... short twist drill GB/T613.2-96 D6~D40 Hss Step drills from 4-39 mm. Coated with a TIN coating ... Best drill results you get with proper lubrication. TheHss step drills have following size:Sizes: 4-12 mm step: 4, ... 21, 24, 27, 30, 33, 36, 39 etc.The Hss step drills are perfectly round holes in steel, brass, copper, aluminum, wood ... slipping in drill chuck or bit extension. high precision Hss 2 step drills