... short twist drill GB/T613.2-96 D6~D40 Hss Step drills from 4-39 mm. Coated with a TIN coating ... , 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 ... , wood and plastic. Drills up to 1/2" holes sizes with one bit. Ideal tool ...