Cutoff wheels are designed specifically for precision cutting. They are formed by setting abrasive grains with a bond of rubber, shellac, or resin. Reinforced cutoff wheels have a filament added throughout, typically fiberglass, for additional strength. Reinforcement is required for portable tools and situations where a workpiece is not securely clamped. Non-reinforced wheels should be used only on stationary machines. Cutoff wheels can have one of two shapes: Type 1 wheels are uniformly flat and Type 27 wheels have a depressed center. Attachment type can be threaded or straight arbor mount or quick change. Common materials include aluminum oxide, silicon carbide, CBN, and diamond. Choose from very coarse to fine grade, or corresponding grit, depending on material being worked.