As a key rotary component of aero-engines, roundness of an impeller is in a close relationship with engine running stability, fuel consumption, and a whole life service. In this paper, organic sweeping is conducted for the main technologies and measurement difficulty of roundness of an impeller, and a comparative study is done for the most popular methods such as contact-type roundness testers, non-contact laser scanning, two-dimensional image methods, and coordinate measuring machines (CMM). It focuses on investigating the roundness measurement scheme of high-precision CMM combined with mathematical fitting algorithms, and verifies experimentally its performance and engineering practicability in micron-level precision control. The final results show that this method possesses high repeatability, high automation, and applicability to complex structures, etc., which provides a sound technological foundation for quality control of impeller manufacturing.
Introduction
In new aircraft engines, impellers are important components that face severe environment such as high temperature, high pressure, and high-speed rotation. Accurate control of their geometry is an important connection to ensure stable engine operation and extend service life. Particularly, the roundness deviation of the impeller cross-section can lead to non-uniform distribution of airflow, abrupt axial force change, and dynamic unbalance, thereby resulting in vibration, wear, and even fatigue failure. Therefore, it is an important technical issue in the aviation manufacturing industry to create a precise, highly repeatable roundness measurement method suitable for complex curved surfaces. Through this research, we attempt to explore and verify an efficient measurement method with coordinate measuring technology as the core and fitting algorithms for aiding analysis.
Overview of Impeller Roundness Measurement Technology
Roundness is a geometrical tolerance used for the measurement of the extent to which a cross-sectional profile deviates from an ideal circle. In manufacturing and precision machining, the real part profile can hardly be an ideal circle, so measuring tools are required to determine its deviation and keep it within the range of acceptable errors.
Definition and Evaluation Standards of Roundness
Roundness is a geometric dimension measuring the difference between a cross-sectional profile and an ideal circle, typically represented by the maximum radial deviation (μm). At aviation manufacturing standards, roundness control on the micron level has been the standard. High-quality measurement methods must meet the following three indicators:
- High resolution and repeatability: Capable of continuously discovering small profile errors;
- Fast data acquisition: Appropriate for batch inspection and on-line real-time quality control;
- Complex geometry adaptability: Suitable for measuring impellers featuring multi-curvature transition structures.
Summary of Main Measurement Methods
(1) Contact-type Roundness Tester Measurement
Roundness testers employ mechanical styluses to radially scan the workpiece profile in order to obtain high-precision roundness data. However, this method is also plagued by such limitations as small number of measurement points, potential surface scratches caused by contact probes, and long measuring time, thus making its application challenging for modern-day high-efficiency manufacturing environments.
(2) Non-contact Laser Scanning Measurement
Laser scanning devices acquire vast point cloud data on the surface of an object by refracting and reflecting laser light, best suited for examining fine free-form surfaces. For instance, the ZLDS202 laser scanner is capable of sustaining high-density sampling of 16,000 points per second while maintaining measurement accuracy at the expense of efficiency. However, the technology is highly dependent upon material reflectance, roughness of the surface, and conditions of measurement, along with complicated preprocessing of data and relatively low result stability.
(3) Coordinate Measuring Machine (CMM)
CMM has emerged as the conventional method of geometric measurement of aircraft components, using highly precise probes to record numerous sample sections of key locations and compare them with theoretical models. Due to extremely good measuring accuracy (±1μm), excellent stability of repetition, and the ability to adapt to complicated structures, this technology has been highly applied in quality inspection of engine impellers.
(4) Two-dimensional Image Measurement Technology
The two-dimensional measurement method by CCD cameras and image processing technology can quickly capture profile images and roundness error measurement. With the advantages of automation and non-contact, it is suitable for low-complexity parts or instant initial inspection use. However, its accuracy relies on pixel resolution and edge detection algorithms, making it difficult to meet high-precision applications.
Research on CMM-based Roundness Measurement Scheme
Design of Measurement System Process
Addressing the stringent requirements for geometric precision of impellers in medical and aviation use, we have designed a precise roundness measurement system in terms of coordinate measuring machines (CMM) with the overall process involving four main steps:
- Special Fixture Positioning: Create special high-rigidity positioning fixtures according to impeller structural features to ensure stable workpiece positioning without displacement, minimizing measurement errors caused by inappropriate clamping or vibration.
- Measurement Section and Sampling Point Planning: Select representative sections (e.g., blade root, blade waist, blade tip) as measurement objects, and distribute an equal number of sampling points (e.g., 12–36) on each section for ensuring the accuracy and integrity of profile reconstruction.
- Data Acquisition and Path Control: Employ high-precision trigger probes and coordinate with pre-established paths in the CMM system to automatically complete probe movement and point cloud acquisition, improving measurement efficiency and consistency.
- Roundness Analysis and Calculation: Export the data acquired to professional measurement software, perform profile fitting and error analysis through geometric algorithms, ultimately obtain the roundness value of each section, and create the report.
Research on Mathematical Fitting Algorithms
Precision of measurement largely relies on fitting algorithms. For describing the geometric deviation of impeller profiles suitably, we selected and studied two popular algorithms:
- Least Squares Circle Fitting: Determines the center and the optimal-fit radius for the best-fitting circle by minimizing the sum of the total squared distance of all measurement points to the circle surface. The method is suitable to measure the general trend of the profile with high stability and is currently used most widely as a roundness test method.
- Maximum Gap Method (Minimum Enclosing Circle Method): Estimates the smallest enclosing circle of all measurement points, whose maximum radial deviation is taken as index. It is more concerned with identifying the most outlying errors or abnormal points of deformation, suitable for limit tolerance analysis in quality control.
In practical applications, we propose using the two algorithms in combination: the first to assess global trend deviation, and the second to identify locally severely distorted regions, ensuring therefore robustness and engineering applicability of results by the measurement system.
Conclusion
This paper uses a systematic study and comparison of several traditional roundness measuring methods of aircraft engine impellers to explain the pioneering position of coordinate measuring machines and mathematical fitting algorithms in high-precision measurement. This method has had good application effects in engineering practice, especially in the quality control of high-performance titanium alloy impellers, to reach the target measurement goals of high resolution and low error.
