Sensing of Drill Wear and Prediction of Drill Life

High-speed steel drill life is found to be a strong function of the work material hardness (T_l α H_B⁻¹⁶) when drilling cast iron. This strong dependence may account for the large variation in the life of the drills, which is observed in industrial conditions. Torque, thrust, and power are observed to be functions of drill wear. In the drilling of cast iron using H.S.S. drills of 10.32 mm (13/32 in.) dia at 690 rpm or 22.25 M/min (73 ft/min) maximum, the following relations were observed: where H_B = Brinell hardness of work material, d = diameter of the drill, f = feed per revolution, w = average flank wear, and r = radius at the cutting edge (0.01 mm or 0.0004 in.) all in consistent units. Flank wear of a drill increases rapidly at the end of its life. Torque and thrust reflect this change but are also influenced by workpiece hardness. Therefore, torque and thrust can be used as variables for drill wear sensing only when there is a close tolerance on the workpiece hardness, of the order of ±5 percent.