Aluminum oxide grain.Soft bond for fast stock removal.Type 27., The "softer" a grinding wheel is the faster it will remove metal.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.Part Numbers 20002 and 20012 are made in Italy.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.