1. Interrupted Hard Turning Collaborating Company: Element Six Ltd.
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.
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
mins
(i)
(ii)
Fig.1 Hard Turning:
(i) 3-D and (ii)Orthogonal Detail
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.
20µm
where equivalent plastic strain at failure is given by;
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.
where d1 – d5 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.
Fig D Tool Wear Plot, obtained using Optical Interferometry
(i)
(ii)
Fig.2 Finite Element Model of Orthogonal Interrupted
Hard Turning showing Equivalent Plastic Strain
(i) with Mesh Shown (ii) No Mesh
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