Research on Cutting Simulation of CNC Lathe

1 Introduction

Virtual machining process simulation mainly includes geometric simulation and physical simulation, as shown in Figure 1. Geometric simulation does not take into account the effects of cutting parameters, cutting forces and other physical factors, only the motion of the tool-workpiece geometry. By translating the NC code to simulate various operations that can be performed by turning machining, such as machining the outer circle, end face, chamfer, thread, curve, etc., verify the rationality and correctness of the NC code and check for collisions and interferences, thereby reducing or eliminating Machine damage, fixture damage and tool damage caused by program errors. Most of the current simulation systems only carry out geometric simulation, that is, the calibration of the tool path and the interference between the workpiece and the tool, and the simulation of the combination of geometric simulation and physical simulation is still relatively rare. The mechanical simulation of the cutting process is one of the physical simulations. Cutting force is indispensable in calculating the cutting power, selecting the tool, designing the fixture and formulating the process. It is very important to determine the reasonable metal cutting amount and optimize the geometric parameters of the tool; it directly affects the cutting heat. Produced and further affected tool wear, tool durability and machined surface quality. Applying the force analysis of the workpiece to the virtual machining process in real time can not only simulate the force change caused by the parameter changes of the workpiece, the tool, etc., but also adjust it in time to help select the most suitable machining parameters.

2. Geometry Simulation (1) Model Establishment The current main 3D modeling methods mainly include CSG (Structural Solid Geometry) method and Brep (Boundary Representation) method, each of which has its own characteristics. The CSG method mainly combines some simple entities into the required objects through set operations, and can also generate some entities by scanning representation. Its main feature is that the coverage area is relatively wide, but it can achieve a limited shape and can be used. The algorithms for generating and modifying entities are limited, and the amount of computation of the graphics is large and time consuming, so a large amount of computation is required during the cutting process. The Brep method accurately describes the 3D model entities based on the surfaces formed by the vertices, edges and faces. According to the body-plane-ring-edge-point hierarchy, the geometric information of all geometric elements constituting the shape and their interconnected topologies are recorded in detail. relationship. This information can be obtained directly in various operations and operations. The advantage of this method is that the stereo or wireframe model can be drawn quickly. The disadvantage of this method is that its data space is large, and the modified design is not as simple as the CGS method. By combining the CGS and Brep methods, the modeling complements the simulation technology, and the functions of cutting the outer circle, the end surface, the grooving and cutting can be realized. For more complicated entities such as threads, the triangle can be approximated. The method implements the simulation.
(2) Tool modeling tools mainly include information such as tool type, tool material, and tool geometry. In the turning process, several kinds of tools such as external knives, cutting knives and thread knives are mainly used. The materials of the knives mainly include high-speed steel and hard alloy, and the angle of the tool includes the main declination, the front angle and the edge. Many angles such as inclination. According to the actual needs, the database of the tool is used. The database contains various parameters that affect the tool performance and the cutting force. It can be pre-set according to actual needs before the cutting simulation processing, as shown in Figure 2.
(3) NC code Compilation of NC code is a combination of uppercase letters that define certain functions, and generally, each instruction completes an action, and several instructions form a program according to a certain structure. It mainly includes preparation function G code, auxiliary function M code and F (speed), S (spindle speed), T (tool number command) and so on. Based on the purpose of numerical control simulation, it is necessary to extract the motion and state information of the workpiece and the tool in the simulation system from the NC program, and convert it into the corresponding state of the workpiece and the tool during the demonstration. The module mainly completes the following functions: 1 Screen editing or input of NC program, or inputting txt file to open; 2 Parsing NC code, mainly checking whether there is instruction mismatch, instruction order error or writing format error; 3 NC program The tool selection is verified; 4 The running NC code program line is highlighted, and if there is an error message, it will automatically alarm, which helps to modify the NC program.
(4) Simulation animation display During the machining simulation process, the tool moves according to the data in the corresponding program of the NC code. When the tool intersects the workpiece, the program re-edits the model data of the workpiece entity by calculation. The interactive part of the workpiece is cut off and redrawed and the current position is recorded in a very short time. At the same time, using OPENGL's double buffering technology, when the front buffer is used to display one frame of the animation, the background buffer is drawing the next picture, which can make the machining process run continuously, so that the discontinuous tool processing track shows approximately continuous. With the simulation effect, the time for executing the entity display is also greatly shortened.

3. Cutting force, cutting power simulation

(1) Cutting force simulation In the process of cutting force simulation, the empirical formula of cutting force is mainly applied. The empirical formula can be divided into two main categories: one is the exponential formula, and the other is calculated according to the unit cutting force. In metal cutting, the use of exponential formulas to calculate cutting forces has been widely used. The empirical formula for the commonly used exponential form is as follows: where: for the cutting force, for the back force, for the feed force. , , and the coefficient determined by the metal being processed and the cutting conditions. , respectively, the product of the correction factor of various factors on the cutting force when the actual machining conditions do not match the empirical formula obtained. When cutting the thread, the amount of backing knife should follow the principle of decreasing, that is, the amount of knife to be used in the back of the knife should be smaller than the previous one. The formula for calculating the cutting force is: where is the pitch and N is the number of passes.
(2) Cutting power simulation The cutting power is mainly the power consumed and the direction is not displaced, so no power is consumed. The cutting power is the rotational speed of the workpiece. During the cutting process, according to the pre-selected workpiece material, tool material, tool geometry parameters, through the NC code, the program automatically queries the spindle speed, the cutting amount of the tool, the feed rate and the corresponding coefficient and index in the database, and substitutes the cutting force. In the empirical formula of cutting power, the cutting force and cutting power are synchronously demonstrated while the tool is cutting the workpiece.

4. Simulated machining examples

Taking a cemented carbide tool with a workpiece strength of 0.65 GMPa as an example, the length of the workpiece is 300 mm and the diameter is 81 mm. The geometric angles of the turning tool are: lead angle, rake angle, and blade inclination angle. After roughing and finishing and finally obtaining the following workpiece (Fig. 5), the cutting force and cutting power are shown in Fig. 6 and Fig. 7, respectively. In the simulation cutting process, at the same time of cutting, the program automatically finds each correlation coefficient from the database and substitutes it into the formula. The parameters are obtained by the cutting point, spindle speed and feed speed of the tool in the NC code. In the process of simulation cutting, the system interface is good, easy to use, through the functions of translation, zooming, rotation, etc., the processing process can be clearly seen, which plays a good role in demonstration processing. The cutting force and cutting power curve are always synchronized with the machining process, and the current force and power conditions can be clearly seen through the numbers in the upper right corner. In this way, it is possible to obtain high-productivity and low-machining cutting amounts by studying the cutting force and the cutting power curve while ensuring the quality.

5 Conclusion

The research of CNC lathe cutting simulation system mainly realizes the following functions:
(1) Verification of the rationality of the NC code, testing the interference, overcutting, collision, etc. that may occur during the machining process, reducing the risk of trial cutting of the CNC machine tool;
(2) A database of some tool and workpiece materials was initially established, which can simulate various states during the cutting process and display various cutting parameters in real time.
(3) Based on the analysis of cutting force and cutting power, the cutting parameters can be optimized and designed from the economic point of view and production efficiency according to the actual production needs, so as to achieve the best turning process.

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