Key Links of Numerical Control Machining Technology of Turbine Runner Blades

Turbine runner blades are key components of water turbines. Their manufacturing accuracy has a direct impact on the efficiency of the unit and other hydraulic performance. The high profile processing accuracy increases the turbine output efficiency by one percentage point, which will bring long-term and huge benefits. Due to the superiority of the cnc machining of the blade profile to the manufacturing quality of the turbine, it has become a key technical requirement for ordering the unit. The numerical control machining of the turbine blade can effectively ensure the machining accuracy and smoothness of the blade profile, thereby improving the stability, efficiency and cavitation of the turbine. The method of using numerical control processing or mould pressing combined with numerical control processing is the most effective and advanced method in the world to solve the processing and manufacturing of hydraulic turbine blades. It is the development direction of the hydraulic turbine manufacturing industry.

1. Measurement of blade

1.1 Purpose of measurement

Accurate measurement of the blade profile is an indispensable and important link in the CNC machining of hydraulic turbine blades. Before processing, it is necessary to understand the distribution of the blade blank machining allowance in order to determine the most reasonable programming processing position. Therefore, there are two main purposes for measuring the blade profile before processing. The first is to send the measured data of the blade blank to the computer, compare and adjust the theoretical shape of the blade, and adjust the theoretical shape and the blank profile to a relative position with the most reasonable machining allowance distribution. The second is to calibrate three alignment points on the measured blank, so that when the machine tool is installed on the blade for alignment, use these three points as a reference to perform three-point alignment, and adjust the theoretical shape of the blade for programming to the blank The actual processing position is completely consistent with the programming processing at the most reasonable relative position adjusted during the measurement. During the machining process, the measuring system equipped with the machine tool must also be used to measure the intermediate results of blade machining to check whether the machining effect and the remaining amount are consistent with the expected effect during programming. At the same time, it can also check the deformation of the blade during processing and whether the blade has unexpected movement, so as to make adjustments in time. After the processing is completed, the processing results of the blades must also be measured and checked with the corresponding measuring device to obtain the final result data of the processing, determine whether it meets the design requirements, and provide actual data basis for the assembly and welding of the following runners .

1.2 Principles and methods of measurement

The main working surface of the turbine blade is a complex three-dimensional sculpture surface. The main purpose of this surface measurement is to obtain the normal distance from the actual position of the measuring point on the surface to the theoretical surface. To complete this kind of measurement, ordinary measuring instruments cannot afford it. At present, two methods are usually adopted at home and abroad, one is the direct measurement method, and the other is the indirect measurement method.

2. Machine selection

The most advanced CNC machining of hydraulic turbine blades at home and abroad are mostly five-axis CNC milling machines. For example, the main power of the German CNC gantry milling machine is LOOKW, the table size is 4.5X13M, and it is equipped with the world’s most powerful CNC universal milling head. The CNC system uses SIEMENS840C. The machine has X, Y, Z, W, U five-axis CNC and A , C two rotating axis CNC milling head.

3. Aligning the blade installation card

The blades of hydraulic turbines are irregular, especially the Francis blades are free-form surfaces without any reference, which makes it more difficult to install and align the blades. It is recommended to use a cnc machine tool milling shaft to measure the coordinate values ​​of the three positioning reference points of the blade mounted on the mold, and set the reference point on the mold for alignment. Solving the problem of blade installation and card alignment is an important task to ensure that the processing position of the blade coincides with the programming position. The German VOITH company adopts the shaft outer diameter and flange positioning and vertical clamping method for the axial flow blade, and the mixed flow blade adopts the adjustable steering head jack positioning and the vacuum suction cup clamping method. Vacuum chuck clamping requires polishing and pretreatment of the suction part, and its system components occupy a large area. We propose to adopt the clamping method of front and back mould positioning, welding nugget lap welding and screw clamping, which is economical, efficient and easy to operate. It is reported that Norwegian GE company and others also adopt this method. The clamping and positioning method has a good effect on determining the reference point of the workpiece, implementing the alignment, and solving the technical difficulty of the dislocation of the front and back surfaces of the blade.

3.1 Design principles of blade tire clamps

In view of the shape characteristics of the turbine blade, the design of the blade tire clamp must follow the following principles:

• (1) Due to the large weight of the blade, the tire clamp must have sufficient strength.
• (2) The supporting position of the tire clamp must ensure that the center of gravity of the blade is placed stably, and each processed part of the blade surface must be within the effective stroke of the machine tool.
• (3) There must be enough supporting points to ensure that the blade does not deform greatly during processing. Firmly hold the position. And it is not easy to cause the deformation of the blade when it is closed. The clamping position should avoid interference with the tool and the milling head during processing.
• (4) Take vibration reduction measures to minimize vibration during processing.
• (5) The card installation is flexible and convenient.

3.2 Principle and method of blade alignment

So far, all manufacturers of water turbine blades in the world have adopted the method of mold alignment. That is, the blades are placed in the fixed position of the mold, and the mold is placed in the fixed position of the machine tool. The advantage of this method is that a set of procedures is used to process the blades of a unit. However, due to the particularity of the blade shape of the Francis turbine, this method is very difficult to align, and it must take up a lot of machine time. At the same time, it is easy to cause the front and back of the blade to be misaligned when turning the blade. Moreover, the cost of the mould used as a benchmark for alignment is extremely high. Since the alignment of the first side of this alignment method is only a rough alignment, in order to ensure that the entire blade can be processed. The blank margin is required to be relatively large, which causes the blade processing cost to be too high. Our company has also used this method for clamping and aligning. As a result, the front and back sides are misaligned, which makes the processing halfway. The computer automatic three-point alignment method is used to solve the above alignment problem.

4. Selection of processing tools

Turbine blades are usually made of stainless steel, which is difficult to process. In order to ensure the efficiency and quality of processing, suitable tools must be selected for processing. From the type of tool, you should choose a cemented carbide machine chuck. This tool has good wear resistance and impact resistance. The size is accurate and the blade can be replaced, which is more suitable for the processing of turbine blades. The choice of tool shape should be determined according to the processed part. The size of the tool (tool diameter, blade height, shank length) is determined by the shape of the processed surface. For curved surfaces with relatively flat shapes and small curvatures, tools with larger diameters can be used to improve processing efficiency. A curved surface with large undulations and large curvatures. A tool with a smaller diameter can be used to avoid tool interference and overcutting. The general principle is under the premise of no tool interference. Try to use a tool with a larger diameter for processing. For blades with very uneven machining allowance distribution, a tool with a higher side edge can be considered to avoid the phenomenon that the depth of the knife exceeds the height of the blade in a local position during processing. When the machined surface or other curved surfaces interfere with the CNC milling head, a tool with a longer shank can be considered for processing. The choice of tool size can be confirmed by interference check on the computer through simulation processing. Commonly used tools for blade machining include: Ø200MM, Ø160MM, Ø125MM, Ø100MM face milling cutters, Ø63MM, Ø80MM rod milling cutters, and Ø50MM ball end cutters. The shape of the blade can be triangular, diamond, square, circular and other different shapes of blades according to the different cutters and the requirements of rough milling and finishing milling.

5. Tool position calculation

The main task of the tool position calculation is: first select the machining coordinate system, and then use the three-point alignment method to align the theoretical shape of the blade. on the basis of. Based on the principle of improving processing efficiency, ensuring processing accuracy and surface quality, select the tool, determine the feed method and the tool axis control method, set the accuracy of the feed direction and the tool row direction, and calculate the tool location path and tool interference Check, and finally generate a tool location trajectory data file for post-processing.

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