Multi-axis NC machining programming technology for sculptured curved mixed-flow blades Abstract: Runner blades are the key components of energy conversion of hydraulic turbines and the most difficult parts to machine. At present, multi-axis NC machining is the most effective machining method to solve this kind of large sculpture curved parts. Multi-axis NC machining programming is the most important link to realize its high precision and high efficiency machining. This paper introduces the key technologies involved in five-axis NC machining programming of large sculptured surfaces of Francis turbine blades, such as three-dimensional modeling of runner blades, tool path calculation, cutting simulation, machine tool collision simulation and post-transformation. Through the link and research of these technologies, the multi-axis linkage machining of large blades is developed and realized. Keywords: Introduction to NC programming: Hydraulic turbine is the prime mover of hydropower generation. The manufacture of turbine runner blades and the quality of runner have great influence on the safe, reliable and economical operation of hydropower units. Turbine runner blades are very complex sculptural surfaces. In the manufacturing process of large and medium-sized units, the long-term manufacturing process of "sand casting-grinding wheel shovel grinding-three-dimensional template detection" can not effectively ensure the accuracy and manufacturing quality of blade profile. At present, five-axis NC machining technology is a cutting-edge high-tech in the field of machining. The NC machining programming of large complex curved surface parts is the most important technical basis to realize its digital manufacturing, and its NC programming technology is a process of digital simulation evaluation and optimization. Its key technologies include: three-dimensional modeling and positioning of complex parts, five-axis linkage tool path planning and calculation, tool axis control technology for machining sculpture surfaces, cutting simulation and interference inspection, and post-processing technology. Multi-axis NC programming technology of large complex curved surface makes it possible to process sculptured curved surface runner blades, which will greatly promote the development and progress of China's hydraulic turbine industry and lay the foundation for the development of China's hydropower equipment manufacturing industry to advanced manufacturing technology. Programming process of multi-axis NC machining for large Francis turbine blades. Five-axis NC programming of large complex curved parts is much more complicated than ordinary parts. According to the characteristics of Francis turbine blade with large volume and large curvature change, the process design is carried out by analyzing the machining requirements, and the machining scheme is determined. The appropriate machine tools, tools and fixtures are selected to determine the reasonable tool path and cutting parameters. The geometric model of the blade is established, and the motion trajectory of the tool relative to the blade during machining is calculated. Then the cutting simulation of the blade and the motion simulation of the machine tool are carried out, and the machining parameters, cutter parameters and cutter axis control scheme are repeatedly modified until the simulation results do not interfere and collide. Then, according to the acceptable program format of machine tool numerical control system, post-processing is carried out to generate blade processing program. The specific programming process is shown in Figure-. Figure-Programming process of five-axis NC machining for large Francis blades! ”！ Three-dimensional geometric modeling of Francis turbine blades This complex sculptural surface of Francis turbine blades is composed of front, back, belt-shaped revolving surface connected with the upper crown, belt-shaped revolving surface connected with the lower ring, and large. You can write a */0 program to read these three-dimensional coordinate points, and then use bicubic multi-patch patches to model the surface through free-form surface features, as shown in figure 1. The blank shape of the blade can be offset calculated from the design data points, or "234$" surface modeling can be determined from the point aggregation method obtained from three-dimensional measurement, and each surface of the blade can be stitched into a solid. ! "# Blade machining process planning, machining scheme and selection of machining parameters determine the efficiency and quality of NC machining. According to the structure and characteristics of the blade to be machined, a large gantry mobile five-axis CNC milling and boring machine can be selected. According to the principle of three-point positioning, after a lot of research and analysis, it is decided to adopt the universal adjustable bracket with balls on the back, position the blade with the positioning pin welded by the blade, weld the necessary process blocks on the blade and clamp it with some universal tensioners. When processing the front side, put the back side of the blade into the clamping fixture with the same clamping fixture as the back side when processing the back side, adjust and align it with the welding process block, and still clamp it with the universal tension and compression device. Because the blade is composed of multiple curved surfaces, in order to solve the collision problem in the machining process, we use tools to machine different areas of the front and back of the blade along the streamline, and use different diameters of tools and different cutter axis control formulas according to the curvature of each curved surface. Generally, each surface is divided into multiple rough milling and one finish milling. Under the condition that the lathe bed does not collide and interfere with the workpiece and fixture, the large-diameter curved surface milling cutter should be used as much as possible to improve the machining efficiency. On the front and back of the blade, we choose the tool diameter! Rough milling of -56 curved surface milling cutter! -16 surface milling cutter finish milling, the cutter head adopts curved surface! 76 curved surface milling cutter processing, water! Five-axis side milling of 76 spiral corn end mill. According to the follow-up simulation, the tool position is edited repeatedly to find a reasonable machining scheme. On the premise of meeting the machining requirements, the normal operation of the machine tool and a certain tool life, the machining efficiency should be improved as much as possible. ! Tool path generation in five-axis machining of blades According to the characteristics of various curved surfaces of large Francis blades, reasonable tool path planning and calculation are the key to make the generated tool path non-interference, collision-free, stable and efficient. Because the cutter position and cutter axis direction of five-axis machining change, five-axis machining is composed of cutter position vector and cutter axis direction vector in workpiece coordinate system, and the cutter axis can be controlled by rake angle and inclination angle, so we can calculate the cutter position vector and cutter axis vector according to the local coordinates of the surface at the cutting point. From the point of view of machining efficiency, surface quality and cutting performance, the parameter line along the blade modeling is selected as the milling direction, which is divided into rough milling and fine milling for many times, and then the machining area is divided to define the parameters related to the machine tool. According to the machining position and clamping diagram of the selected blade, the tool path of mixed-flow blade is generated and positioned, and the blade is divided into multiple combined surfaces for machining respectively. Through the analysis of the curvature distribution of the surface, different face milling cutters are used for different areas. Roughing gives the allowance for each machining, and finishing uses milling cutters with the same diameter. Given the residual height according to the roughness requirement, the cutting type, cutting parameters, cutter axis direction, cutter advance and retreat mode and other parameters are selected according to the specific situation, and the generated tool path is shown in the figure. However, for sculptural curved parts with large and uneven curvature changes like blades, we need to do a lot of tool position editing according to the situation, and further modify and edit the tool path through cutting simulation and interference and collision inspection. !" Numerical control machining simulation of five-axis linkage of blade NC machining simulation verifies and optimizes the machining scheme through software simulation of machining environment, tool path and material removal process. Computer simulation verifies the tool path of multi-axis linkage machining, assists the interference inspection of machining tools and the collision inspection between machine tools and blades instead of trial cutting or trial machining process, which can greatly reduce the manufacturing cost, shorten the development cycle, avoid the collision between machining equipment and blades and fixtures, and ensure the safety of machining process. Machining parts "! Code usually needs to be trimmed before it is put into practical processing. Turbine blade is a very complex sculptural surface, and the development and utilization of NC machining simulation technology is the key to its successful use of five-axis NC machining. Here first analyze the process system and define the machine tool! ”！ System model, machine tool structure and size, machine tool motion principle and machine tool coordinate system. Use three dimensions! -The software establishes the solid geometric models of the moving parts and the fixed parts of the machine tool, and converts them into the format available in the simulation software, then establishes the tool library, creates a new user file in the simulation software, and sets up the used! "! System, and establish the machine tool operation model, that is, the component tree, add the geometric model of each component, and locate it accurately, and finally set the machine tool parameters. Next, the blade model is converted to the machining position to calculate the tool path, and then the three-dimensional dynamic simulation of blade cutting process, tool path and machine tool movement is carried out based on this path. This can clearly monitor the interference and collision between over-cutting and under-cutting, between the tool holder and the connecting system and the blade, and between the moving parts of the machine tool and the blade and fixture, thus ensuring the quality of NC programming, reducing the workload and labor intensity of trial cutting, improving the one-time success rate of programming, shortening the product design and processing cycle, and greatly improving the production efficiency. If it is popularized in NC machining industry, it can produce huge economic and social benefits. The cutting simulation of the blade is shown in the figure. The machining simulation of the blade by the machine tool is shown in Figure/. Figure. Francis blade cutting simulation diagram/Francis blade machining simulation! Post-processing of blade tool path is an important content of NC programming, which converts the tool path data generated in front of you into data suitable for a specific machine tool. The two basic elements of post-processing are tool path data and post-processor. First of all, we should understand the structure of gantry mobile five-axis CNC milling and boring machine, the auxiliary equipment equipped with the machine tool, the function of the machine tool and the way to realize the function, as well as the CNC system equipped with the machine tool, and be familiar with the system! Programming includes the composition and meaning of functional codes. Then, based on the above knowledge, a customized special post-processor is developed by applying the general post-processor positioning template. Then, input the cutter location source file we got and convert it into a "controllable machine tool"! Code. Conclusion Multi-axis NC programming of complex surfaces is a complex process involving multi-domain knowledge, and it is also a process of digital simulation and optimization. The multi-axis linkage programming technology of large turbine blades introduced in this paper has been used in NC programming of large turbine blades in engineering practice, and the tool path calculation and machining simulation of five-axis linkage NC machining of large runner blades have been realized, ensuring the quality and efficiency of subsequent NC machining. It has been used as a programming tool for five-axis NC machining of large turbine blades in practical production.