Multi-axis linkage CNC machining programming skills for large mixed-flow blades

I. Introduction
The hydraulic turbine is the prime mover of hydropower generation. The quality of the turbine runner blades has a huge impact on the safety, reliability, and economical operation of the hydropower station unit. Turbine runner blades are very complex sculptural surfaces. In the manufacturing process of large and medium-sized units, the long-used 21 Kinds Surface Treatment Processes For Sheet Metal Manufacturing

process of “sand casting → grinding wheel grinding → three-dimensional prototype inspection” cannot effectively guarantee the accuracy and manufacturing quality of blade profiles. The current use of five-axis linkage CNC machining technology is the cutting-edge high technology in today’s mechanical processing. CNC machining programming of large complex curved surface parts is the most important technical basis for realizing digital manufacturing. Its CNC programming technology is a digital simulation evaluation and optimization process. Its key technologies include: three-dimensional modeling and positioning of complex-shaped parts, five-axis linkage tool position trajectory planning and calculation, tool axis control technology for processing sculptured curved surfaces, cutting simulation and interference inspection, and post-processing technology. Multi-axis linkage CNC programming technology for large and complex curved surfaces makes it possible to process multi-axis CNC high speed machining of sculptural curved runner blades. This will greatly promote the development and progress of my country’s water turbine industry and lay the foundation for the development of my country’s hydropower equipment manufacturing industry towards advanced manufacturing technology. .
2. Multi-axis CNC machining and programming process of large Francis turbine blades
The five-axis linkage CNC programming of large and complex curved surface parts is much more complicated than the programming of ordinary parts. In view of the characteristics of mixed-flow blades that are large in size and have large changes in surface curvature, process design is carried out by analyzing the processing requirements, determining the processing plan, and selecting appropriate machine tools. , tools, fixtures, determine the reasonable tool path and cutting amount, etc.; establish the geometric model of the blade, calculate the movement trajectory of the tool relative to the blade during the processing, and then perform cutting simulation of the blade and motion simulation of the machine tool, and repeatedly modify the processing parameters, tool parameters and cutter axis control scheme, until the simulation results confirm that no interference collision occurs, post-processing is performed according to the program format acceptable to the machine tool CNC system to generate a blade processing program. The specific programming process is shown in Figure 1.

Figure 1 Five-axis linkage CNC machining programming process for large mixed-flow blades
1. Three-dimensional geometric modeling of Francis turbine blades
The complex sculptural curved body of the Francis blade consists of a front, a back, a strip-shaped rotating surface connected to the upper crown, a strip-shaped rotating surface connected to the lower ring, an inlet edge curved surface, an outlet edge curved surface, and an inlet edge head. Surfaces, etc. Since the amount of data of the three-dimensional coordinate points described by the blade along the flow surface is too large, a Grip program can be written to read in these three-dimensional coordinate points, and then a bicubic multi-patch surface patch is used to perform surface modeling through the curve method of free-form features, such as As shown in Figure 2. The blank shape of the blade can be calculated from the design data points, or determined from the point cloud obtained from the three-dimensional measurement. NURBS surface modeling can be performed on each surface of the blade and stitched into a solid body.

Figure 2 Three-dimensional modeling of mixed flow blades
2. Blade processing process planning
The selection of processing plans and processing parameters determines the efficiency and quality of CNC processing. Water turbine blades are very complex sculptural curved surfaces. We can choose a large-scale gantry mobile five-coordinate CNC milling and boring machine according to the structure and characteristics of the blades to be processed. Based on the three-point positioning principle and after extensive research and analysis, we decided to use a universal machine on the back side. It has a spherical adjustable support and a positioning pin for blade welding to position the blade. The necessary process blocks are welded on the blade and some universal tensioning devices are used for clamping. When processing the front side, use a mold that is milled out with the same profile as the back side when processing the back side. Put the back side of the blade into the mold, use the welded process block to adjust and align, and still use a universal tension and compression device for assembly. folder. Since the blade is composed of multiple curved surfaces, in order to solve the collision problem during the processing, we can use the tool to move along the streamline, process the front and back of the blade in separate areas, and use tools with different diameters according to the different curvatures of the curved surfaces. , different tool axis control methods for processing. Each surface is generally divided into multiple rough milling and one fine milling. When there is no collision or interference between the machine tool and the workpiece and fixture, try to use a large-diameter curved surface milling cutter to improve processing efficiency. For the front and back of the blade, we use a φ150 curved surface milling cutter for rough milling and a φ120 curved surface milling cutter for fine milling. The blade head surface is processed with a φ80 curved surface milling cutter, and the water outlet edge is processed with a φ80 spiral corn end mill for five-axis linkage side milling. . Repeat tool position editing based on subsequent simulation situations to find a reasonable processing plan. Improve processing efficiency as much as possible under the premise of meeting processing requirements, normal operation of machine tools and a certain tool life.
3. Generation of tool position trajectory for five-axis simultaneous machining of blades
Based on the characteristics of each curved surface of large-scale mixed-flow blades, reasonable tool trajectory planning and calculation is the key to making the generated tool trajectory interference-free, collision-free, good stability, and high programming efficiency. Since the tool position and tool axis direction of five-axis machining change, five-axis machining is composed of the tool position point position vector and the tool axis direction vector in the workpiece coordinate system. The tool axis can be controlled by the forward inclination angle and the inclination angle. , so we can calculate the tool position vector and tool axis vector based on the local coordinates of the surface at the cutting point. From the perspective of machining efficiency, surface quality and cutting process performance, the parameter line along the blade shape is selected as the direction of forging can be divided into milling forging die forging. It is divided into multiple rough milling and one fine milling. Then the processing area is divided and the parameters related to the machine tool are defined. According to the above Select the processing part of the blade, the clamping and positioning method, the machine tool, the cutting tool, the cutting parameters and the margin distribution, and divide the blade into multiple combined surfaces for processing respectively. By analyzing the distribution of surface curvature, different face milling cutters are used for different areas. Rough machining gives the allowance for each machining. Finishing machining uses a milling cutter of the same diameter. The residual height is given according to the roughness requirements. The cutting type, cutting parameters, cutter axis direction, tool advance and retreat methods and other parameters are selected according to the specific situation to generate The tool position trajectory is shown in Figure 3. However, for sculptural surface parts such as blades with large and uneven curvature changes, we have to edit a large number of tool positions according to the situation, and we must further modify and edit the tool path through interference and collision checks through cutting simulation.

Figure 3 Tool path generation for mixed flow blades
4. Blade five-axis linkage CNC machining simulation
CNC machining simulation uses software to simulate the machining environment, tool path and material removal process to test and optimize the machining program. It simulates and verifies the tool path of multi-axis linkage machining on the computer, assists in checking the interference of the machining tool and the collision check between the machine tool and the blade, replacing The trial cutting or trial processing process can greatly reduce manufacturing costs, shorten the development cycle, avoid collisions between processing equipment and blades and fixtures, and ensure the safety of the processing process. The NC code of machined parts usually requires trial cutting before being put into actual processing. Water turbine blades are very complex sculptured curved surfaces. The development and utilization of CNC machining simulation technology is the key to the successful use of five-axis linkage CNC machining. Here, we first conduct a process system analysis to clarify the machine tool CNC system model, machine tool structural form and size, oem tool steel propeller rope guard cutters motion principle and machine tool coordinate system, use 3D CAD software to establish a solid geometric model of the machine tool moving parts and fixed parts, and convert it into a simulation The format available for the software, then establish a tool library, create a new user file in the simulation software, set up the CNC system used, and establish a machine tool motion model, that is, a component tree.