1/4" (6.35mm) Shaft Diameter EXTREME BLACK Carbide Tipped Kutzall Burrs. FOR EXTREMELY AGGRESSIVE CARVING!  Long lasting tool designed for use with rotary hand tools/drills with speeds of 4000 rpm or more. Effective on "difficult to cut" materials such as: Fibre glass, Wood, Rubber, Silicone, Plastic, Polyurethane, Epoxies. Extremely sharp, long wearing conical teeth provide superior stock removal and WILL NOT LOAD/CLOG on most materials. They remove material with smooth cutting action, making them very easy to control. Available in a wide range of shapes and shaft sizes. NOT FOR USE ON METAL.