3 HSS Step Drills available in a Steel or Plastic Case. Step Drill Bit hole diameters (mm) include: 4-12, 4-20 and 4-30. The CBN ground and spiral flutes guarantee quiet running and high cutting performance. Especially the chip flow is optimized, so even long, non-breaking chips will be removed easily. The optimized chip flow protects the cutting edges and reduces built-up edges and cold weld marks. The cone makes it easier to withdraw the tool from the material.