Five axis NC machining technology of the hottest w

Five axis NC machining technology of water turbine blades

1 Introduction

the runner of hydraulic turbine is the heart of hydraulic generator unit. The manufacturing technology and quality of its runner blades directly affect the hydraulic performance and reliability of the unit. Turbine runner blades are very complex sculptured curved surface parts. In the manufacturing process of large and medium-sized units, the manufacturing process of "sand mold casting - grinding wheel shovel grinding three-dimensional template detection" has been used for a long time. Its production efficiency is very low, and the blade surface accuracy is difficult to ensure. In addition, the manual grinding wheel shovel grinding has high labor intensity and a very bad working environment, which can no longer meet the requirements of technological progress, It can not effectively ensure the accuracy of blade surface and manufacturing quality, nor can it meet the requirements of today's power generation equipment market competition. "Five axis CNC machining technology of large turbine blades" is one of the key technologies in the world power generation equipment manufacturing industry, and it is also the cutting-edge high technology in today's machining technology. It involves three-dimensional modeling of computer-aided products, computer simulation and machining, five axis CNC technology, complex metal cutting technology, three-dimensional surface measurement and positioning technology, as well as blank manufacturing. Foreign world-class power generation equipment manufacturing companies have invested a lot of human and financial resources in a large number of research and development since the 1980s. Due to considerable technical difficulties, only a few companies have mastered the technology and are extremely confidential. As a powerful technical means of market competition, it is also one of the effective technical means to ensure the hydraulic performance of hydraulic turbines. With the construction of the Three Gorges project, the research and development of NC machining technology for large turbine blades, as one of the key projects in the manufacturing technology of the Three Gorges unit, has been included in one of the major projects jointly funded by the National Natural Science Foundation and the machinery industry development foundation after strict evaluation, and has been supported by the two foundations, At the same time, it is also a key project of the Three Gorges unit designated by the Three Gorges Construction Commission of the State Council. The research and development of "five axis CNC machining technology for large turbine blades" is of great significance not only for the manufacturing of Three Gorges units, but also for the technological progress of the whole turbine manufacturing industry and improving the market competitiveness of China's hydropower equipment manufacturing industry. Our company has organized a certain amount of manpower to carry out research and development on the relevant technologies in the NC machining of large blades, and has been successfully used in the manufacturing of a large axial-flow turbine blade (Gaobazhou power station). For the first time in China, the whole machine adopts NC machining blades. The key lies in the successful application of computer simulation machining technology

2. Structural characteristics of large-scale axial-flow turbine blades

Gaobazhou unit is a high head axial-flow unit, and its turbine runner diameter is f5.8m. The blades are highly twisted from the water inlet edge to the water outlet edge, with an outer skirt on the back rim of the blades and a large inner skirt on the hub side of the blades. The inner skirt is a constant R (front r265.2mm, back r232.0mm) transition surface in the flange spherical area, and the surface r from the flange spherical area to the inlet and outlet edges transitions to zero along the spatial curve. The diameter from blade flange to outer edge is f1180mm. Because this blade has some particularity compared with the general axial-flow blade in structure, it makes the machining process and simulation machining programming more difficult. Gaobazhou turbine blade is shown in Figure 1

Figure 1 Schematic diagram of blade design

3 According to the structural characteristics of the blades to be processed and the structure of the 5fzg gantry mobile NC machine tool used, as well as the specific conditions of the blade blank, the NC machining process plan

carefully studied, analyzed and compared, and decided to use the blade rotation axis and flange end face as the positioning benchmark. There are many curved surfaces of the blade that need to be machined. The large overflow surface is milled by five axis linkage, and the cutter is moved along the flowline "raster". In order to solve the problem of anti-collision of machine tools in the process of machining, the large surface of the front and back of the blade is divided into areas, and the tools with different diameters and different tool axis control methods are used for machining. The division principle of area is to use large-diameter curved surface milling cutter as far as possible without collision. Since the diameter of the NC milling head flange of the machine tool is f1.23m, the division of the area must be determined by multiple simulations of the simulation processing described later. Due to the uneven distribution of blank allowance, multiple rough milling and one fine milling are adopted. The whole processing scheme must be formulated on the basis of computer simulation, verification, modification and perfection by using simulation processing technology, otherwise there may be many unexpected problems in processing

clamping positioning and fixture

based on the rotating axis line of the blade, weld the process shaft on the blade flange, rough turn the flange end face, and then drill two center holes on the process shaft and flange end face. The corresponding fixture adopts two center seats, one is the fixed center seat, which is placed at the flange end, and the other is the axially adjustable center seat, which is placed at the process shaft end on the rim. The flange end face is marked with the blade processing position (relative to the design position - 10 °, determined by the simulation processing described later). Using the fixture and mark, with the help of the function of the machine tool, find the machining position, determine the zero point of the workpiece, weld the lug on the opposite side of the machining surface, and use the universal Jack and pull rod to pull and press and clamp. This set of fixture is simple and easy to operate, and can be used for machining axial flow blades

division of processing parts and processing areas

the processing parts of axial-flow blades include the front and back surfaces of blades, inlet and outlet water edges, inner and outer skirt transition surfaces, rims, etc. In order to avoid collision and tool interference, the simulation processing described later has been modified and verified for many times. As shown in Figure 2, the front and back of the blade are divided into three areas for processing. The principle is to maximize the AP1 and AS1 areas to improve the processing efficiency under the condition that the machine tool does not collide and interfere with the workpiece and fixture. AP1 and AS1 adopt φ 200 curved surface milling cutter five axis linkage machining, ap2 and as2 adopt φ 125 curved surface milling cutter five axis linkage processing, AP3 and As3 adopt φ 100 ball head cutter 3.5 axis linkage processing. Water inlet and outlet edge φ 100 spiral corn end milling cutter five axis linkage side milling

Figure 2 Schematic diagram of machining area

machining tools

more factors should be considered in selecting tools for machining blades than in general machining. First of all, the geometry of the milling cutter head used should adapt to surface machining, have good cutting performance, chip removal and chip breaking performance, and be suitable not only for convex surfaces, but also for concave surfaces, which is a very important factor for tool interference. When selecting tools, we should not only calculate according to the power of the machine tool, the rotation speed of the milling head, the material of the blade and the relevant cutting parameters of the tool and blade, but also further simulate and inspect the cutter head, blade, arbor and milling head according to the simulation processing described later, so as to comprehensively consider. If the tool interferes, the tool scheme and processing method must be modified, that is, the finally determined tool can be known only after the simulation and interference inspection verify that there is no problem. Under the condition that the power of the machine tool, the rotating speed range of the milling head and the rigidity of the machine tool are sufficient, different diameter tools are used for calculation in the simulation processing, and large diameter tools are used as far as possible to improve the processing efficiency. For Gaobazhou blade, after calculation and simulation processing verification, for AP1 and AS1 areas, heavy cutting with four edges with 60 ° main deflection angle is adopted φ 200 curved surface milling cutter, AP3 and As3 adopt special long cutting edge φ 100 ball head cutter, ap2 and as2 adopt large feed cutting arc blade with 8-sided edge φ 125 surface milling cutter. The inlet and outlet water side adopts the one with large feed volume φ 100 spiral corn end mills

Figure 3 blade five axis NC machining tool

blade measurement

blade measurement includes blank measurement before machining and profile detection after machining. In foreign countries, photoelectric theodolite measurement system is used for blank measurement and sampling inspection after processing [1]. At present, our company does not have the conditions in this regard. For the blank measurement, according to the blank casting technology level, we will offset the normal direction of the blade surface to a given value, and prepare the inspection program according to the similar processing method to determine the residual distribution of the processing part, and then decide which program to start processing according to the distribution. After machining the profile, the three-dimensional measurement technology is used to detect the profile of the blade after machining. The accuracy of the machined profile is very good, which is much higher than the requirements of iec193 standard

4. Simulation processing and programming

hydraulic turbine blades are very complex sculptured curved surfaces. The key to the successful use of five axis NC machining is to develop and utilize computer simulation processing technology, which is particularly important for the NC machining of large blades. Moreover, it is also an advanced technical means of five axis NC machining programming. There are many problems to be considered in the automatic programming of five axis NC machining of large blades, which is much more complex than the general automatic programming, and must be verified by computer simulation. The computer simulation processing of blade is the most critical and technical work in the multi axis NC machining process of blade, and it is the basis of assisting in formulating the process plan and compiling the machining program. Through repeated modification and improvement of simulation processing, a reasonable processing scheme and specific processing methods are sought. The main work is to use the software of camandcad/cam of SDRC company to realize simulation processing on the basis of secondary development. The simulation processing and programming of blades are shown in Figure 4

Figure 4 simulation processing and programming of blades

three dimensional modeling of blades

the profile and surface of hydraulic turbine blades must be simulated strictly according to the design requirements, because this is to affect their hydrodynamic performance and even the performance of the whole unit. The following engineering requirements should be considered in modeling

the blade surface body is reasonably divided into a combination of multiple surfaces in order to accurately simulate the blade surface body

the selection of modeling methods for each surface should be able to accurately define and reflect the requirements of the actual project

error control of surface modeling, surface operations such as surface clipping and extension

according to the process plan during simulation processing, divide each blade surface according to the processing method of each surface

the blade of axial-flow turbine is a curved part composed of a flange with a spherical surface and a number of sculptured surfaces, which cannot be defined by analytical equations. How to accurately use numerical methods to simulate each blade surface, especially for large blades, is extremely important. The NURBS surface approximation method can accurately simulate each surface on the blade, and it is also convenient for the calculation of tool path in multi axis NC machining. The axial flow blade is composed of front and back surfaces with sculptured surfaces, surfaces with variable arc radius at the inlet edge, surfaces at the outlet edge, rim sphere, hub and flange sphere, rim skirt surface, hub and flange transition surface with front and back surfaces, etc. The front and back of the blade are based on the type value points given in the cylindrical coordinate system. Write a program to read the type value points according to the cylindrical section, convert them to the rectangular coordinate, convert them to the revpost format of the Cayman, make the lofted spline curve along the cylindrical section line, and then make the loft surface according to the curve passing method, so as to make the front and back, and extend the surface to the rim and hub. Make the spherical surface of the rim and hub according to the drawing, then make the flange shaft with the cylindrical surface, and use the spherical surface of the hub to remove the trim flange shaft. According to the requirements of the drawing, make the equal radius fillet surface of the flange and the front and back, the variable radius fillet surface of the flange to the water inlet and outlet edges, and the rim hub sphere, extend the front and back of the blade rim, make the skirt at the back of the rim according to the drawing, use the skirt and the trim rim sphere at the front and back of the blade, and then use the wheel