Grinding wheels are fabricated out of abrasive particles that are bonded together by a material, according to Sizes.com. Abrasive particles are slowly worn away over time as work pieces are applied to them. As particles wear away, they expose new, unworn particles. The bond between abrasive and bonding agent is meant to fail at a calculated pace. "Hardness " and "softness" are terms that do not refer to the hardness of abrasive particles but rather their rate of wear. There are a large number of acceptable bonding agents for grinding wheels from rubber to certain glasses.
According to Global Spec, there are several types of grinding wheels available to consumers, including straight wheels, type 5 wheels, type 7 wheels and cone wheels. Straight wheels feature no recesses, cups or flaring. Type 5 grinding wheels feature a recess on one side and type 7 grinding wheels feature recesses on both sides. Cone wheels can have a curved or straight edge.
There are a number of abrasive grain materials that are commonly used in the construction of grinding wheels. According to Global Spec, aluminum oxide is used in ceramic grains and silicon carbine is traditionally utilized with brass, aluminum, titanium and other nonferrous materials. Grinding wheels made with aluminum oxide are not appropriate for carbide bit and blade sharpening applications. Alumina-zirconia grains improve performance on stainless steel and other materials. Cubic boron nitride increases the grinding performance on alloy steels. Cubic boron nitride is rated just below diamond in terms of hardness, according to Global Spec.
According to Sizes.com, grinding wheels are typically manufactured to diameters of 5, 6 and 8 inches. Corresponding wheel thicknesses are 1/2, 3/4 and 1inches. Grinding wheels used for side grinding applications are usually manufactured to a diameter of 6 inches with a wheel thickness of 1 inch.
All grinding wheels feature markings that indicate their abrasive type, grain size, grade, structure and bond type. Aluminum oxide (A) and silicon carbide (C) are two types of abrasives. Grades range from A to Z, where grinding wheels grades close to "A" are the softest grades. Structure corresponds to the spacing between abrasive grains. Bond type refers to the bonding agent used to contain the abrasives. Common bonds are rubber (R), silicate (S), vitrified glass (V), elastic (E), resins (B) and oxychloride (O).