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.Type 27., Highest quality 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 are internally reinforced with the highest quality fiberglass for maximum safety.Our unique Super-Lock??????????Hub combines both a mechanical locking system and chemical bond providing one of the safest, vibration-free designs in the industry.