Abrasive flap discs are commonly used for grinding and finishing in the same step, helping reduce finishing time and cost, such as when removing a weld bead and smoothing the workpiece around the weld. They use grains of abrasive grit material adhered to a backing of paper, plastic, or cloth, forming a saucer-shaped disc of overlapped pieces of coated abrasive. The action of a flap disc is much like that of a sanding disc: the grinding or polishing occurs on the round face of the disc. Flap discs are commonly used with tools such as vertical shaft grinders and right-angle grinders.
When selecting a flap disc, consider the grit material type. For example, aluminum oxide is the most common abrasive material, chosen for its durability and its compatibility with many different materials. Each grit material has different attributes, like brittleness, toughness, and hardness. Tough grit materials like ceramic aluminum oxide last longer than grit material such as aluminum oxide. Ceramic aluminum oxide is often used for medium-to-high material removal on metals.
The next consideration is grit size. 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.
Another important factor is how the disc is constructed. Flap discs are coated, meaning an adhesive is used to attach the abrasive grains to a backing.
Lastly, consider the fastening type, which indicates how the flap disc attaches to the tool. There are various fastening types, such as metal hub quick-change, plastic snap quick-change, threaded, and adhesive-backed. A standard fastening type refers to a single hole, called an arbor hole, in the center of the disc.