... twist drill GB/T613.2-96 D6~D40 Hss Step drills from 4-39 mm. Coated with a ... 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, ... contractor, auto mechanics, and general contractors. Surface treatment available: White Finish, Black Oxide, Titanium Coated, Cobalt Coated, TiALN Coated ...