Effects of Friction and Cutting Speed on Cutting Force

Observations are made regarding the influence of cutting speed and friction on cutting force by way of finite element modeling. Simulations are validated by comparison of cutting forces and chip morphologies for the Al6061-T6. Analysis of cutting forces over a wide range of cutting conditions suggests an important role of the secondary shear zone in the decrease of cutting force as a function of speed, even well into what is considered to be the adiabatic machining regime. The proposition is supported by a decrease in chip thickness and significant increase in temperature at the tool-chip interface as the speed is increased. Temperatures in the primary shear zone rise only modestly and cannot account for the change in cutting force. Furthermore, the effect contributes to the nonlinear increase of forces with respect to feed as opposed to a plowing force by the cutting edge radius. INTRODUCTION In order to improve metal cutting processes, i.e. lower part cost, it is necessary to model metal cutting processes at a system level. A necessary requirement of such is the ability to model interactions at the tool chip interface and thus, predict cutter performance. Many approaches such as empirical, mechanistic, analytical and numerical have been proposed. Some level of testing for model development, either material, machining, or both is required for all. However, the ability to model cutting tool performance with a minimum amount of testing is of great value, reducing costly process and tooling iterations. In this paper, a validated finite element-based machining model is presented and employed to determine the effects of friction and workpiece strength in the secondary shear zone on cutting forces. Typical approaches for numerical modeling of metal cutting are Lagrangian and Eulerian techniques. Lagrangian techniques, the tracking of discrete material points, has been applied to metal cutting for over a decade (Strenkowski and Carroll, 1985; Komvopoulos and Erpenbeck, 1991; Sehkon and Chenot, 1993; Obikawa and Usui, 1996; and Obikawa et. al 1997.). Techniques typically used a predetermined line of separation at the tool tip, propagating a fictitious crack ahead the tool. This method precludes the resolution of the cutting edge radius and accurate resolution of the secondary shear zone due to severe mesh distortion. To alleviate element distortions, others used adaptive remeshing techniques to resolve the cutting edge radius (Sehkon and Chenot, 1993; and Marusich and Ortiz, 1995). Eulerian approaches, tracking volumes rather than material particles, did not have the burden of rezoning distorted meshes (Strenkowski and Athavale, 1997). However, steady state free-surface tracking algorithms were necessary and relied on assumptions such as uniform chip thickness, precluding the modeling of milling processes or segmented chip formation . In this paper, a Lagrangian finite element-based machining model is applied to orthogonal cutting of Al6061-T6. Techniques such as adaptive remeshing, explicit dynamics and tightly couple transient thermal analysis are integrated to model the complex interactions of a cutting tool and workpiece. MACHINING SIMULATION SYSTEM Simulations were performed with Third Wave Systems AdvantEdge machining simulation software, which integrates specialized finite element numerics and material modeling appropriate for machining. The orthogonal cutting system is described in Fig. 1 where the observer is in the frame of reference of the cutting tool with the workpiece moving with velocity v. The cutting tool is parameterized by rake and