Study on Virtual Design Method of Metal Cutting Tools

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0 Introduction Modern metal cutting technology is moving in the direction of high speed, high efficiency, high precision, low cost, resource conservation, and environmental protection. The traditional tool development process needs to go through materials development, tool design, manufacturing molding, cutting test, feedback modification, and production. The development cycle is very long and it is difficult to meet the requirements of modern cutting technology development. In recent years, there has emerged a new technology based on multidisciplinary theory such as modern mathematics and mechanics, computer technology and advanced algorithms—virtual design technology. The application of this technology can be used for numerical simulation of many engineering problems, which can speed up the design of the product and improve the design accuracy and reliability. Virtual design technology can also be applied to the research and development of metal cutting tools. By inputting material performance parameters into computer, establishing finite element model, loading, calculation and other steps, the entire cutting process can be realistically simulated, and the geometric parameters of the tool can be optimized. Applying this technology can not only shorten the design and development cycle of the tool product, but also increase the success rate and reliability of the design. Numerical simulation technology is the core technology of virtual design, and the main analysis method used in numerical simulation technology is the finite element method. In recent years, finite element analysis technology has been continuously developed under the promotion of the development of computer technology, and many excellent finite element analysis software have been developed, which has effectively promoted the popularization and application of virtual design technology. In this study, the internationally-adopted large-scale finite element software ANSYS was used to simulate the changes in the stress of the tool and the formation of the shear angle in the metal cutting process. A series of calculations were performed using the tool rake angle as a variable to verify the tool. The relationship between the rake angle and the shear angle. In the cutting process, there are many factors that affect the above-mentioned change process, not only depends on the tool geometry parameters, cutting amount, etc., but also closely related to the performance of the workpiece material. In the process of numerical simulation, not only material nonlinearity, geometrical nonlinearity, and state nonlinearity have to be considered, but also the selection of the solver and the control of the load step are strictly required. The powerful nonlinear processing capability of ANSYS software can provide powerful help for numerical simulation of cutting process. 1 Modeling and Calculation Finite Element Modeling Establishing the correct finite element model is the key to achieving numerical simulation. In combination with the actual situation of metal cutting, the following issues should be considered when modeling:

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Fig.1 Comparison of yield trajectories of two yield criteria

Establishing a reasonable material model is the primary condition for simulating the cutting process. Considering that the hardness of the tool material is much greater than the hardness of the workpiece material, the tool can be considered as an elastic body when modeling, and the workpiece is constructed as an elasto-plastic model. During the entire cutting process, the material change is non-linear, so it is necessary to determine the material's yield criteria, flow criteria, and reinforcement criteria. In complex stress states, criteria for determining the continuum's entry into the yield state are usually the Tresca yield criterion and the Von Mises yield criterion. The Tresca yield criterion states that the maximum shear stress in a deformed body reaches a certain threshold and enters the yield state. The VonMises yield criterion considers that the deformation body enters the yield state when the unit volume of the deformable body changes elastic energy or the elastic deformation energy per unit volume reaches a certain limit value. Since the Tresca yield criterion does not reflect the effect of the intermediate principal stress s2 on the yield criterion, it is mathematically and geometrically discontinuous; the geometry of the Von Mises yield criterion is an external one in the s1-s2 stress plane. In Tresca's hexagonal ellipse, this eliminates corner points on the yield trajectory and solves the mathematical discontinuity problem. The yield trajectory of the two yield criteria is shown in Figure 1. For most metallic materials, the Von Mises yield criterion is more consistent with the experimental data. Therefore, the Von Mises yield criterion was chosen as the criterion for determining whether the material entered the yield state. The material will be fluid in the process of thermal softening. The development direction of the single plastic strain components eplx and eply can be described by the flow criterion. Its expression is

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(1) where: epl—the plastic strain of the material l—the plastic increment coefficient Q—determines the stress function in the direction of the strain of the material. With the development of the plastic strain, the yield criterion can be enhanced by two kinds of isotropic and follower reinforcements. The criteria are described in this study. The multi-linear isotropic reinforcement criterion (MISO) is selected in this study. It adopts the method of inputting up to five stress-strain data points to represent the stress-strain curve. It is applicable to comply with the Von Mises yield criterion and is proportionally loaded. Situation and large strain analysis. In the process of chip formation, the displacement of the element in the chip and the change in orientation of the element affect the overall stiffness of the model. This is a geometrically nonlinear problem including large strain and large deflection. For such problems, the large strain effect can be activated. The equation iterates a correct solution. There is friction between the tool rake face and the chip and between the tool flank face and the machined surface. In order to correctly describe the friction model, we must consider the state of the whole process of non-linear contact problems, this study selected the rigid body contact mode of the flexible body. Since there is a sticking area and a sliding area on the rake face, and the positions of the two areas are different depending on the cutting conditions, it can be controlled by setting a maximum allowable shear stress tmax, ie, the area where the interface shear stress is lower than tmax is sticky. In the junction region, the region where the interface shear stress is higher than tmax is the sliding region. In order to make the numerical simulation more representative, the cemented carbide WC-TiC-TaC-Co was selected as the tool material. The elastic modulus E was 550 GPa, the Poisson's ratio was 0.3, and A3 steel was selected as the workpiece material. The elastic modulus E is 210 GPa, the Poisson's ratio is 0.3, the ultimate stress is sb=520 MPa, the yield stress is ss=320 MPa, and the ultimate deformation is 20%.

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Fig. 2 Right-angle free cutting finite element model

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Fig. 3 Formation of shear angle

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Figure 4 knife specific effective stress distribution

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Fig. 5 Relationship between the effective stress of each point on the tool rake face and the distance from the tool tip

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Figure 6 Lee & Shaffer shear angle theory

The table corresponds to the cutting angles of the rake angles of different tools. Tool rake angle -15° -10° -5° 5° 10° 15° Shear angle 38° 41° 44° 48° 52° 56° Established as shown in Figure 2 Right angle free cutting 2D finite element model. In this model, the workpiece is divided into 1750 units and the tool is divided into 100 units; the X, Y two degrees of freedom are constrained at the bottom of the workpiece, and the X degrees of freedom are constrained on the left side of the workpiece; the tool is constrained to Y degree. Load and Calculation The horizontal right shift is applied at the right end of the tool. The tool moves to the right at a given speed and with different displacement boundary conditions to form the cutting process. Then it is calculated by the calculation software's solver. The ANSYS postprocessor can observe the formation of shear angles during cutting (as shown in Figure 3) and the effective stresses experienced by the tool (as shown in Figure 4). From this, the size of the shear angle can be measured with AutoCAD software. At the same time, the instantaneous effective stress that the tool receives at any moment can be obtained. The effective stress analysis of the rake face of the tool can be seen from the distribution of the stress lines of the tool shown in Fig. 4. The effective stress at the tool tip is the largest, and it gradually decreases along the rake. The relationship between the effective stress of each point on the tool rake face and the distance from the tool tip is shown in Figure 5. This result is consistent with the conclusion in BA Dynamica's "Kinematic Dynamic Strength Calculation" (published by Machinery Industry Press, 1982). 2 Lee & Shaffer shear angle theory verification Lee & Shaffer shear angle theory material into the yield state, the plastic zone in each point along their mutual maximum shear stress plastic shear, the point of the maximum shear stress The directions are connected to form an orthogonal network, which is a plastically sheared slip line field. Thus, the slip line field in the cutting layer ahead of the rake face of the tool can be constructed as shown in FIG. 6 . AB in the chip layer is the boundary between the plastic zone and the rigid zone, ie, the shear plane. The angle between the shear plane and the direction of the tool displacement is the shear angle. The geometric relationship can be obtained from Figure 6.
f+b-g0=45° (2) where f——shear angle b—friction angle g0—the tool front angle is known from equation (2). When the tool front angle g0 increases, the shear angle f With this increase, the thinner the chips formed, the smaller the deformation and vice versa. This is the Lee & Shaffer shear angle theory. If the shear angle and the yield shear stress of the sheared material are known, the cutting force can be calculated. Therefore, studying the shear angle is an important way to study the cutting mechanism. Verifying the Lee & Shaffer shear angle theory virtual design method with a virtual design method provides a more concise method for the study of the shear angle. Based on the above studies, we kept the other factors in the cutting process unchanged, changed the tool rake angle within the range of -15° to 15°, and established different cutting simulation models for calculation. The variation range of the shear angle was 38°~ 56°. The calculation results of shear angles corresponding to different tool rake angles are shown in the table. The calculation results show that when the tool rake angle increases, the shear angle increases and the deformation decreases. This verifies the correctness of the conclusion. 3 Conclusions Through the finite element simulation and results analysis of the metal cutting process, the following conclusions can be drawn: The finite element method was used to successfully simulate the formation process of the cutting layer during the metal cutting process and the stress-strain change process of the entire shear deformation zone. . The simulation of the formation process of the three deformation zones in the cutting proves that it is feasible to simulate the large strain plastic deformation using this method. When the tool strength is studied, the load condition of the tool must be known firstly, and the boundary conditions of the tool force during the cutting process are very complicated. The finite element method can apply transient boundary conditions to the tool through the simulation experiment process, and can acquire various stress and strain values ​​of any part of the tool at any time during the cutting process, thus providing a series of numerical solutions for the optimal design of the tool. Therefore, the numerical simulation method as an important part of the computer-aided engineering system will certainly become a more effective and more reliable research method in tool theory research and product development. This article is only a preliminary attempt to apply numerical simulation methods to tool research. On this basis, the distribution of the temperature field during the cutting process and its effect on the cutting performance of the tool can be further studied. The fracture mechanics theory can also be used to analyze the tool damage and wear. Based on the results of the comprehensive numerical analysis, Tool structure and geometry parameters are optimized. In addition, the 3D finite element model can be applied to a more comprehensive structure and performance analysis of some complex tools.

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