With the acceleration of aerospace, energy power, and high-end manufacturing industries, diversified structures, rigorous precision, and high efficiency are the characteristics of impeller components. Special measurement fixtures of conventional design possess a complete set of bottlenecks in inspecting multi-model impellers, such as low flexibility, complex replacement, and great variability in measuring accuracy.
Introduction
In novel manufacturing, as an integral component of turbomachinery, centrifugal pumps, compressors, etc., the complexity of the geometric structure and high-precision performance requirements of the impeller determine the substitutability of its measurement link. Especially in the conditions of different models and elastic batches during production, the traditional special measurement fixtures are strained. They not only lack good clamping versatility and lengthy debugging cycles but also (hardly) provide consistency in positioning accuracy and repeatability and significantly impact the enterprise’s strengths in lean production and quality control.
To this purpose, in actual engineering practice, I have analyzed step by step a measurement fixture design methodology based on the modular concept. The basic aim is to develop a “multi-model general, fast switching, and high-precision clamping” adaptable measurement platform to meet the urgent needs of the production line for high-reliability efficient measurement systems. In this paper, the structural structure, application domain, technical performance, and digital cooperation of the modular fixture system are discussed in detail.
Analysis of Multi-Model Impeller Measurement Requirements
As the increasingly important development trend of precision manufacturing and bespoke production develops, the varieties of impeller parts keep growing, and their geometric structures also exhibit diversified and non-standard attributes. Efficient switching and precise clamping of different models of impellers in the measurement link are how to become the major problem to be solved in the design of the measurement system and fixtures. The following talks about from two aspects: the structural difference of the impeller and the elasticity of the fixture.
Diverse Impeller Structural Features
There are many types of modern impellers. Standard structure changes include a broad range of diameters (Φ50mm ~ Φ200mm), blade numbers and directions, complex shaft hole structures (spigot holes, stepped holes, tapered holes), and asymmetrical keyways or complex connection features are embedded in some structure. The randomness of the above structures makes the traditional one-to-one specially designed fixturing difficult to meet the double demands of the rapid switching and high-precision control.
Common Requirements for Fixture Design
Aside from obvious differences in structure, requirements for impeller measurement clamping have some universal laws:
- Strong concentric positioning: Rotary measurement through center hole or axis shall ensure high coaxiality;
- High clamping stability: Prevent measurement error due to uneven clamping force or unstable structure;
- Unobstructed measurement area: There is a requirement to analyze the probe path and obstacle-free contact in the critical zones;
- Strong operation speed: To meet the rhythm needs of replacing the multi-model workpieces frequently.
From the technical perspective, how to obtain the “multi-adaptability” and “high efficiency” of the fixture without losing the measurement accuracy has been the key point for the research and development of the impeller fixture system.
Design and Composition of Modular Fixture System
The modular fixture system is a main facility for achieving effective clamping, rapid model change, and intelligent measurement. Its design concept should always depend upon the basic principles of “combinability, interchangeability, and adjustability”. Through the combination of standard interfaces and functional modules, quick fitting and high-precision measurement are enabled for complex parts with different specifications, which is especially suitable for the inspection of complex parts such as aviation impellers, turbine disks, and pump shells. The overall system arrangement can be divided into four functional core modules with a positional and cooperative relationship, to form an expandable, dependable, and flexible measurement fixture system.
Standard Base Module: The Positioning and Bearing Core of the System
Because it is the installation reference and functional bearing core of the fixture system, the standard base module needs to have high platform compatibility and stability. The module is generally designed as a matrix positioning hole plate or a T-slot platform of high rigidity, to be directly mated with the measurement equipment such as coordinate measuring machines and optical probe platforms. Its main function is to provide accurate installation locations and recurrence positioning accuracy for every kind of fixture components at the top layer, so every module of the system can be rapidly assembled and disassembled to various tasks. Meanwhile, the bottom is mainly made of high-strength cast iron or micro-deformation granite to ensure the structural stability and flatness standards in long-term operations.
Positioning Module (Flexible Core Shaft): A Fast Center Positioning Unit Adapted to Diverse Structures
To enable diversification of the central hole structure of different types of impellers, turbines, and shaft components, the positioning module is designed very flexible. The system can quickly replace all kinds of positioning pieces such as core shafts, taper positioning columns, and floating core columns according to the product structure to meet the central positioning requirements of keyway type, spigot type, and keyless type. The modules are designed by a sliding or quick-locking plug-in type, and the users are able to complete the replacement within seconds, which greatly improves the model change efficiency and reduces the operation complexity.
Adjustable Support Module: High-Degree-of-Freedom Accurate Support to Ensure Posture Stability
According to the requirements of supporting workpieces with different diameters, heights, and blade structures, the system designs multiple adjustable support modules. Typical forms include slide rail lifting supports, V-groove supports, ball head universal adjusting seats, and threaded fine-tuning columns, etc., and micron-level adjustment at the support points is realized through the use of the fine-tuning mechanism. The module is generally mounted with a quick-locking device and a clamping limit device to ensure the supporting rigidity and avoid workpiece deformation caused by over-clamping. The support module is adjustable, and the dynamic layout can be performed based on the specific product and measurement requirements to make the workpiece have the optimal posture after clamping, enhancing the accuracy and repeatability of measurement.
Quick-Change Interface and Identification Module: Realize Intelligent Linkage of the Measurement System
On the architecture of the intelligent measurement platform, the identification module and the quick-change interface are the key bridge between the “physical fixture” and the “virtual program”. There is a fixed QR code label or an internal RFID chip at the bottom of each fixture module group. After the module is mounted onto the base, the coordinate control system will read the current fixture configuration from the code reader automatically, to link the measurement software (e.g., PC-DMIS, QUINDOS) with automatically loading the relevant measurement program, coordinate conversion information, and tolerance definitions, which greatly improves the operation efficiency and data reliability. In the application of practical project work, I found that this function not only can reduce human operation errors but can also achieve the inevitable process evolution of measurement from “manual drive” to “system drive”, which is a crucial guarantee for automatic and unmanned measurement.
Application and Empirical Analysis of Modular Fixtures
The measurement object of three generally used sizes of centrifugal impellers (Φ80, Φ120, Φ180mm) were adopted in the process of implementation of a manufacturing enterprise, and the three original special tooling were substituted by the modular fixture system.
Comparison of Measurement Switching Efficiency and Accuracy
| Index Project | Special Fixture | Modular Fixture System |
| Single replacement time | 30 min | 5 min |
| Module universal adaptation model | 1 | ≥3 |
| Positioning repeat accuracy (radial) | ±0.03 mm | ±0.01 mm |
| Operator technical requirements | 1 technical worker | 1 ordinary worker |
Improvement of Measurement Consistency and Repeatability
By concentrating control on the module placement system and optimizing structural stability, measurement repeat error is brought down from the original ±0.025 mm to ±0.01 mm, and measurement consistency is enhanced by over 60%.
Cost and Management Benefits
The total production cost of the modules is reduced by 40%, the maintenance cycle is reduced by 50%, the fixture inventory management is eased, and operations and maintenance are improved.
Integration of Module Identification and Intelligent Measurement System
In addition to the module design, combined with Hexagon’s intelligent measurement software system based on PC-DMIS and QUINDOS platforms, I have also facilitated greater integration of information of the fixture system and automation of measurement. With functions like module identification technology, automatic programming parameter matching, and program execution automatically, “one-click measurement by non-professional operators” has been achieved.
For example, the impeller detection software solution supports:
- Probing protection and path planning automatically;
- Scanning of principal geometrical features such as flow channels, leading edges, and P points in multi-direction;
- Six degrees-of-freedom best fit and automatic report output of evaluation;
- Export of measurement result in structured format (TXT, 3D report).
The deep integration of the modular fixture system with the measurement software platform has formed a closed chain of “fixture-measurement-analysis” in completeness, and thus greatly increased the intelligence level of the system.
Conclusion
The modular impeller measurement fixture system is a key method to realize multi-model flexible detection. Through the combination of technologies such as structural reconfigurability, interface standardization, high-precision clamping, and information integration, the system has significantly improved the flexibility and efficiency of the measurement process and surmounted the primary problems of poor versatility, slow response, and unstable precision of traditional special fixtures.
