Aluminum oxide grain. Hard bond designed for edge and bevel grinding and other applications where a harder wheel is preferred. Longer life for fewer wheel changes. Ferrous Metals: iron, steel, welds, etc. Specifications: Max RPM - 12200, 5/8-11 Super-Lock hub. Tech Info: Virgin aluminum oxide grain assures consistent high stock removal for tough applications. All sizes are made with uniform grain throughout, no filler grain used in the backing. All wheels internally reinforced with the highest quality fiberglass for maximum safety. Super-Lock Hub on all 5/8-11 threaded wheels is the safest, vibration-free design.