Early Test Results; Hypotheses for tool failure due to entry (impact) and exit (shear plane rotation) effects have been discussed since the 1960s, with most research papers propounding Pekelharing’s exit failure mechanism [1]. However, these tests demonstrated the entry phase to be more influential on tool life when machining carburised AISI 1040 steel. Fig.3 Tool Life in Machining Minutes against Interrupt Geometry for High CBN Content (CBN-H) and Low CBN Content (CBN-L) Tools. (Also shown is the novel workpiece design which allowed the differentiation of entry and exit effects) Fig.4 Exit Plane Surface of an Interrupted Workpiece after Hard Machining (X600 mag.) Note the Brittle Fracture Surface. Previous Authors have Reported Evidence of Intense Shearing on the Exit Plane Surface [2], pointing to a Workpiece Specific Tool Failure Mechanism Further Work; An extensive research program has been initiated with state-of-the-art parameter monitoring including optical profilometry (Fig.5) tool wear measurement, and high frequency direct cutting force monitoring using a custom designed toolholder and a high speed load cell. References; [1] Pekelharing A. J., 1984, The Exit Failure of Cemented Carbide Face Milling Cutters Part 1 – Fundamentals and Phenomena, Annals of the CIRP, 33 (1), pp. 47 – 50 [2] Olvera O., Barrow G., 1998, Influence of exit angle and tool nose geometry on burr formation in face milling operations, Proc. Inst. of Mech. Eng., 212 (Part B), pp. 59 – 72 Interrupted Hard Turning Timothy Halpin Supervisor: Prof. G. Byrne Collaborating Company: Element Six Ltd. Research Premise and Objective; Polycrystalline cubic boron nitride (PCBN) is an ultrahard tool material commonly used in the machining of hardened steel. When the process is discontinuous, as with interrupted hard turning, the transient cutting effects can cause premature brittle fracture of these tools. The precise mechanisms involved are not well understood, yet the majority of industrial applications of PCBN tools are interrupted to some extent. This project aims to provide fundamental, rather than empirical, understanding of interrupted machining para-meters and their influence on PCBN tool performance. Finite Element Process Modelling; A mathematical simulation of interrupted hard turning is built to obtain detailed dynamic tool stress and temperature field data as process parameters are systematically changed. The model is 2-D, lagrangian/ explicit. The chip is formed using the Johnson-Cook shear failure parameter ‘ω’, with no predefined crack path. where equivalent plastic strain at failure is given by; where d 1 – d 5 are user defined parameters, ‘p/q’ is the deviatoric stress stress ratio, is the reference strain rate and is a Johnson-Cook non-dimensional temperature parameter. (i) (ii) Fig.1 Hard Turning: (i) 3-D and (ii)Orthogonal Detail Fig.2 Finite Element Model of Orthogonal Interrupted Hard Turning showing Equivalent Plastic Strain (i) with Mesh Shown (ii) No Mesh (i) (ii) Fig.5 3-D Tool Wear Plot, obtained using Optical Interferometry mins 20µm