Some Aspects of Machinability Data Optimisation

Abstract

Face milling is considered for this work in two main parts. Firstly, the cutting geometry of face milling was examined. As the one independent variable, the author used equivalent chip thickness which is the ratio of the area cut (which is a measure of heat generated) to the cutting edge engaged with the workpiece (which is a measure of heat transferred to the chip, to the cutter and to the workpiece) by each tooth, because cutter wear is a function of cutting temperature. The machinability data such as cutter life, power required at the cutter, surface finish and vibrations, were obtained using two different face milling cutters (the Walter Cutter and the Sandvik cutter), two different types of tool steel material (B.H.N. 238 and B.H.N. 197 as used by G.K.N. Ltd) in down-cut and up-cut face milling. The cutter life tests were planned and performed only as a function of cutting speed and of equivalent chip thickness in order to reduce testing time and number of workpieces required. The validity of the equivalent chip thickness was proved. It was found that the equivalent chip thickness gives a guide to the selection of the geometry of teeth commercially available. Cutter life equations were obtained using statistical techniques. The power required at the cutter was expressed only as a function of metal removal rate. The results show both feed and cutting speed affect surface quality and vibration is generally generated by cutting force not by the chatter phenomenon. With the necessary backlash eliminator, down-cut face milling showed better performance (in terms of machinability data, initial contact point at entry conditions) than up-cut face milling. Shorter cutter life was obtained in central milling than in up-cut and down-cut milling, because the width of the workpiece was relatively small according to the cutter diameter used. Secondly, using the cutter life, metal removal equations and T - MR (T is cutter life; MR is metal removal rate) characteristic function idea, MR was expressed only as a function of T. Then the economics of face milling were examined.