In a grinding wheel, the abrasive performs the same function as the teeth in a saw. But unlike a saw, which has teeth only on its edge, the grinding wheel has abrasive grains distributed throughout the wheel. Thousands of these hard, tough grains move against the workpiece to cut away tiny chips of material.
Abrasive suppliers offer a wide array of products for a wide array of grinding applications in metalworking. Choosing the wrong product can cost the shop time and money. This article presents some of the fundamentals of selecting the best grinding wheel for the job.
Abrasives—Grits and Grains
Grinding wheels and other bonded abrasives have two major components-the abrasive grains that do the actual cutting and the bond that holds the grains together and supports them while they cut. The percentages of grain and bond and their spacing in the wheel determine the wheel's structure.
The particular abrasive used in a wheel is chosen based on the way it will interact with the work material. The ideal abrasive has the ability to stay sharp with minimal point dulling. When dulling begins, the abrasive fractures, creating new cutting points.
Each abrasive type is unique with distinct properties for hardness, strength, fracture toughness and resistance to impact.
Aluminum oxide is the most common abrasive used in grinding wheels. It is usually the abrasive chosen for grinding carbon steel, alloy steel, high speed steel, annealed malleable iron, wrought iron, and bronzes and similar metals. There are many different types of aluminum oxide abrasives, each specially made and blended for particular types of grinding jobs. Each abrasive type carries its own designation-usually a combination of a letter and a number. These designations vary by manufacturer.
Zirconia alumina is another family of abrasives, each one made from a different percentage of aluminum oxide and zirconium oxide. The combination results in a tough, durable abrasive that works well in rough grinding applications, such as cut-off operations, on a broad range of steels and steel alloys. As with aluminum oxide, there are several different types of zirconia alumina from which to choose.
Silicon carbide is an abrasive used for grinding gray iron, chilled iron, brass, soft bronze and aluminum, as well as stone, rubber and other non-ferrous materials.
Ceramic aluminum oxide is the newest major development in abrasives. This is a high-purity grain manufactured in a gel sintering process. The result is an abrasive with the ability to fracture at a controlled rate at the sub-micron level, constantly creating thousands of new cutting points. This abrasive is exceptionally hard and strong. It is primarily used for precision grinding in demanding applications on steels and alloys that are the most difficult to grind. The abrasive is normally blended in various percentages with other abrasives to optimize its performance for different applications and materials.
Once the grain is known, the next question relates to grit size. Every grinding wheel has a number designating this characteristic. Grit size is the size of individual abrasive grains in the wheel. It corresponds to the number of openings per linear inch in the final screen size used to size the grain. In other words, higher numbers translate to smaller openings in the screen the grains pass through. Lower numbers (such as 10, 16 or 24) denote a wheel with coarse grain. The coarser the grain, the larger the size of the material removed. Coarse grains are used for rapid stock removal where finish is not important. Higher numbers (such as 70, 100 and 180) are fine grit wheels. They are suitable for imparting fine finishes, for small areas of contact, and for use with hard, brittle materials.
