Some observations on the flow stress of metals during metal cutting

Abstract

Orthogonal cutting experiments have been carried out on a number of high purity metals using low cutting speeds (0.1–1 mm s −1) and a large range of depths of cut (1μm–100 μm). The metals used were copper, gold, aluminium, iron, magnesium, zinc and titanium. Cutting was carried out on two machines, an ultramicrotome and a planing machine, to cover the range of depths of cut. In most cases the metals were cut both in the annealed condition and in the fully work-hardened condition. Calculation of the shear stress on the shear plane during cutting showed that this was approximately constant for a particular metal and did not depend on the cutting machine used or the prior condition of the specimen. Comparison of the shear stress with the flow stress obtained from indentation hardness measurements on fully work-hardened metals showed good agreement. Experiments were also carried out on copper single crystals orientated in two positions in relation to the cutting geometry. The first orientation favoured easy chip formation and had the (111) plane parallel to the shear plane; the second orientation was unfavourable for chip formation and had the (111) plane parallel to the direction of cutting. The shear stress was found to be approximately 40% lower in the favourable orientation than that in the unfavourable orientation, with the value in the unfavourable orientation in good agreement with that obtained on the polycrystalline specimen. Scanning electron micrographs of the free surface of the chips cut in the two orientations showed a difference in the characteristic lamellae spacing, from about 5 μm in the unfavourable orientation to 1 μm in the favourable orientation.