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Adaptive Techniques in CNC Machining of Aeroengine Precision Forging Blades


The self-adaptive CNC machining technology of aero-engine precision forging blades integrates a number of technologies in the field of digital manufacturing such as digital inspection, workpiece positioning and model reconstruction. A system solution for machining. The research and application of this technology is of great significance for improving the current situation of my country’s aero-engine precision forging blade manufacturing field and improving the level of advanced manufacturing technology. In order to further promote the weight reduction, efficiency increase and performance improvement of major equipment in the domestic aerospace and other fields, a large number of new blades under the background of composite manufacturing processes are applied to the aero-engine fans and compressors currently in service or under development. With the continuous improvement of the bypass ratio, thrust-to-weight ratio and service life requirements of aero-engines in the development of large aircraft, precision forging methods are often used to manufacture blade blanks for new aero-engines. Precision forging is an important part of advanced manufacturing technology.

It is widely used in the processing and manufacturing of complex parts in aviation, aerospace, shipbuilding and other industries. It can effectively save energy and raw materials, simplify the production process, and its technical and economic benefits are both. Very impressive. Compared with ordinary die forging blades, precision forging blades can save about 20% to 25% of metal, and the material utilization rate is increased by more than 50% [1]. The use of precision forging technology to manufacture blade blanks reduces the cost of blade manufacturing, and at the same time puts forward more stringent requirements for the efficient and precise CNC machining technology of precision forged blades. In the process of precision forging blade manufacturing, due to the complex processing technology, poor blank consistency, and low clamping and positioning accuracy, the processed blades have poor precision, low efficiency and high rejection rate.

Research on how to use digital detection methods to quickly measure and locate parts blanks, and further realize the self-adaptive processing technology integrating digital detection, model reconstruction and CNC machining [2] to improve the manufacturing accuracy of aero-engine blade parts, The processing efficiency and automation level are of great significance. Adaptive CNC machining technology has become a key technical problem restricting the efficient and precise machining of aero-engine blade parts in my country. This paper will analyze the structural characteristics of aero-engine precision forging blades and the problems existing in the current manufacturing process, and discuss the application of adaptive CNC machining key technology in CNC machining of aero-engine precision forged blades. Problems Existing in the Manufacturing of Precision Forged Blades of Aero-engine.

Aero-engine precision forged blades and airfoil profiles

It is a complex space curved surface, and the arc radius of the intake/exhaust edge is small, and the curvature and torsion of the entire blade part change greatly, which is a typical thin-walled complex curved surface part. The quality of the precision forging blade profile is guaranteed by the forging die, and the forming precision is high. After forming, the blade body profile does not need secondary processing. Limited by the current precision forging technology, the blade tenon and inlet/exhaust edges cannot be precision forged. Forming requires CNC machining. In the process of precision forging blade manufacturing, due to the complex processing technology, poor blank consistency, and low clamping and positioning accuracy, the processed blade has poor precision, low efficiency and high rejection rate. Therefore, the precise positioning of the surface of the blade body of the precision forged blade and the smooth transition processing of the intake/exhaust edges are the key issues of its CNC machining technology. The geometric model of the precision forged blade

At present, precision forging blades are usually preliminarily clamped using special fixtures and calibration tools, and then operated by the operator.
The staff repeatedly debugged to achieve the final clamping and positioning. However, the precision forged blades are thin-walled parts, and the inconsistency of the precision forged blade body profile and the fixture tooling and other factors lead to low positioning accuracy and efficiency, and even cannot meet the requirements of machining accuracy. The blade tenon is processed by using a special fixture to locate the blade body surface, which is prone to problems such as out-of-tolerance between the tenon and the blade body, low processing efficiency, and poor product consistency.

The intake/exhaust edge processing of aero-engine blades mainly adopts manual polishing, numerical control controlled milling and other manufacturing processes [3]. The traditional manual polishing method is to control the blade section according to the template shape, processed precision forging blade inlet/exhaust edge surface precision is low, easy to burn, low efficiency and unstable product quality. The use of CNC grinding technology to process the intake/exhaust edges of precision forging blades cannot solve the problems of uneven distribution of allowances, cross-sectional surface shape and position tolerance. Using CNC milling to process the intake/exhaust edges can well solve the problems existing in the above processing technology.

However, precision forging blades belong to thin-walled parts. After precision forging, the geometric parameters of the blade profile and the theoretical model will be different due to uneven stress distribution and clamping deformation. According to the theoretical model for CNC milling, there will be a problem that the transition area cannot be smoothly spliced, which affects the processing quality of precision forging blades.

At present, it is difficult to meet the surface integrity of precision forging blades by using traditional blade processing methods.The requirements of performance and precision control restrict the improvement of the high-efficiency precision machining and manufacturing level and manufacturing capacity of blade parts under the background of composite manufacturing technology.

Application of Adaptive NC Machining Technology

Adaptive machining can be divided into process adaptive and geometric adaptive. Process adaptation can be divided into optimal adaptive control system (Adaptive Control Optimization, ACO) and constraints

There are two types of adaptive control systems (Adaptive Control Constraint, ACC). ACO pursues an optimal machining process index, such as machining time, removal rate or surface quality to achieve an optimal index. ACC is to keep a certain constraint constant, such as torque, cutting force or cutting power, in order to improve the machining efficiency, maintain the stability of the machining process and ensure the machining quality. Geometric adaptation is machining that changes with the shape or position of the part, also known as adaptive CNC machining. Adaptive CNC machining technology integrates a number of technologies in the field of digital manufacturing such as digital inspection, workpiece positioning and model reconstruction.
A system solution for efficient and precise machining of sheet parts. Given the adaptive CNC machining technology

In view of the broad application prospects of the technology, various developed countries have carried out related technology research. For example, the aerospace project prioritized under the EU’s sixth framework – the automatic repair system for aero-engine turbine components
AROSATEC is a typical case of the application of adaptive machining technology [4]. The system integrates

The self-adaptive machining technology of the German company BCT is used to automatically generate the machining path by detecting the geometry of the blade repair area online. The adaptive machining technology of the British TTL Company is quite mature, and has been successfully applied to the CNC machining and repair machining of blade parts [5]. The PowerINSPECT adaptive processing module launched by the British company Delcam monitors the actual position of the part in the process of digital inspection, and automatically establishes the position correspondence between the part and the processing path [6]. The domestic research on adaptive machining technology started late. GAO et al. [7] realized the adaptive repair of complex aviation parts by re-planning the tool position trajectory based on the reconstructed model through the precise detection of the parts to be repaired. The Key Laboratory of Modern Design and Integrated Manufacturing Technology of Northwestern Polytechnical University of the Ministry of Education has accumulated rich experience in efficient and precise CNC machining of thin-walled blades, and has carried out a lot of research work in adaptive CNC machining technology. The key technical problems of CNC machining, the establishment of blade class zero
The self-adaptive CNC machining system for parts has passed the test verification.

Adaptive CNC machining technology is suitable for machining complex curved surfaces with uneven margins, repairing blisks and CNC machining of hollow blades. It is the best way to improve the accuracy and efficiency of traditional CNC machining.

From the perspective of technical realization, the adaptive CNC machining technology is based on the design model of the part to be machined and the corresponding machining program, and obtains the orientation change and shape deviation of the part model according to the actual measurement results of the machining area, and automatically based on the nominal machining program. The machining code of the actual machining area is adaptively generated, and the smooth transition with the pre-process machining surface model is guaranteed. This technology covers many key technologies in the field of CAD/CAM, such as CNC machining programming, surface modeling, digital inspection, reverse engineering, workpiece clamping and positioning, and is an important part of advanced manufacturing technology.

The adaptive CNC machining system is mainly composed of 4 modules:

  • (1) The digital detection module obtains the feature point sets required for registration and positioning and model reconstruction respectively by means of three-coordinate measurement or on-machine measurement;
  • (2) The registration and positioning module, based on the measured feature point set, determines the position of the theoretical model in the blank through the registration algorithm, and realizes the self-adaptive optimization of the machining allowance;
  • (3) The process geometry model reconstruction module, based on the measured feature point set, regenerates the process geometry model of the area to be processed.
  • (4) The automatic generation module of the processing trajectory, based on the reconstructed process geometric model, automatically generates
    CNC machining tool position trajectory, and finally complete the adaptive CNC machining of parts. The basic workflow of adaptive CNC machining is shown in Figure 2.

The key technology of self-adaptive CNC machining for precision forging blades

Based on the structural characteristics and manufacturing process of precision forging blades, adaptive CNC machining is based on blade theory
Based on the model, the feature point set based on registration and modeling is obtained through digital detection, and the feature point set of the blade is registered with its theoretical model to realize the rapid positioning of the blade. On this basis, the self-adaptive optimization of the margin of the blade to be processed area is realized by the registration method, and the process geometry model is automatically constructed in combination with the theoretical model of the blade. Finally, according to the theoretical model of the blade and the geometric model of the process, the blade machining trajectory is automatically generated online and sent to the CNC system to realize the high-efficiency, precise and adaptive CNC machining of the precision forged blade. Key Technologies Involved in Adaptive NC Machining of Precision Forging Blades

There are the following 3 aspects:

1 Clamping scheme

The CNC machining of precision forged blades is usually based on the precision forged blade body surface for positioning. When planning the clamping scheme, it is not only necessary to ensure the accuracy and stability of the positioning and the clamping deformation control of the free-form surface of the blade caused by clamping. Moreover, it must meet the requirements of convenient loading and unloading, high positioning accuracy, uniform clamping force, and good consistency of repeated clamping. In this paper, the method of hard clamping is used to plan the clamping scheme of the precision forging blade, and the fixture is required to meet the repeatability and precision of the adaptive CNC machining requirements by positioning and clamping the blade body of the precision forging blade under the premise of ensuring the positioning accuracy of the surface. Stability requirements. The precision forging blade self-adaptive CNC machining fixture consists of two parts, which respectively realize the CNC milling of blade tenon, blade tip and intake/exhaust edge. The fixture design model is shown in Figure 3.

The precision forging blade adaptive CNC machining fixture is mainly composed of a turntable, a fixture box, a box cover, a process

Table positioning block, fastening bolts, tapered positioning pins, positioning blocks, etc., are designed to meet the six-point positioning principle. The contact surface between the box body and the blade is obtained by processing the blade body, which plays the role of fitting and clamping; the blade is clamped through the transmission of the hexagon socket bolt to the parts, which has a uniform and stable clamping force. Function; when clamping the blade, use a torque wrench to tighten the bolt to realize the clamping process of the blade, which is convenient for clamping and avoids the pinching and falling off of the blade; the clamp is connected to the machine tool through a pneumatic quick-change chuck, which is convenient for the loading and unloading of the robot in the automatic production line . The fixture is simple in structure, convenient in installation, high in positioning accuracy, can be assembled and disassembled many times without affecting the positioning accuracy, and meets the requirements of self-adaptive CNC machining of precision forging blades.

2 Digital inspection

Digital inspection is to obtain the discrete surface of the blade through specific measurement equipment and measurement methods.

Measurement process of point data. By obtaining the three-dimensional information of the surface of the part, it provides basic data for the registration, modeling and CNC machining of complex surfaces. According to the structural characteristics of precision forging blades and the requirements of efficient and precise self-adaptive CNC machining, the combination of three-coordinate measurement and on-machine measurement is an ideal measurement method at present. At the same time, the distribution and number of measurement points directly affect the accuracy of registration and positioning and the accuracy of surface model reconstruction. Therefore, it is very important to achieve high-efficiency and high-precision data point collection on the surface of the part. The number of measurement points is usually determined according to the profile tolerance, machining accuracy, measurement system and shape and size of free-form surfaces given in the design drawings. The distribution form of the measurement points has a great relationship with the curvature of the free-form surface. How to automatically adjust the distribution of the measurement points to be consistent with the characteristics of the surface according to the change of the curvature of the surface, so as to realize the self-adaptive distribution of the measurement points, has been carried out by many domestic and foreign scholars. corresponding research.

According to the distribution form of measuring points on the surface, the surface sampling methods mainly include uniform sampling method, curvature sampling method, mixed sampling method and so on. In view of the structural characteristics of precision forging blades with small arc radius and large curvature change at the intake/exhaust sides, the blade body surface can be divided into four areas: intake side, exhaust side, blade basin, and blade back. The measurement points are distributed according to the curvature sampling method when measuring the inlet/exhaust sides of the blade, while the blade basin and blade back are distributed according to the mixed sampling method of equal arc length and equal parameters, as shown in Figure 4.

3 Model registration and positioning

Commonly used registration methods can be summarized as
The next 3 categories:

  • Marker-based registration method. This method is a simple and effective registration and positioning method, which is generally used for the combined positioning of multi-view measurement data in product geometry detection;
  • Feature-based registration method. The method uses the product measurement data and the geometric features on the model as a reference to perform the registration and positioning of the model;
  • The registration method based on the surface point set. This method uses the surface measurement data points of the product to perform model registration operations to determine the spatial transformation matrix between the models to be registered [8]. The Iterative Closest Point (ICP) algorithm is a typical registration method based on the surface point set [9]. Due to the good versatility of this algorithm, it has been widely used in the workpiece positioning and detection of free-form surfaces.

Model registration is a prerequisite for realizing part positioning, model evaluation, error analysis, etc.The research of the problem is to improve the precision and efficiency of digital detection. For precision forged blade geometry inspection

According to the design requirements of different tolerances in different areas of the blade profile, the registration method based on the constrained area can be used on the basis of the ICP algorithm to realize the rapid matching between the measurement data and the theoretical model of the precision forging blade. allow. Taking the theoretical model surface S as the benchmark, the area formed between the inward and outward error surfaces S 1 and S 2 is called the constrained area, and the area outside the constrained area is the free area, as shown in Figure 5. By judging whether the data point is located in the area between the equidistant surfaces S 1 and S 2 , the different weights of the point in the registration process are determined. Among them, the weight factor is a function related to the distance from the point to the surface,can be expressed as:

Among them, ε1 is the distance between the surfaces S 1 and S; ε2 is the distance between the surfaces S 2 and S; and both ε1 and ε2 are greater than 0, dist(t)(P, P1 ) represents the t-th iteration process , the distance from the data point P to the projected point P 1 on the theoretical model; dist(t−1)max represents the maximum distance from all data points to the corresponding projected point of the theoretical model during the t-1th iteration. The mathematical model of the objective function of the introduction of the constraint area weight factor is as follows:

In the formula, Pi (i=1, 2, …, N) is the measurement data on the model to be registered, Pi’ is the closest point of Pi on the surface of the theoretical model, R and T are the rotations during registration, respectively matrix and translation matrix. By solving the rotation matrix R and translation matrix T, the objective function F 1 is minimized. The transformation matrix R and T in solving the objective function is a constrained optimization calculation problem, and the mathematical model of the objective function can be solved based on the ICP algorithm. The ICP algorithm reduces the average error of the corresponding point set in each iteration process by finding the least square sum, and reduces the distance between the corresponding point pairs by finding the nearest neighbors. Data point sets are related.

The surface roughness value of the part is 1~2 grades, and the surface stress state is all compressive stress. The surface morphology after vibration polishing is greatly improved, the surface texture is uniform and regular, and the surface quality and surface integrity of the part are greatly improved.

The edge of the tongue and groove adopts CNC automatic chamfering machine to realize the automatic mechanical forming processing. Compared with the traditional manual processing, the surface roughness of the tongue and groove edge is good, the rounding size is uniform, and the edge size consistency between different tongue and groove is more than 90%. The processing quality is greatly improved to meet the performance requirements.

The surface integrity manufacturing technology application research carried out above has been applied in the mass production of turbine disk parts, and through the optimization and curing of process parameters, a typical processing technology has been formed, and high-quality and qualified product delivery has been achieved.

The technical research still needs to be further deepened and popularized, especially the influence of the surface integrity index on its performance and life needs to be systematically studied and tested. By vigorously carrying out innovation and exploration of surface integrity manufacturing technology, it can meet the needs of high reliability and long life of a new generation of aero-engines.

In Conclusion

This paper analyzes the problems existing in the manufacturing process of aero-engine precision forging blades, and systematically expounds the key technologies such as clamping scheme, digital inspection, registration and positioning, and process geometry modeling in adaptive CNC machining.

The in-depth research and application of this technology can not only effectively solve the technical problems existing in the machining of precision forging blades, but also have certain guiding significance for the efficient and precise machining of blade parts under the background of composite manufacturing technology.

As an important part of intelligent machining, this technology realizes the self-adaptive control of process parameters on the basis of satisfying geometric self-adaptation, which is the development direction of future self-adaptive machining technology.

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