Proxxon 28858 Tungsten vanadium micro twist-drills are manufactured from one piece blanks in tungsten vanadium steel for highest concentricity. Tunsten vanadium provides safety from breakage and allows for a longer bit lifetime. These bits are designed for drilling metal, non-ferrous metals, plastic and wood. Recommended work speeds: soft materials approximately 8,000 rpm and hard materials approximately 3,000 rpm.