Chip formation in the machining of hardened steel

With the availability of polycrystalline cubic boron nitride (PCBN) it is possible to machine very hard gears, etc. at speeds of (60-150 m/min = 200-500 fpm). When this is done using PCBN tools in face milling, chip formation is of a cyclic saw toothed type. This type of chip formation is reviewed in relation to other types of cyclic and noncyclic chip formation. The root cause of high frequency, saw toothed chip formation is found to be periodic gross shear fracture extending from the free surface of the chip toward the tool tip and not adiabatic shear as commonly believed.