, ceramic, silicon carbide, and alumina zirconia. Choose from very coarse to medium grade, or corresponding grit, depending on material being worked.