Heat shrinking disc works on steel, stainless, aluminum and brass. Creates heat through friction to allow shrinking of the sheetmetal. Works with compressed air or water cooling (spray bottle or rag). Is good for shrinking, smoothing, blending and relieving surface tension created from damage. Use with electric sander/grinder/polishers and suitable right angle air die grinders. 4 1/2" Discs, recommended RPM 6-8000 rpm, Max RPM 8500. 9" Discs, recommended RPM 3,000 - 4,000, Max RPM 4,500