Master Mechanic, 4″ x 1/4″ x 5/8″ Arbor, Masonry Depressed Center Grinding Wheel, Double Reinforced With Industrial Grit & Glue For Increased Life & Durability, Superior Friability Allows Wheel To Break Down Quickly, Consistently Exposing New Grain For Increased Sharpness & Longer Life, For Low Angle & Vertical Grinding, Max RPM Rating 15,200.