... straight shank short twist drill GB/T613.2-96 D6~D40 Hss Step drills from 4-39 mm. Coated ... , 15.3, 19, 24.5, 28 mm 40-30 mm step: 4, 6, 8, 10, 12, 14, 16, 18 ... 20, 22, 24, 26, 28, 30 mm 4-39 mm step: 4, 6, 9, 12, 15, 18, 21, 24, ... 27, 30, 33, 36, 39 etc.The Hss step drills are perfectly round holes in steel, brass, copper, ... slipping in drill chuck or bit extension. high precision Hss 2 step drills