What Type of Grinding Wheel Should I Use?
Finding the best type of grinding wheel for your application.
Operators use grinding wheels for both non-precision and precision grinding tasks. Non-precision grinding refers to using a power tool to manually grind welds, casts, and tools. Precision grinding relies on machine grinding, and allows for cylindrical, internal, centerless, surface, and thread grinding. The process of precision grinding involves specific parameters that control the surface finish, size, and geometry of a workpiece.
Select Abrasives and Grit Size
Selecting a grinding wheel for non-precision or precision grinding depends on the type of material, the hardness of the material, the amount of stock removal, and the desired surface finish. Material type and its hardness indicate the selection of the abrasives and grit size. Grinding wheels use abrasion to remove material. By using the abrasive grains found on either the face or the sides of a bonded grinding wheel, the process of grinding cuts chips from a workpiece. The type and arrangement of abrasive grains define the structure, allowing operators to match the structure of specific grinding wheels with applications.
Most grinding wheels consist of either aluminum oxide or silicon carbide abrasives. The process of manufacturing these synthetic abrasives provides precise control over the properties of the abrasive grain. Therefore, manufacturers can produce grinding wheels specific to each application. Aluminum oxide works for grinding most steels; non-ferrous cast alloys; and anneal, malleable, and ductile iron. Variations of aluminum oxide, such as white aluminum oxide and brown aluminum oxide, contain enhanced cool cutting and grinding properties needed for grinding hardened or high-speed steel. Silica carbide grinding wheels are the best choice for crushing softer metals such brass, soft bronze, aluminum, and gray iron, as well as stone, non-ferrous materials, and rubber.
Other types of abrasives include variations of zirconia alumina, used for cut-off and rough grinding of steel and steel alloys. Some precision grinding applications use ceramic aluminum oxide, providing the exceptional hardness needed for grinding firm, durable metals.
Abrasives used for grinding wheels hold specific qualities that affect how the grinding wheel works with each material, including hardness, strength, fracture toughness, and resistance to impact. The performance of a grinding wheel depends on its cutting point. The use of high quality abrasives, which resist dulling and fracturing, prevents the creation of multiple cutting points. Manufacturers classify abrasive grains according to grit size, with higher grit sizes designating finer grains. Coarse abrasive grains have lower numbers and work best for grinding softer materials. Hard or brittle work pieces require grinding wheels with fine abrasive grains.
Select the Bond Material
Finding the optimal amount of stock removal and the desired degree of surface finish depends not only on grit size, but also on the type of material used for bonding. An application that requires the removal of more stock requires the use of coarse grit wheels. The use of finer grits yields close surface tolerances and finer finishes. The wheel consists of abrasive grains bonded together with materials such as vitrified ceramics, phenolic resins, rubber, silicate, natural and synthetic shellac, Magnesite Oxychloride, and metals. Increasing the amount of bond increases the hardness of the grinding wheel.
Selecting the correct grinding wheel also depends on other factors: the decision to utilize wet or dry grinding; the size of the grinding contact area; the type of equipment used for grinding; and the peripheral speed of the wheel. Dry grinding applications use softer wheels to minimize generated heat. In contract, wet grinding applications use coolants to reduce generated heat and can use harder grinding wheels. An application that requires grinding a large contact area should employ soft grinding wheels, while smaller contact areas – used for precision work – should employ hard grinding wheels.
Combined with the grinding application, the type and power of the grinding machine determines the category of abrasive material, grit, bond, and hardness needed for the grinding wheel. An application that requires heavy feeds, high speeds, and severe grinding will utilize a harder grinding wheel. The power of the grinding machine must also match the type of wheel. For example, a grinding machine with a lower power will cause a harder wheel to generate more heat. In turn, the cutting machine speed cannot surpass the maximum speed in revolutions per minute (RPM) of the grinding wheel.
The peripheral speed of the grinding wheel refers to the speed of the grinding edge as it passes the work surface. Reducing the peripheral speed of the grinding wheel lessens the need for a harder grinding wheel. Grinding wheels bonded with vitrified ceramics can turn at a maximum speed of 60 meters/second, while phenolic resins-, rubber-, or shellac-bonded grinding wheels can turn at speed up to 48 meters/second.
Select the Shape
Manufacturers produce straight, tapered, cylinder-shaped, cup-shaped, cone-shaped, plug-shaped, and dish-shaped grinding wheels. Each form provides a different type of cutting face and works for specific applications. For example, cone-shaped and plug-shaped grinding wheels serve as excellent choices for precision grinding. Straight wheels work best with bench or pedestal grinders, while machines used for sharpening tools often utilize cup-shaped wheels.