The Irwin Step Drill Bits are designed for precise drilling in thin materials and have a single flute for maximum drilling control into sheet metal. Their unique flute design ensures perfect round holes in sheet metal. A fluted shank also prevents the drill bit from slipping in the chuck, along with a self centring point that will not walk and eliminates centre punching. Made from industrial grade M7 high-speed steel. Irwin Step Drill Bit. Number of hole sizes 13. Hole diameters 5, 7.5, 10, 13, 16, 19, 21, 23, 26, 29, 31, 33, and 35.