The flap wheel is a uniquely designed abrasive tool. It is intended to help workers grind and finish hard-to-reach areas, such as the insides of pipes. It is used to blend edges, clean metal surfaces, provide a final finish for metals, or before painting or plating. The flaps on most flap wheels are made of cloth covered in aluminum oxide, which provides a suitably abrasive surface. There are no pieces that can shatter and fly off, so they're generally safer than other kinds of grinding solutions. Here are some pointers for selecting a flap wheel suitable for one's purposes.
Grit and finish
The flaps on flap wheels are available in a range of 'grits' or coarsenesses. They may be as low as 40 grit or as high as 240 grit, with higher grit flap wheels providing a more refined finish. As these tools are used primarily in fine finishing work and pressure can be varied to get a coarser or finer effect with the same grit, it's possible to work flexibly with grit options to reach a satisfactory result. The flaps are flexible, making them especially suited for contours and complex shapes, and less so for stock removal work.
The shank on a flap wheel is slotted into a collet on a grinder or into an electric drill. To minimize vibration, the shank has to be well-balanced and fit snugly into the collet. While most flap wheels come in a universal ¼" shank diameter, it's still useful to check the specifications of one's tools to make sure they're selecting compatible products, as ⅛" shanks are also available on the market. It's also possible to obtain unmounted flap wheels. These don't come with an attached shaft and are meant to be used with bench/pedestal grinders.
The length and width of each flap determine the size of the wheel as a whole. Smaller wheels can be used with hobbyist multitools (such as Dremel tools) for fine finishing work and can fit into narrow pipes and tubes for blending and cleaning applications. Larger wheels (having diameters of 3" and above) are used for more general-purpose surface finishing with tools like hand drills since more powerful motors are required to spin larger flap wheels. The wheel's diameter also impacts how long it can be used, as the material wears with use and will eventually wear down the entire flap.