FEM simulation of orthogonal cutting: serrated chip formation
Introduction
The use of the finite-element method (FEM) has proven to be an effective technique to investigate and optimize forming processes. Simulation of cutting processes will be effective for improving cutting tool design and selecting optimum working conditions, especially in advanced applications such as in the machining of dies and moulds. This technique will reduce time consumption and expensive experimental testing.
In plastic deformation processes, such as cutting, extrusion and shearing, the prediction and the control of fracture are a critical issue for producing defect-free parts. Prediction of damage and fracture is necessary to investigate the surface finish and the integrity of the parts produced.
A commercial, general purpose FE code, DEFORM 2D, has been modified to study the material damage and the fracture propagation in orthogonal cutting 1, 2. In the numerical model, material fracture is simulated by deleting the mesh elements that have been subjected to high deformation and stress. Fracture occurs when the critical damage value, calculated using a ductile fracture criterion, is satisfied. The definition of the proper damage value for the separation criterion is a crucial point because it is not easily measurable by experiment. This study is introductory and a more detailed study on the breakage mechanism and on the critical damage values is required.
Section snippets
The customized FEM model
To simulate orthogonal cutting with serrated chip formation, the FEM code DEFORM 2D has been used and customized. New sub-routines have been written and linked to the original code. This new modified version has the potential of simulating material breakage by deleting the mesh elements of the workpiece material when their damage is greater than a defined critical value. The modified code differs from the original code in the remeshing module, where new features are highlighted.
Fig. 1 shows the
Damage criteria
The Cockroft and Latham [3]damage criterion has been used. The damage is evaluated according to the equationwhere Ci is the critical damage value given by a uniaxial tensile test, εf the strain at the breaking condition, the effective strain, the effective stress and σ* is the maximum stress.
The criterion predicts the material damage when the critical value Ci is exceeded. To optimize the material fracture in cutting operations a combined criterion has been used. The Cockroft
Orthogonal cutting simulation
The cutting operation shown in Fig. 2 was simulated. When the diameter of the cylinder is large compared to the depth of cut, the plane-strain condition is satisfied [5]. The orthogonal cutting operation is modelled as shown in Fig. 3. The width of the cutting tool is larger than the engaged workpiece. The model of the simulation is plane-strain and non-isothermal. In the simulations the workpiece is assumed to be rigid–plastic and to have a rectangular shape, see Fig. 3. The latter figure
Simulation set-up
The workpiece material is steel AISI 1045 and the cutting tool material is a high-speed steel (H11). Different cutting tools were used and different cutting speeds were selected. No lubricant is used at the tool–workpiece interface. The cutting conditions are shown in Table 1. Simulation set C refers to a chip breaker tool.
Simulation results: chip formation
In the following figures the chip formation is shown for different tool paths.
Conclusions
The results of this study demonstrate the effectiveness of the customized FEM model (for simulating serrated chip formation in orthogonal cutting) in: (i) predicting chip shapes and the influence of cutting conditions, and (ii) predicting cutting forces and process parameters.
The definition of the critical values is crucial. According to Kim et al. [11], it has been noted that the critical values depend on the operation simulated. For instance the critical damage value in compression or
Future work
In order to predict fracture in cutting more accurately, it is necessary to determine the critical values under realistic deformation conditions (high strain rate and temperatures) and conduct additional studies to compare FEM-based results with the results of experiments.
Different experimental observations were found for orthogonal cutting operation with a tool with a positive rake angle, so that an accurate analysis of the damage critical values is required. Chip segmentation similar to that
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