High Speed Steel Titanium Step Drill Bit 4-39mm ?Unique structure of the ladder, variety of standard size aperture, equal to a box of drills Used to cut holes in a variety of material like steel, wood, thin iron, PVC sheet, etc Cone cutter diameter: 10mm hex shank More different sizes, more choices, step sizes: 4, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39 mm Double flute for easy control, safe used