Matériaux & Techniques
Volume 87, Number 5-6, 1999
|Page(s)||33 - 41|
|Published online||19 April 2017|
Tournage dur : formation du copeau et endommagement d'un outil PcBN
Hard turning : chip formation and PcBN tool damage
LaBoMaP, ENSAM Cluny
2 MécaSurf, ENSAM Aix-en-Provence
Machining materials featuring high mechanical properties (Rm > 2000 MPa) is becoming a process with increasing industrial use. The cutting tool able to cut these very hard materials (> 55 HRC) requires exceptional physical properties. The more widely used are ceramic and PcBN tools. The resistance of these tools during machining depends of course on the cutting parameters and above all on the workpiece hardness.
Intensive research works concerning hard turning technology have addressed fundamental questions concerning chip formation mechanisms, tool wear, surface integrity and geometrical accuracy of the machined components. The major consideration for the user of this technology is the quality of the parts produced. One conclusion of this research is that flank wear of the cutting tool has a large impact on the quality of the part produced (surface finish, shape accuracy and surface integrity). The process reliability is largely controlled by crater wear leading to fracture at the cutting edge. For components with surface, dimensional and shape requirements (bearing surfaces), hard turning technology is often not economical compared with grinding because tool life is limited by the required tolerances (i.e. high flank wear rate).
In hard turning, cutting materials must be improved to reduce the flank wear rate to become suitable for such precision applications. To improve the performance of PcBN while hard turning, it is necessary to understand the wear mechanisms. The conclusion of wear studies reported thus far are contradictory. Sirotin et al.  observed a coating on the flank and the rake face consisting in compounds of elements of the tool and the workpiece material. Narutaki and Yamane  did not find a chemical reaction between cBN grains and pure iron in a diffusion test. A similar test with cBN and a bearing steel was performed by König and Neises  in which a recrystallisation of the binder and some changes of the chemical composition of the binder were observed. The cBN grains themselves showed no chemical reactions with the work- piece material. However, steady-state diffusion reaction trials are not an accurate methodology for predicting wear mechanisms in machining operations. In a diffusion couple test, the reaction products change the reaction rates whereas, in machining, fresh unreacted material is continually presented to the tool as the chip and the workpiece move by.
The lack of knowledge concerning this new process leads us to be interested in the large stresses that the tool can bear. In particular, the specific cutting force applied on the rake face, the slip velocity at the interface chip/rake face, the high dynamic stresses endured by the cutting tool due to the chip formation mechanisms. These different statements lead us to study precisely the evolution of the wear and the damage caused to the cutting tool in order to establish a reliable wear modelling. This modelling will allow us to predict the tool life as a function of the cutting parameters and the workpiece hardness. These research studies have enable us to find a fundamental aspect during hard machining: the effect of cutting speed is similar to the effect of the workpiece material hardness. This main observation is developed in this paper.
© SIRPE 1999
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