Grinding, or abrasive machining, is the process of removing metal in the form of minute chips by the action of irregularly shaped abrasive particles. These particles may be in bonded wheels, coated belts, or simply loose.
Grinding wheels are composed of thousands of small abrasive grains held together by a bonding material. Each abrasive grain is a cutting edge. As the grain passes over the work piece it cuts a small chip, leaving a smooth, accurate surface. As each abrasive grain becomes dull, it breaks away from the bonding material.

Types of abrasives

Two types of abrasives are used in grinding wheels: natural and manufactured. Except for diamonds, manufactured abrasives have almost entirely replaced natural abrasive materials. Even natural diamonds have been replaced in some instances by synthetic diamonds.
The manufactured abrasives most commonly used in grinding wheels are aluminum oxide, silicon carbide, cubic boron nitride, and diamond.
Aluminum oxide: Refining bauxite ore in an electric furnace makes aluminum oxide. The bauxite ore is heated to eliminate moisture, then mixed with coke and iron to form a furnace charge. The mixture is then fused and cooled. The fused mixture resembles a rocklike mass. It is washed, crushed and screened to separate the various grain sizes.
Aluminum oxide wheels are manufactured with abrasives of different degrees of purity to give them certain characteristics for different grinding operations and applications. The color and toughness of the wheel are influenced by the degree of purity.
General-purpose aluminum oxide wheels, usually gray and 95 percent pure are the most popular abrasives used. They are used for grinding most steels and other ferrous alloys. They are used for grinding most steels and other ferrous alloys. White aluminum oxide wheels are nearly pure and are very friable (able to break away from the material easily.) They are used for grinding high-strength, heat-sensitive steels.
Silicon carbide: Silicon carbide grinding wheels are made by mixing pure white quartz, petroleum coke and small amounts of sawdust and salt, and then by firing the mixture in an electric furnace. The process is called synthesizing the coke and sand. As in the making of aluminum oxide abrasive, the resulting crystalline mass is crushed and graded by particle size.
Silicon carbide wheels are harder and more brittle than aluminum oxide wheels. There are two principal types of silicon carbide wheels: black and green. Black wheels are used for grinding cast irons, non-ferrous metals like copper, brass, aluminum, and magnesium, and nonmetallics such as ceramics and gemstones. Green silicon carbide wheels are more friable than the black wheels and used for tool and cutter grinding of cemented carbide.
Cubic boron nitride (CBN): Cubic boron nitride is an extremely hard, sharp and cool cutting abrasive. It is one of the newest manufactured abrasives and 2.5 times harder than aluminum oxide. It can withstand temperatures up to 2,500°F. CBN is produced by high-temperature, high-pressure processes similar to those used to produce manufactured diamond and is nearly as hard as diamond.
CBN is used for grinding super-hard, high-speed steels, tool and die steels, hardened cast irons, and stainless steels. Two types of cubic boron nitride wheels are used in industry today. One type is metal-coated to promote good bond adhesion and used in general purpose grinding. The second type is an uncoated abrasive for use in electroplated metal and vitrified bond systems.
Diamond: Two types of diamond are used in the production of grinding wheels: natural and manufactured. Natural diamond is a crystalline form of carbon, and very expensive. In the form of bonded wheels, natural diamonds are used for grinding very hard materials such as cemented carbides, marble, granite and stone.
Recent developments in the production of manufactured diamonds have brought their cost down and led to expanded use. Manufactured diamonds are now used for grinding tough and very hard steels, cemented carbide and aluminum oxide cutting tools.

Grinding wheel grade

The grade of a grinding wheel is a measure of the strength of the bonding material holding the individual grains in the wheel. It is used to indicate the relative hardness of a grinding wheel. Grade or hardness refers to the amount of bonding material used in the wheel, not to the hardness of the abrasive.
The range used to indicate grade is A to Z, with A representing maximum softness and Z maximum hardness. The selection of the proper grade of wheel is very important. Wheels that are too soft tend to release grains too rapidly and wheel wear is great. Wheels that are too hard do not release the abrasive grains fast enough and the dull grains remain bonded to the wheel causing a condition known as "glazing."

Grinding wheel structure

The structure of a grinding wheel refers to the relative spacing of the abrasive grains; it is the wheel's density. There are fewer abrasive grains in an open-structure wheel than in a closed-structure wheel. A number from 1 to 15 designates the structure of a wheel. The higher the number, the more open the structure will be; and the lower the number, the more dense the structure will be.

Grinding wheel specifications

Grinding wheel manufacturers have agreed to a standardization system to describe wheel composition as well as wheel shapes and faces.

Grinding wheel markings

Abrasive grinding wheels have a different marking system than CBN and diamond wheels.
Abrasive grinding wheels: This marking system is used to describe the wheel composition as to type of abrasive, grain size, grade, structure and bond type.
CBN and diamond wheels: The same standardization is applicable to CBN and diamond wheels. Wheel markings are a combination of letters and numbers.

Grinding wheel shapes and faces

Most grinding wheel manufacturers have adopted eight standard wheel shapes and 12 standard wheel faces for general use. The illustration shows the most common standard wheel shapes used on all types of grinders. A following illustration shows the standard wheel faces used on most grinding wheel shapes.