What are recommended maximum speeds (rpm and m/s) for Grinding Discs? Size of Disc Maximum RPMs 3”, 75mm 21,800rpm 4”, 100mm 15,200rpm 4.5”, 115mm 13,600rpm 5”, 125mm 12,250rpm 7”, 180mm 8,500rpm 9”, 230mm 6,500rpm Why is the maximum speeds (rpm or m/s) for grinding wheels so important? What material should I use a Grinding Disc on?