Influence of the undeformed chip cross section in finishing turning of Inconel 718 with PCBN tools

Abstract Inconel 718 is a difficult-to-cut material due to its poor thermal conductivity, severe work hardening and high strength at high temperature. Machining of Inconel 718 with Polycrystalline Cubic Boron Nitride (PCBN) tools at high speed in finishing operations may lead to brittle fracture in the cutting tool because of the low toughness of PCBN inserts in comparison to other cutting materials. PCBN inserts shaped with large tip radius are commonly used to remove small values of depth of cut in order to counteract these aspects. When applying multipass turning strategies with this configuration, a significant peak may occur in the response of the machining forces at the end of the pass in the second and successive passes. This fact may also be present when turning inner profiles or into a shoulder. The increase of the undeformed chip cross section and the high specific cutting forces of Inconel 718 give rise to this peak of the machining forces. In this work, different tests involving multipass finishing turning in Inconel 718 have been carried out in order to study the magnitude of the peak forces for different cutting conditions. A PCBN tool at high cutting speed with coolant has been employed at different feed rates and depths of cut. On the other hand, an analytical study of the undeformed chip section at the end of each turning pass has been performed in order to determine its relation to the machining peak forces. The results have shown that these peaks in the machining forces are strongly related to the tool tip radius and the cutting parameters. The machining force at the end of a turning pass can increase significantly their values during a short interval. Consequently, this effect may influence tool wear progression leading to a premature breakage of the tool.