Machinability tests of several carbon steels, chromium steel, chrome-molybdenum steel and stainless steel are carried out with radioactive carbide tools. The correlation of carbon contents in the carbon steels and variation in heat treatment of the chrome-molybdenum steel with the tool lives are investigated. By such quick tool-life tests, reasonable tool-life equations are obtained for the aforementioned materials. Besides, the relation between machinability in terms of tool life and flow stress in metal cutting is demonstrated experimentally.