- Detail

Finite element simulation of ultra precision single point diamond turning

1 overview

ultra precision machining represents the highest stage of development in terms of accuracy level. Generally, according to the machining accuracy grade, machining can be divided into three different stages: general machining, precision machining and ultra precision machining. With the continuous development of production technology, the boundaries of division are gradually moving forward. In terms of machining accuracy grade, it is generally believed that the accuracy of precision machining is 1m μ、 The surface roughness is RA 0 025m μ； The precision of ultra precision machining is higher than 0.1M μ、 Surface roughness Ra is less than 0.025m μ。 Precision and ultra precision machining mainly include the following three different technologies: (1) ultra precision machining; (2) Precision and ultra precision grinding and grinding; (3) Precision special processing, such as electron beam and ion beam processing technology. Single point diamond turning (SPDT) processing technology (Figure 1) is a common technology in ultra precision machining. Due to the high hardness, strong wear resistance and superior thermal conductivity of diamond, the cutting edge of diamond tools can be very sharp (the cutting edge radius can be less than 0.05m μ Even smaller), and the affinity between diamond and non-ferrous metals is small. For non-ferrous metals such as copper, aluminum and plastics, the method of single point diamond turning can be used for numerical control processing, and ultra precision optical surfaces can be obtained directly

Figure 1 diamond tools and single point diamond turning equipment

finite element method, as a computer simulation technology and solution method, has been widely used in various fields of scientific research. The method of computer simulation experiment reduces the cost of physical experiment and accelerates the process of experiment. In recent years, finite element simulation method is also widely used in the simulation of machining process, as a tool to predict cutting force and workpiece surface quality. This paper mainly introduces the simulation method of single point diamond turning principle using RC

2 principle of ultra precision single point diamond turning

in an ideal state, when ultra precision cancellation machining is carried out with a circular arc edge single point diamond tool, contour peaks and contour valleys are formed on the machined surface of the workpiece, and the distance between them is the so-called theoretical residual height or theoretical roughness (as shown in Figure 2a)

Figure 2 Schematic diagram of single point diamond cutting principle

in the actual ultra precision cutting of plastic metal, the main task of the main cutting edge and the rake face is to remove the metal, the cutting layer occurs shear slip and plastic deformation under the extrusion of the rake face, and then forms chips to flow out along the rake face (as shown in Figure 2b). The shape of the rake face directly affects the degree of plastic deformation, the curl form of chips and the friction characteristics between chip tools, and directly affects the cutting force, cutting temperature, chip breaking mode and machined surface quality. The main cutting edge is the intersection of the rake face and the flank. In fact, the intersection line between the rake face and the rake face cannot be an ideal straight line, but a micro intersection curve. The shape of the curve can be approximately reflected by the average radius of curvature of the intersection line with its normal plane at different positions, which is called the edge radius ρ。 During cutting, the operation of the plastic granulator in front of the cutting edge touches an extremely wide range of areas, and the stress state in the area is very complex. The stress concentration causes the concentration of dislocations in the metal, resulting in plastic deformation and slip separation of the metal. Some metals become chips and flow out along the front cutting surface, while the other part of the metal is ironed and pressed on the machined surface by the rear cutting surface. Because the two parts of metal move in different directions, the metal in front of the cutting edge of the tool must be in a tensile state. The tensile stress reduces the shear resistance of the metal in front of the cutting edge. Under the direct action of the cutting edge, the metal will slip and separate. The smaller the radius of the cutting edge, the more concentrated the stress, the easier the deformation, the smaller the cutting force, and the better the quality of the machined surface. In addition, the metal of the cutting layer is divided into two parts by the shunting line passing through the shunting point O and parallel to the machined surface. The material above the shunting line flows out along the rake face, and the plastic deformation layer below the shunting line becomes the machined surface after being ironed by the blade below the o point. After ironing, the material under the blade produces serious compression deformation, which has a direct impact on the quality of the machined surface

3 finite element simulation of cutting process

3.1 selection of finite element simulation platform

large general commercial software for finite element simulation includes NASTRAN, Aska, sap, ANSYS, Marc, ABAQUS, JIFEX, etc. these software include many unit forms, material models and analysis capabilities, and have pre and post-processing functions such as automatic lattice division, result analysis and display [2]. The finite element simulation of cutting process is a nonlinear problem, and the material will have large deformation, so the simulation platform needs to have the function of lattice adaptive re division. The global lattice division function of RC provides necessary support for this requirement, and RC has strong ability to solve nonlinear problems, and also provides users with rich user subroutine interfaces, so that users can carry out secondary development through FORTRAN program and customize complex material models. Compared with the previous finite element simulation models using ABAQUS, defo cooling and mining 2-element freezing system rm2d, thirdwave advancededge and other software, RC provides users with a more powerful and simpler solution

3.2 establishment of cutting model

one of the key problems of finite element simulation of cutting process is to model and simulate the principle of cutting. In the old modeling method, separation criteria (including geometric separation criteria and material separation criteria) should be preset between chip and workpiece, which can create a lot of economic benefits for enterprises;, That is, when the deformation reaches a preset condition or a physical quantity (such as stress, equivalent plastic strain, or strain energy density) reaches a predetermined value, the chip will be disconnected from the preset position

because RC provides a powerful two-dimensional and three-dimensional global lattice division function, the cutting model will not depend on the separation criteria, which also makes the cutting model more able to describe the actual machining process. In addition, RC has a powerful function of contact simulation, which also greatly simplifies the definition of tool workpiece contact when using RC modeling compared with other software such as ABAQUS

Figure 3 cutting geometric model and boundary conditions

according to the actual machining conditions, use Mentat to carry out two-dimensional geometric modeling, as shown in Figure 3. Among them, the horizontal displacement and growth rate of all nodes on the left side of the workpiece increased by 38.25 percentage points compared with 2014, and the displacement of all nodes on the bottom of the workpiece in both directions in the plane was set to zero. The tool on the right is assumed to be a rigid body, and the workpiece material uses the macro elastic-plastic model provided by RC to input the corresponding material constants, and the workpiece model is globally re divided by lattice. In the model, it is assumed that the cutting edge radius of the tool is zero, and the stress distribution diagram shown in Figure 4 can be obtained by changing the rake angle of the tool for cutting simulation

Figure 4 Comparison of two-dimensional cutting simulation results of different tool rake angles

Figure 5 three-dimensional simulation results figure

if the two-dimensional simulation is similar, the single point diamond orthogonal cutting process can be abstracted as a plane strain problem for three-dimensional finite element simulation. Tetrahedral elements are used in the model to divide the workpiece into lattices. The simulation results are shown in Figure 5. Using the mental post-processing function, the change diagram of cutting force in the cutting process can be obtained (Fig. 6)

Figure 6 cutting force change result figure

4 summary

this paper uses the finite element simulation technology to carry out two-dimensional and three-dimensional modeling and Simulation of the single point diamond turning process, simulates the chip generation and growth process, analyzes the stress distribution in the workpiece and the change of the cutting force of the tool during the machining process, and also discusses the influence of different tool rake angles on the chip shape in the two-dimensional simulation. (end)

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