Abrasive wheels and discs are often used for similar applications like sanding or grinding metal, wood and other materials. They both use grains of abrasive material, including aluminum oxide, silicon carbide, and zirconia alumina, but abrasive wheels and discs are manufactured differently.
Abrasive wheels are bonded, meaning the grit material is mixed with a binder of resin for resin bond wheels, clay for vitrified bond wheels, and rubber for rubber bond wheels. Resin bond is the most commonly available bond. The binder and grit mixture is shaped into wheels with a variety of profiles, including flat, depressed center, saucer, and tapered.
To create resin bond wheels, the wheel is "cured," meaning it is heated to less than 500 degrees F. Resin bonds soften under the heat of grinding, releasing dulled abrasive grains to expose new grit, and are more resilient than vitrified wheels. Some wheels bound with resin are fiber reinforced.
To create vitrified bond wheels, the wheel is "fired," meaning it has been heated to more than 2,000 degrees F. Vitrified bond wheels are less affected by the heat of operation and are harder than resin bond wheels, but they are broken down by the pressure of grinding and need to be dressed to expose new grit.
Rubber bond wheels are commonly chosen for use in wet cutoff applications and for very thin wheels.
Bonded abrasive wheels are given a manufacturer grade, which refers to the hardness of the bonding material in the wheel, and rated between D (softest) and Z (hardest). Also consider the profile or shape of the bonded abrasive wheel which conforms to a universal standard. For example, a Type 01 straight (or flat) wheel from one manufacturer is essentially the same as a Type 01 from other sources.
Sanding discs are coated with abrasive grit material adhered to a backing of paper, plastic or cloth. They are used with tools such as vertical shaft grinders, right-angle grinders, orbital sanders, and disc sanders. Discs may be used with backing pads, also called back-up pads, (sold separately), depending upon the application.
Non-woven abrasives can be made into either wheels or discs. Thinner, non-woven shapes are generally called discs; thicker, non-woven shapes are often called wheels. Non-woven abrasives have grit held in a flexible nylon web. They are considered less aggressive than bonded or coated abrasives, although there is some overlap. Due to the distribution of abrasive grain throughout the non-woven disc or wheel, the abrasive performs consistently throughout the life of the product. The open structure of nylon makes the wheel or disc clog-resistant and helps it conform to most workpiece surfaces.
When selecting a disc or wheel, consider the grit size, or grit number. With a lower grit size like 40, there are fewer and larger grains in a given area, making it "coarse" and commonly chosen for removing excess material and shaping a workpiece. With a high grit size like 400, there are more and smaller grains in a given area, making it "fine" and creating a smooth surface finish.
The next consideration is grit material type. For example, aluminum oxide is the most common abrasive material, chosen because it is durable and compatible with many different materials. Zirconia alumina has a faster cut rate than aluminum oxide and is self-sharpening. Zirconia alumina is most commonly used on metal and wood. Each grit material has different attributes, like brittleness and hardness. Hard grit materials, like silicon carbide, last longer and are useful for removing rust, grinding masonry, and other rough work.
Another important factor is how the wheel or disc is constructed: coated, bonded, or non-woven. Coated abrasive discs use an adhesive to attach the abrasive grains to a backing. Bonded abrasive wheels consist of abrasive grains distributed throughout a binder material. Non-woven discs and wheels have abrasive grains held in a flexible nylon web.
Another consideration for coated abrasives is backing type. Paper backing is flexible, and offers a more consistent finish than a cloth or fiber backing, while being thicker and more durable than a polyester film backing. Cloth backing is stiffer, tougher, and thicker than a paper or film backing. Cloth resists tearing while bending and flexing during use. Fiber backing is more durable, stiffer, harder, and thicker than other backings, for a more aggressive action that is often intended for heavy material removal applications. Plastic film backing is thin, flexible, and uniform. This means that film backings offer more control than other backings, for a more consistent surface finish.
Another consideration for discs and wheels is fastening type, which indicates how a disc or wheel attaches to a tool or backing pad. There are various fastening types, such as metal hub quick-change, plastic snap quick-change, hook and loop, threaded, and adhesive-backed. A standard fastening type refers to a single hole, called an arbor hole, in the center of the disc or wheel.
Lastly, consider the arbor hole diameter, which indicates the size of the central hole in the disc or wheel, and the thickness of the disc or wheel, which refers to its smallest dimension.