Aluminum oxide grain.Medium bond for longer life and fewer wheel changes.Use this saucer shape (Type 28) for uniform grinding of large flat surfaces.Type 28., Type 28 saucer shape allows greater surface area coverage and provides the operator with greater grind visibility.Additional fiberglass reinforcement 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.