These Shrinking Discs work on aluminium and steel and the secret to them is that they create heat through friction when spinning and touching the metal. When the area is cooled by either a squirt of soapy water or compressed air the heated area cools and shrinks. Discs are available in 2 sizes, 230mm and 125mm. This one is the larger one suitable for use in a 7″ or 9″ grinder or sander. This kit includes the rubber sanding disc that you require to support the disc and also the tapered nut to attach it to your grinder or buff.