As the most critical component of fluid machinery, impellers are widely used in high-speed rotation equipment such as aero-engines, gas turbines, and turbopumps. With their intricate geometric structure and high-precision machining, they put extremely rigorous demands on testing technology. Especially with the widespread application of advanced materials such as ceramic matrix composites and titanium alloys, inspection difficulty of impellers has also increased further. German-manufactured ZEISS coordinate measuring machines (CMMs) have become essential tools for high-end contemporary impeller quality assurance due to their nanometer-level accuracy measurement, excellent scanning performance, and intelligently designed software systems.
Introduction: Technical Background and Challenges of Impeller Inspection
For precision manufacturing industry, impellers are prime candidates for quality control given that they possess vast free-form surfaces, wide spatial curvatures, and high assembly requirements. Specifically in power machinery such as aero-engine core stages, gas turbine turbines, and water pumps, their operating conditions are extremely severe with high stressing expectations for symmetry, profile accuracy, and balance. The traditional contact measurement tools such as micrometers, calipers, and projectors have measurement blind spots, low repeatability, and low efficiency in measuring complex 3D surfaces, far from the needs of modern manufacturing for “whole-process digital inspection”.
During an inspection project for gas turbine rotor impellers in a plant that produces energy equipment, I was presented with the same challenge: In the initial stage, there was manual projection with calipers utilized to measure only blade tip height and rim thickness, unable to detect form and position errors, not even managing complete-surface contour variation. After integrating ZEISS CMM with CALYPSO and PiWeb systems, we first achieved complete contour digital reproduction of the impeller, batch statistics of form and position errors, and closed-loop feedback for quality traceability. Inspection efficiency was increased by as much as 60%, and traceability of data was 100%.
Measurement Preparation: Environmental, Workpiece, and Equipment Prerequisites
Before high-accuracy impeller measurement, preparation of the environment, workpiece-to-be-tested, and measuring equipment has to be done systematically to ensure data stability and reproducibility. The following are most crucial:
Environmental Control Requirements
ZEISS CMMs are very sensitive to the environment in terms of temperature and humidity. The measurement room should be (thermostatically controlled) at (20±1)°C, relative humidity between 45%–65%. High-temperature fluctuations cause thermal drift, leading to deviation in the data. In the meantime, equipment should be mounted on an anti-vibration platform, separated from machining workshops, to prevent mechanical vibration from interfering with measurements. To guarantee measurement precision, the area of inspection must be clean and free of dust as well in order not to have oil mist, metal dust, etc., stick to the surface of the probe or workpiece.
Workpiece Inspection Preparation
Workpieces of impellers must be cleaned before checking to avoid any burrs, oil marks, or water marks on the surface. When the workpiece is heat-treated, it should cool down to room temperature naturally before being measured to avoid thermal expansion errors. The workpiece positioning reference surface must be intact and transparent, as it is used as a reference for modeling measurement and arrangement measuring points, which is closely related to the measurement model precision.
Equipment and Probe Status Inspection
ZEISS CMMs need to carry out daily automatic calibration, including checking of three-axis linear accuracy, ball bar testing, and probe calibration. High-sensitivity active scanning probes such as VAST XT gold with temperature compensation and ongoing surface scanning are optimally designed for geometric inspection of complex surface impellers. All the measurement accessories, comprising normal spheres and measurement blocks, shall have national traceability metrology certificates to ensure traceability of the system accuracy as well as to carry out process control.
Fixture Design and Clamping Specifications: Stability and Accuracy as Premises
Fixtures form the basis for achieving repeat positioning and measurement reproducibility during inspection on CMM. For multi-surface asymmetric parts such as impellers, the design and stability of the accuracy of fixtures directly determine the ultimate accuracy of measurement.
Selection of Positioning References
Impellers typically take the center hole of mounting as the reference for coordinates, with the added axial reference surface for 3D positioning. In practical application, I adopt the “three-surface six-point” method of constraint: bottom surface height limitation, center hole location constraint, and blade angle rotation constraint, keeping the workpiece in a stable position without freedom in the fixture.
Fixture System Selection
ZEISS UMM modular fixture system is preferred due to its modularity and extremely high flexibility for dealing with multi-specification impeller workpieces. Large non-standard workpieces require flexible fixture systems tailored to their needs, with no metal in the way of the measuring point area and clamping force not causing micro-deformation of the workpiece body. The pre-tightening force test should be performed on all fixture components before measurement to prevent micro-movement errors.
Program Modeling and Measurement Execution: Integration of Digital Path and Intelligent Scanning
Due to the complex geometry and steep surface transition of impellers, traditional point measurement methods cannot meet accuracy and efficiency requirements. With the help of CMM systems and digital software platforms, the whole intelligent inspection process can be designed to achieve automated closed-loop control from model modeling to path scanning.
CAD Modeling and Feature Selection
ZEISS CALYPSO software can import 3D CAD models directly in STEP or IGES format to be used as a reliable reference for measurement path building. In my model, I usually select the following significant dimensions as inspection targets: rim coaxiality, blade height, blade root thickness, inlet angle, outlet angle, pitch circle diameter, and blade profile error. For free-form surface areas, it is suggested to turn on the VAST XT active scanning capability, substituting single-point detection with continuous path sampling to maximize data density and contour reduction precision.
Path Planning and Probe Parameter Setting
The blade area has (sharp) changes of curvature, which calls for high-precision scanning while protecting probes. I typically assign a scanning point at each 5° with a dense probe path and control the probe scanning speed between 3 mm/s to avoid probe rebound and lag errors. In measuring deep grooves or blade root regions of small widths, the multi-probe array function of the VAST probe can be called on to dynamically set alternative probe directions and lengths, thus facilitating greater access to compound areas.
Automated Measurement Execution
After path preparation and interference simulation, execute the measurement program by utilizing CALYPSO software. The system will carry out all scanning processes automatically without any intervention and generate structured inspection reports in real time. During the process, the chamber door should remain closed and no mid-course intervention is permitted to upset the probe positioning accuracy. The last inspection outcomes can be uploaded to the PiWeb quality analysis platform at the same time in order to realize cross-batch data comparison, trend monitoring, and visual summarization, offering smart data assistance for quality control.
Data Evaluation and Error Handling Mechanism
The core of high-precision impeller measurement is not just data collection, but also a rational assessment of geometric deviations and closed-loop control of abnormal results. In traceability of inspection results and machining consistency, the following points are particularly crucial:
Key Geometric Error Evaluation
- Profile Error: Used in measurement of the overall deviation from the design profile to the free-form surface of the blade and typically kept within ±0.02 mm. Surface drift and local deformation in any direction can be quickly identified through the “Profile” surface comparison function.
- Position Error: With regard to verifying assembly offset in the direction of blade thickness and rim runout, especially in impellers with exacting static balance precision requirements, this is a critical control index.
- Angle Error: Such as installation blade angle, helix angle, etc., which should be rigorously controlled to ±0.1° to prevent loss of aerodynamic performance caused by rotational surface misalignment.
In a previous work on a turbocharger project, I discovered 3 products with local profile drift by the above factors. We found after back-checking that support points of fixture were looseness, which was totally settled by structural readjustment.
Consistency and Stability Verification
Apply batch sampling inspection, at least 3 pieces per batch for repeatability analysis, where measurement standard fluctuation is controlled at ±15%. Further, perform a monthly system linear drift analysis to compensate for potential deviations owing to environmental changes or equipment aging.
Closed-Loop Handling of Abnormal Data
If data are outside of specified control limits, verify sequentially fixtures, probes, temperature control, and calibration procedures. Abnormal records are twice verified by quality engineers, initiate (begin) a quality anomaly analysis process as required, and recommend feedback process adjustment.
Safety Specifications and Equipment Maintenance
ZEISS CMMs are precise instruments, and proper operating procedures and maintenance systems form the foundation for guaranteeing their long-term reliable operation:
- Prevent handling probes bare-handed to avoid static electricity and contamination;
- Daily routine operations include probe homing, self-inspection, and air filter drainage;
- Periodical quarterly calibration is conducted on systems by certified ZEISS service personnel;
- Prohibit any spraying, grinding, vibration-type processes in the measurement area to prevent environmental interference;
- It is recommended to use ZEISS original probe systems and spare parts to ensure integrated system performance.
Application Cases and Trends of Intelligent Measurement: Insights from CPS Company
As a high-precision component supplier to the energy industry, Italy’s CPS Company has particularly stringent quality control of key components like large impellers. To overcome the challenges of time-consuming and inefficient inspection of complex components and intangible errors, our company selected ZEISS MMZ T and MMZ G series large bridge-type CMMs, complemented by VAST scanning systems and PiWeb platforms, to implement full-process quality data traceability and tolerance control closed loops in production.
With the bridge-type CMM system, CPS has been able to measure long workpieces in 14.8m³ range in the production factory, not just saving measurement time but also reducing rework rates for products by nearly 30%. Its management pointed out that “precision inspection is not merely a qualification test but a condition for customer confidence.” This entirely conforms to my own practice in production enterprises—correct inspection has already evolved from “end-stage quality control” into a primary driving force for “front-end design.”
Conclusion
ZEISS CMM technology is an orderly, standardized, and digital integrated solution for modern impeller precision control. From inspection preparation to program development, from measurement execution to data evaluation, and subsequently to equipment maintenance and smart closed loops, it constructs not only a technical process set but a manufacturing philosophy of “measurement-driven quality.” With aviation production, energy equipment, and high-speed power systems advancing towards even higher performance and more precise structures, ZEISS CMM smart measuring systems will remain an indispensible technical support in the precision manufacturing domain.
