Quality control of turbine blades in automated production cell

Optical measuring machines offer advantages in the quality inspection of turbine blades. Therefore the CORE High Speed Scanning Systems from the Swiss Starrag AG are used in an automated production cell for the final quality control of turbine blades. The core requirements for the measuring machines are: high accuracy, minimum measuring time and high temperature stability.

Starrag Group has been the leading technology partner when it comes to the productive manufacture of gas and steam turbine blades for over 50 years. Turbine blades, compressor blades, impellers, blisks and complex structural parts of the highest precision are manufactured by Starrag machines. In addition, Starragkomplette develops flexible manufacturing systems, so-called FMS (Flexible Manufacturing Systems), which allow the production of a wide variety of parts in automated mode.

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Two CORE systems were successfully integrated into one FMS for final quality control. The production cell manufactures a wide variety of complex blade types from high-precision forged blanks and consists of four milling machines, which produce the turbine blades with a special clamping concept, as well as two cleaning systems and the two measuring machines. The individual stations are operated by a robot. Communication with the CORE measuring machines is controlled via the cell controller.

The CORE is a coordinate measuring machine with an optical white light point sensor. The optical white light point measurement is performed with a double-eye sensor. This is similar to the measurement principle with a tactile probe, but with a tiny white light spot whose diameter is about 40 µm small. This enables the precise detection of free-form surfaces or measurement of minute features, such as the small radii of the leading and trailing edges on turbine blades. Using the double-eye sensor and a stable measurement routine of point triangulation, even highly reflective surfaces can be measured without spraying the surface.

Time saving for shorter measuring cycles

The CORE is a 5-axis measuring machine with three linear axes and two rotary axes. The rotary axes are formed by the rotary table and the rotary joint on the sensor head. This combination and simultaneous control of the linear and rotary axes enables the best accessibility, optimized traverse paths and thus time savings for shorter measuring cycles. Particularly when measuring small leading and trailing edges on turbine blades, the optical metrology eliminates the need for probe radius compensation.

"The decision in favor of CORE was made after a rigorous selection process," explains Stefan Mahr, Sales Manager for Optical High Speed Scanning Systems at WENZEL. Roland Ziltener, Head of Quality Management at Starrag AG confirms, "We decided on an optical solution because no tactile system could measure the number of features required by the end customer in the required time. The CORE system allows us to measure the smallest features, where we would reach the limit with a tactile system."

"An important reason for our measuring solution was that we could meet the demanding cycle time," adds Mahr "In addition, the design of the CORE is extremely stable. Large linear guides with wide bearing spacings guarantee a long service life and stable operation, even under harsh production conditions. Furthermore, the CORE offers a compact footprint and does not require compressed air. The measuring system is therefore extremely mobile."

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The CORE measuring machine is operated fully automatically at Starrag and is ideally suited for direct integration into the production process. On the one hand because of the large temperature range, on the other hand the open design of the CORE with the large accessibility area from three sides allows easy automatic loading and unloading with an industrial robot. This communicates with the coordinate measuring machine via an I/O interface or Ethernet connection.

The rotary table of the CORE was modified according to customer specifications so that the pneumatic chuck used was integrated directly into the rotary table of the measuring machine. The system also interacts with the complex cell control system. This takes over the automated remote control of the CORE. The measuring device provides automated feedback on cell control, starts and stops measurements on command, transmits measurement logs and reports changes in the machine's status.

"Our experience of working with Wenzel on this project has been extremely positive," says Ivo Ritter, project manager at Starrag. He is in charge of the overall management of the FMS project. "According to our experience, the communication channels at Wenzel are short and the hierarchy structure is very flat. This allows us to react very quickly to changing conditions, especially in project work, which is immensely beneficial to the progress of the project. We really enjoy working with Wenzel and are looking forward to future challenges together," praises Ritter.

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