HSS6542 M2 stee Spiral Groove Step Drill Bit,4-20mm Suit for use in hardness under 28 artifacts,(Such as: Stainless steel , iron plate, aluminum, wood ) You can use water to cooling it when it is work and it can be used for even longer 4mm to 20mm 9 Steps (4mm, 6mm, 8mm, 10mm, 12mm, 14mm, 16mm,18mm,20mm)