... short twist drill GB/T613.2-96 D6~D40 Hss Step drills from 4-39 mm. Coated with a TIN ... 10, 12, 14, 16, 18, 20 mm 5-28 mm step: 5; 8.8, 11.8, 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, aluminum ...