Unmounted Aluminum Oxide Flap Wheels are built to last. Manufactured using premium aluminum oxide grains and x-weight cloth flaps which is more stiff, durable and thick than a paper or film backed alternative. Each 4 inch unmounted flapper wheel is designed with their flaps arranged in a circular fan shape around the center arbor hole. This design increases the useful life of the flapper wheel and provides flexibility to grind flat or curved work pieces. As the flap wheel with a hole is used the flaps wear down, exposing new fresh aluminum oxide grains which provides a long duration of consistent cuts. Additionally, the fan shape of these 4 inch flap wheels allows for added flexibility which makes them ideal for grinding irregular shaped or contoured work pieces. Unmounted large flap wheels can conform to most shapes and are especially effective on pipes, tubes and other cylindrical items and are designed to be used on a bench grinder, pedestal grinder, or straight finishing tool with a 5/8 inch arbor. So if you have a flat or irregular surface made of metal, wood, rubber, fiberglass, or plastic that you need to grind, blend, deburr, clean, polish, or sand the flapper wheel is the abrasive you're looking for.