Five-axis linkage machining technology of Francis turbine blades

Introduction The CNC machining technology of large hydraulic turbine runner blades is one of the important signs that hydraulic turbine manufacturing has reached the advanced level in the world today. The runner blade is a key component of the hydraulic turbine. Its manufacturing accuracy has a direct impact on the hydraulic performance such as the efficiency of the unit. High surface processing accuracy can increase the output efficiency of the hydraulic turbine by one percentage point, which will bring long-term and huge benefits. The CNC PC machining of blades involves many aspects of high and new technology. Only France’s Alstom, Norway’s Kvarner, Canada’s GE, Germany’s Voytz, and Japan’s Hitachi have matured this manufacturing process abroad. Due to the superiority of CNC machining of blade profiles to the manufacturing quality of hydraulic turbines, it has become a key technical requirement for ordering units.
As the world’s largest water conservancy project – and the Three Gorges hydrogenerator unit with the largest Francis turbine unit capacity in the world today, it cooperated with Alstom to manufacture 8 of the 14 units of the first phase of the left bank project. The CNC machining technology of the runner blades, a key component of the Three Gorges turbine, is of great significance to improving my country’s power plant equipment manufacturing level and entering the ranks of the world’s advanced technologies. There are several types of water turbines, including Francis type, axial flow type, and tubular type. Domestic water turbine blades have long been manufactured using sand casting, manual shovel grinding and three-dimensional prototype testing. In recent years, blades have been molded and then shoveled. The technology of reprocessing after molding and CNC machining after casting.
The manual shovel grinding method not only has a harsh working environment, high labor intensity, and low efficiency, but also has poor blade profile quality, which reduces the efficiency of the turbine. In order to realize CNC machining of blades, our factory introduced a five-axis CNC boring and milling machine from Wotan Company in Germany in 1987, and introduced UKLID software from FIDIS Company in Switzerland. In 1996, it purchased a CNC gantry milling machine and CNC gantry milling machine from Waldrich Kobold Company in Germany. The introduction of CAMAX software provides technical guarantee for CNC machining of blades. Subsequent special projects conducted in-depth research on curved surface modeling, tool position calculation, tool position interference inspection, blade measurement, automatic alignment adjustment and processing program preparation, process clamping, processing methods and tool system configuration, etc., and now has five-axis Production and technical capabilities of linked CNC machining blades. So far, we have completed the processing of Francis turbine blades for Tianshengqiao, Zhelin, Xinfengjiang and Three Gorges turbines and axial flow Mujing turbine blades. Key technologies and processes implement blade CNC machining process. Blade CNC machining process.
Five-axis linkage CNC programming technology Blade profile measurement Due to the inconsistent machining allowances distributed in the casting blank, a three-dimensional measuring machine is used to measure the three-dimensional profile of the blank, and a measurement method that is efficient and convenient for computer data acquisition and processing and benchmark transformation is selected to achieve the margin. The best clamping position for uniform distribution. The surface modeling creates a three-dimensional profile of the blade based on the original data to meet the next step of generating the code program for CNC machining. The tool path calculation is performed based on the three-dimensional profile data after reading the blade shape, setting the machining profile, cutting parameters and tool data.
Tool position trajectory simulation and tool interference check simulate processing of graphics on the computer based on the tool position trajectory, and check the tool interference to ensure safe operation during real machine processing. Post-processing performs post-processing on the tool position trajectory to generate NC code processing programs for specific machine tools. The machining process and processing equipment purchased a German CNC gantry milling machine with a main engine power of 100kW and a workbench size of 4.5×13m. It is equipped with the world’s most powerful CNC universal milling head. The CNC system adopts SIEMENS840C. The machine tool has X, Y, Z, W , U five-coordinate CNC and A and C two-axis CNC milling heads, forming the equipment conditions for five-axis linkage CNC machining of blades.
The blank quality control blade is a cast structure and the material is martensitic stainless steel. Roughly grind the blades to remove sand inclusions to avoid damaging the tool. Then carry out UT and MT flaw detection to check the defect status. Due to poor blade casting quality and uneven margins, it is required to control the machining margins on the front and back sides of the blades and the peripheral margins to reduce CNC machining hours. Use a theodolite to measure the machining allowance on the front and back sides of the blade. Mark the three positioning reference points for measurement and processing alignment on the front and back sides of the blade and attach their coordinate values. And pass the measurement data to the next program in the form of disk. Blade clamping The blade shape is irregular, especially the mixed-flow blade is a free-form surface without any reference. The research on the clamping method is the first problem to be solved when implementing programming and processing. The German VOITH company uses shaft outer diameter and flange positioning and vertical clamping methods for axial flow blades, while mixed flow blades use adjustable steering head jack positioning and vacuum suction cup clamping methods.
​ Vacuum suction cup clamping requires polishing and pre-treatment of the adsorption part, and its system components occupy a large area. Our study proposed the use of front and back tire positioning, welding block lap welding and screw clamping methods, which are economical, efficient and easy to operate. It is reported that Norwegian GE Company and others also use this method. The clamping and positioning method is effective in determining the reference point of the workpiece and performing alignment, and is effective in solving the technical difficulty of misalignment of the front and back profiles of the blade. For blade alignment and measurement, use a CNC machine tool milling axis to measure the coordinate values of the three positioning reference points of the blade mounted on the mold, and set reference points on the mold for alignment. The RENISHAW measurement system is used to develop blade measurements for 3-dimensional curved surfaces, providing advanced detection technology for processed blade profiles.
The programming of the workpiece uses the aforementioned programming technology to compile the program for processing the front and back sides of the blade, the inlet and outlet water edges, and the welding grooves of the upper crown and lower ring. The processing parts of the blades processed by the five-axis CNC gantry milling machine are the front and back profiles, the inlet and outlet water edges, and the upper and lower welding grooves. The tools used are rough and fine surface milling cutters, spiral end mills, ball end mills, etc. The cnc milling copper method adopts down milling and milling with coolant added. The feed direction is selected to feed along the parameter line of the blade shape, which is beneficial to the milling performance such as tool diameter and interference requirements. The determination of the milling width interacts with the surface processing quality and processing efficiency. Since the milling cutter is tilted in the normal direction, the surface of the workpiece after processing will be slightly concave arc. The larger the value, the greater the polishing workload in the next sequence.
Therefore, it is necessary to reasonably determine the width of the tool row for each program segment. The development and application of high-efficiency CNC cutting tools. CNC blade machining technology is expensive to process. Taking the statistics of processing a Three Gorges turbine blade, it takes 10 days to process one blade, and the entire processing cycle is more than 5 months. The price of a CNC gantry milling machine is about 40 million. Yuan, the working time costs thousands of yuan per hour, and the processing fee is more than 4 million yuan. Improving processing efficiency and reducing processing costs are of particular significance. Research, development, and application of efficient CNC tools and processing methods are one of the key technologies that need to be solved. . The development of domestic face milling cutters and spiral end mills requires face milling cutters to process the curved surface of the blade. Spiral end milling cutters are required for entry and exit, water edge and welding grooves.
Foreign cutting tools are expensive. In order to be suitable for processing stainless steel blades, we cooperated with Harbin University of Science and Technology and Mianggong Tool Factory to develop a flat-mounted blade structure, an upper pressure clamping method, a main deflection angle of 75°, and double positive fronts. Angle face milling cutter; cooperated with Taiyuan Ace Tool Factory to develop a 4-edged interchangeable head cutting tool, the peripheral edge is equipped with a square blade, and a spiral end mill that can be indexed 4 times; cooperated with Harbin Gongliang CNC Tool Factory to develop CNC cutting tools System development and development. The test selects cnc milling brass blades to solve the problem of difficult processing of stainless steel. The key is the performance of the blades. There is a considerable gap between domestic blades and foreign blades. The durability of foreign blades can be more than 5 to 10 times that of domestic blades. After testing 9 types of blades from domestic cemented carbide manufacturers such as Zhuzhou, Zigong, Shanghard, and Shaanxi Hard Alloy, and 10 types of blades from foreign companies such as Sweden’s Sandvik, Germany, the United States Kennametal, Israel’s Iska, Hitachi, and Mitsubishi, etc. Extensive testing was conducted on brand blades, and blades with excellent cost performance were selected. In addition, in cooperation with Harbin University of Science and Technology and Mianggong Tool Factory, a new face milling cutter with a double-edge stepped structure was developed to solve the problem of empty stroke in rough milling due to uneven machining allowances of blade shapes.
We also optimized the milling parameters and milling methods to achieve high-efficiency and low-consumption tool milling of blades. 3 Conclusion The successful five-axis CNC machining of turbine blades such as the Three Gorges and Tianshengqiao turbines marks that our factory and my country’s large-scale hydraulic turbine manufacturing have reached the international advanced level. Innovation in blade technology has significant economic benefits in improving quality, improving processing efficiency, and reducing tool costs. Every percentage point increase in the runner output efficiency caused by the CNC machining of the blades will bring huge economic benefits to the long-term operation of the unit. It is of great significance to the manufacturing quality of the Three Gorges unit and my country’s power generation equipment manufacturing industry to enter the international market after entering the WTO.