Evaluation of zircon ia-toughened alumina tool inserts during machining of high-strength steel

Abstract

Previous publications relating to ceramic cutting tools have been concerned mainly with the origin and the evaluation of various types of wear according to the workpiece material [1-4] and the ceramic composition of the tools [5-8]. However, some studies have investigated the influence of cutting parameters on the cutting performance (e.g. cutting force and surface roughness) of ceramic tools. For cemented carbides the situation is somewhat the reverse, with numerous papers published on their cutting performance. Since ceramic cutting tools are more likely to be used under negative rake angle and higher cutting speeds, the cutting performance data for cemented carbide may not be used directly for ceramic cutting tools. In order fully to utilize the inherent potential of ceramic tools in machining modern high-strength materials, it is of value to seek a systematic correlation between the cutting parameters and the cutting performance of ceramic tools. This letter describes the acquisition of cutting performance data for zirconia-toughened alumina (ZTA) ceramic inserts in the machining of hightensile steel. The effects of cutting parameters (i.e. cutting speed, feed rate and depth of cut) on cutting force, surface roughness and tool wear are discussed. Two grades of ZTA cutting tool, namely SN60 and AZ5000, were used in this investigation. They were supplied by Kyocera Corporation (Japan). Less than 10 wt% ZrO2 was present in SN60, whereas AZ5000 contained approximately 15 wt % ZrO2. The inserts were 12.7mm square and 4.76 mm thick and conformed to the International Organization for Standardization specification SNGN 120408. The mechanical properties of these inserts are shown in Table I. Machining tests were conducted on an AIS14340 steel bar (Rockwell hardness 95 + 2). The initial diameter was 150 mm and the length 660 mm. The chemical composition of the steel is shown in Table II. Machining was performed on a Macson lathe. This machine was powered by an 11 kW motor which provided stepwise speed control throughout the range 47-1600 r.p.m. The cutting conditions are shown in Table III. The tool holder used was CSBNR2525N43 (NTK) and the tool angles were ( -6 , -6 , 6, 6, 15, 15, 0.8). The orthogonal cutting force components were measured during turning using a three-axis piezoelectric dynamometer. The signals from the dynamometer were fed through a charge amplifier and recorded on a personal computer. The three force components, viz. feed force (Fx), thrust force (Fy) and principal force (Fz) were measured. At the end of each test the surface roughness of the workpiece was measured with a Surftest 211 (series 178) surface roughness tester. The direction of roughness measurement was perpendicular to the cutting velocity vector. A total of five measurements of surface roughness were taken at random on each machined surface. The tool life of an insert was determined by either the total tool failure or the critical flank wear (VB--0 .3mm) criterion. The flank wear was measured with a toolmaker's microscope. The influences of cutting speed, feed rate and depth of cut on surface roughness of the workpiece are shown in Figs 1, 2 and 3, respectively. The surface finish improved slightly as the cutting speed increased (Fig. 1). With the increase in the feed rate,