Turbine runner blades are a sculptural curved body part with complex shapes, which is very difficult to manufacture. In the past, the traditional manufacturing process of “sand casting → manual shovel grinding → three-dimensional model inspection” was used in China. It has low machining efficiency, high labor intensity, and It is difficult to guarantee the accuracy of the blade profile. At present, computer simulation machining technology has been used in foreign countries to achieve five-axis simultaneous CNC machining of turbine runner blades, thereby greatly improving the machining efficiency and blade profile machining accuracy.
The three Gaobazhou hydropower generating units undertaken by our company are high-head axial flow units. The diameter of the turbine runner is 5.8m, and the blades of the runner have a large degree of distortion from the water inlet to the water outlet. The outer skirt has a larger inner skirt on the hub side of the blade, and the curved surface R from the flange spherical area to the water inlet and outlet edges gradually changes to zero along the space curved surface. According to user requirements, the complete set of blades (3 × 5 pieces) is processed by 5FZG gantry mobile five-axis linkage CNC machine tool. The development and application of CNC tools has become one of the key factors for the success of machining.
Selection of cutting tools
The following factors need to be considered when selecting CNC blades for machining blades:
The geometry of the tool should be able to adapt to the machining of convex and concave surfaces, and it should have good cutting performance and chip breaking and chip removing performance.
When selecting a tool, in addition to calculating the relevant parameters of the tool and insert according to the power of the machine tool, the speed of the milling head, and the blade material, it is also necessary to perform simulation machining verification and interference check and verification of the insert, tool bar, and milling head.
You can choose tools with different diameters for simulation and check calculations. Under the conditions of sufficient machine tool power, rigidity, and milling head speed, large diameter tools are used as much as possible to improve machining efficiency.
After design calculation and simulation machining verification, the following four tools were selected to process the blades of Gaobazhou unit:
①60 ° D200 face milling cutter for rough milling of blade surface;
②D100 corn milling cutter for machining blade inlet and outlet edges and Outer edge;
③ D100 ball-end milling cutter, used to process the transition surface between the blade surface and the flange spherical surface;
④ D125R surface milling cutter, used to finish milling the blade surface.
Localization of cutting tools
In order to ensure the machining quality, imported CNC tools (Kennametal, Sandvik products) were all used when machining the first unit blades. Since the blade material is stainless steel containing Ni and Cr, the material has good toughness, high machining temperature and large tool consumption. Although imported tools can be processed lightly and with high cutting efficiency, they are expensive (5,000 to 32,000 yuan each) and have a long supply cycle, which is not conducive to reducing machining costs and ensuring production progress. To this end, we have developed localized development of some tools.
Cooperated with Mianjiang Tools Factory to develop a 60 ° face milling cutter. Kennametal’s imported cutter head is the right cutter head (positive cutter head). The left cutter head (reverse cutter head) is not produced, and the blade can only be used 4 times. The domesticized positive and negative milling cutters can increase the number of inserts to 8 times. In addition, the imported tools use a lever-type compression structure, which is more complicated to design and manufacture. We have changed to a screw-type compression structure and have proved successful through trial machining.
Cooperated with Mianjiang Tools Factory to develop D100 spiral corn milling cutter. Due to the large and uneven machining volume of the spiral corn milling cutter, we chose a blade with a vertical structure and a chip separation slot to improve the strength of the tool and the smoothness of chip removal.
For large-size non-standard tools, because of the long cutting edge structure, it is difficult to develop domestically, so imported tools are still used.
The R125 face milling cutter is a precision milling cutter with a small machining volume and is not easily damaged, so no localization is required.
Localization of blades
Although the imported blades used for machining blades have excellent performance, they consume large amounts and are expensive, and it is necessary to carry out domestic development. All imported blades are coated blades made of stainless steel, and the blade geometry is complex. For example, a 60 ° face milling insert uses a curved 15 ° large rake angle; a corn milling insert has 3 chipbreakers on one side and 2 chipbreakers on the other side; the rake face of the R surface milling insert has a complex pattern. Therefore, the development of blade molds is very difficult. Localized blades must meet the following requirements: ① good material, suitable for blade machining; ② high shape and dimensional accuracy, can meet the machining requirements.
After careful inspection of the machining capacity, we chose Zhuzhou Cemented Carbide Factory for domestic development of the blade. The specific blade grades developed are: CN35160 ° face milling inserts; CN351 corn milling inserts; CN351 ball-end milling inserts; YB235R face milling inserts. The localized tools and blades have been produced and used with good results, proving that the localized development is successful.
Repair of cutters
In the production process, the ball-end milling cutter was severely damaged and consumed a lot. The imported unit price of this tool is nearly 20,000 yuan. In order to reduce the machining cost, it is necessary to repair the damaged ball-end milling cutter.
First, repair the defective part of the tool, then grind the contour of the tool with a grinder, and repair the file holder with a cyclone file. In order to correct the welding deformation, it is necessary to use M3.5 taps to re-tighten the blades against the screw holes. Although the accuracy of the repaired tool is not as good as the original tool, it can completely meet the machining requirements. We have repaired 14 knives and saved a total of 277,000 yuan.
Benefit analysis
The local development of CNC tools for hydraulic turbine blades has greatly reduced the cost of tools during machining. Unit price of domestic 60 ° face milling cutter is 1580 yuan (imported milling cutter is 12500 yuan); unit price of domestic D100 spiral corn milling cutter is 3000 yuan (imported milling cutter is 31980 yuan); unit price of 60 ° face milling insert is 75 yuan (The imported blade is 174.75 yuan); the unit price of domestic corn milling inserts is 45 yuan (the imported blades are 83.97 yuan); the unit price of domestic ball-end milling inserts is 25 yuan (the imported blades are 62.24 yuan); the unit price of domestic R face milling blades It is 32 yuan (80 yuan for imported blades). The machining efficiency of domestic blades can reach 70% to 80% of imported blades, and the price is less than 1/2 of imported blades. In the machining of the blades of the three Gaobazhou hydro turbine units, the blades of the first unit were all processed with imported blades, and the cost of the blades was 89,000 yuan; the blades of the second unit were mostly processed with domestic blades, and the blade cost was reduced to 37,000 yuan. ; The blades of the third unit are all processed with domestic blades, and the blade cost is only 32,000 yuan.
