Key link of numerical control machining technology for turbine runner blades

The turbine runner blade is a key component of the turbine. Its manufacturing precision has a direct impact on the hydraulic performance of the unit. The precision of the profile processing is high, and the efficiency of the turbine output is increased 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 quality of the turbine manufacturing, it has become a key technical requirement for the ordering of the unit. The numerically processed turbine blades 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 numerically controlled machining or molding combined with numerical control machining is the most effective and advanced method for solving the turbine blade processing and manufacturing in the world. It is the development direction of the turbine manufacturing industry.

1 , the measurement of the blade

1.1 Purpose of measurement

Accurate measurement of the blade profile is an indispensable part of the NC machining of the turbine blade. Before processing, it is necessary to know the distribution of the machining margin of the blade blank in order to determine the most reasonable programming position. Therefore, there are two main purposes for measuring the blade profile before machining. First, the data of the blade blank measurement is sent to the computer, and compared with the theoretical shape of the blade, and the theoretical shape and the blank profile are adjusted to the most reasonable relative position of the machining allowance distribution. Second, on the measured blank, three calibration points are calibrated so that when the machine is loaded and aligned on the blade, three points are corrected based on the three points, and the theoretical shape of the blade for programming is adjusted to the blank. The actual machining position is fully programmed in accordance with the most reasonable relative position adjusted during the measurement. During the machining process, the intermediate results of the blade machining must also be measured with the measuring system equipped with the machine tool to check whether the machining effect and the remaining amount are consistent with the expected results during programming. At the same time, it is also possible to check the deformation of the blade during processing and whether the blade has unexpected movements in order to make timely adjustments. After the processing is completed, the processing results of the blade must also be measured and checked with the corresponding measuring device in order to obtain the final result data of the machining, determine whether it meets the design requirements, and provide the actual data basis for the welding of the lower-order runner. .

1.2 Principles and methods of measurement

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

2 , machine selection

The more advanced machine tools for numerical control machining of turbine blades at home and abroad are mostly five-axis CNC milling machines. For example, Germany's CNC gantry milling machine host power LOOKW , table size 4.5X13M , equipped with the world's most powerful CNC universal milling head, CNC system using SIEMENS840C , the machine has X , Y , Z , W , U five-axis CNC and A , C two rotary axis CNC milling head.

3 , the blade is installed correctly

The turbine blades are irregular, especially the mixed flow blades are free-form surfaces without any reference, and it is more difficult to install the card. CNC milling machine recommended blades mounted on the measuring shaft tire having three positioning reference point coordinate values, and sets a reference point on the tire tool for alignment. Solving the problem of correcting the mounting of the blade is an important task to ensure that the machining position and the programmed position of the blade are consistent. VOITH of Germany adopts shaft outer diameter and flange positioning and vertical clamping method for axial flow vanes. The mixed flow vane adopts adjustable steering head jack positioning and vacuum suction cup clamping. The clamping of the vacuum chuck requires pre-treatment of the adsorption site, and the system components occupy a large area. We propose the use of positive and negative tire positioning, solder lap welding and screw clamping, which is economical, efficient and easy to operate. It is reported that Norwegian GE and so on. The clamping positioning method has a good effect on determining the reference point of the workpiece and performing the alignment, and solving the technical difficulty of solving the misalignment of the front and back profiles of the blade.

3.1 Design principles of blade tire clamps

In view of the shape characteristics of the turbine blades, the design of the blade clamps must follow the following principles: ( 1 ) Because of the large weight of the blades, the tire clamps must have sufficient strength. ( 2 ) The support position of the tire clamp must ensure that the center of gravity of the blade is stable, and each processed part of the blade surface must be within the effective stroke of the machine tool. ( 3 ) There must be sufficient support points to ensure that the blades do not undergo large deformation during processing. Put the position firmly. And it is not easy to cause deformation of the blade when it is combined. The joint position should avoid interference with the tool and the milling head during machining. ( 4 ) Take vibration reduction measures to minimize vibration during processing. ( 5 ) The card is flexible and convenient.

3.2 Principle and method of blade alignment

So far, all manufacturers of turbine blades in the world have adopted the method of aligning the tires. That is, the blade is placed in a fixed position of the tire, and the tire is placed in a fixed position on the machine tool. The advantage of this method is that a set of processes processes the blades of a unit. However, due to the particular shape of the Francis turbine blade, this method is very difficult to find, and it must occupy a large amount of machine time, and at the same time, it is easy to cause the front and back misalignment when the blade is turned over. Moreover, the tires used as a basis for correction are extremely expensive. Since the correction of the first side of this method of aligning is only a rough alignment, in order to ensure that the entire blade can be processed. The margin is required to be relatively large, which results in too high blade processing costs. Our company has also used this method to fix the clamping, resulting in a misalignment of the front and back, which caused the processing to be abandoned halfway. The computer automatically three-point alignment method is used to solve the above-mentioned alignment problem.

4 , the choice 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 the machining, it is necessary to select the appropriate tool for processing. From the type of tool, the carbide machine card cutter should be selected. 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 is determined by the part being machined. The size of the tool (tool diameter, blade height, and tool bar length) is determined by the shape of the surface being machined. A curved surface with a relatively flat shape and a small curvature can be used with a larger diameter tool to improve machining efficiency. A curved surface with a large change in shape and a large curvature. Smaller diameter tools can be used to avoid tool interference and overcutting. The general principle is that without the intervention of the tool. Try to use a larger diameter tool for machining. For blades with a very uneven machining allowance distribution, a tool with a higher side edge can be considered to avoid the phenomenon that the depth of the blade exceeds the blade height during the machining. For interference with machined surfaces or other surfaces and CNC milling heads, consider tools with longer shanks. The choice of tool size can be determined by performing interference checks on the computer through simulated machining. The common knives for machining blades are: Ø200MM , Ø160MM , Ø125MM , Ø100MM end mills, Ø63MM , Ø80MM rod milling cutters, Ø50MM ball cutters. The shape of the blade can be made into different shapes of blades such as triangles, diamonds, squares, and circles according to the different tools and the requirements of rough milling.

5 , tool position calculation

The main task of the tool position calculation is to 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. In order to improve the processing efficiency, ensure the machining accuracy and the quality of the profile, select the tool , determine the cutting mode and the tool axis control mode, set the accuracy of the tool direction and the tool direction, and calculate the tool path and tool interference. Check and finally generate the tool path data file for post processing

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