Integrated Quality Control Of Turbine Blades In An Automated Production Cell

A high speed optical measuring system allows for rapid measurement in an automated cell that produces turbine blades. Contributed by Wenzel

Optical measuring systems offer various advantages for inspection of turbine blades. Key requirements for the deployed measuring machines are: high accuracy, minimal measuring time and high temperature stability.

Swiss company Starrag Group has been producing gas and steam turbine blades for the past 50 years. Turbine and compressor blades, impellers, blisks and complex structural parts are manufactured using their machines. The company also develops complete flexible manufacturing systems (FMS), which allow the production of various parts in an automated mode. The company chose Core high speed scanning systems by Wenzel for an automated production cell for the final quality control of turbine blades.

Integration In Flexible Manufacturing Systems

The optical high speed system scanning a turbine blade.

Two such systems were integrated for quality control into a FMS. The production cell transforms high-precision forged blanks into various complex types of blades and consists of four milling machines with a special clamping concept, which machine the blades, as well as two washing systems and the two measuring machines. The individual stations are loaded by robot. The measuring machines are controlled via the cell controller.

The coordinate measuring solution has an optical white light sensor, and the optical white spot measurement is performed by a dual-eye sensor. The white spot measurement is similar to the measuring principle with a tactile probe, but with a tiny white light spot whose diameter is about 40 μm. This allows precise detection of free-form surfaces or the measurement of minute features, such as the small radii of leading and trailing edges on turbine blades, complex structures of artificial knee joints or any other part with similar features and measurement demands.

With the support of the dual eye sensor and a stable measuring routine of point triangulation, highly reflective surfaces can be measured. There is no need to spray or powder the measuring objects. The application of the white light sensor therefore ensures rapid data acquisition and reduces inspection time drastically. Reconfiguration to a new part is easily achieved by software reprogramming hence converting times are reduced to a minimum. Two different sizes are available for the measuring system and different configurations can be chosen. All configurations offer a large working distance and field of measurement.

Shortening Measuring Cycles

The 5-axis measuring system has three linear axes and two rotation axes. The rotation axes consist of the rotary table and the rotary joint on the sensor head. This combination and simultaneous control of the linear and rotary axes allows for accessibility, optimised travel distances and thus time savings for shorter measuring cycles. Especially when measuring small leading and trailing edges on turbine blades, the probe tip radius errors as well as out-of-plane errors are eliminated by the application of optical measurement.

Roland Ziltener, head of quality management at Starrag, said, “We had decided on an optical solution because no tactile system was able to measure the amount of features demanded by the end customer, in the required time.”

Production cell at Starrag.

“An important reason that favoured our measuring solution was that we could meet the demanding cycle times“, said Stefan Mahr, sales manager for optical high speed scanning systems, Wenzel. He added that large linear guide ways with wide bearing spreads allow for operation under harsh production conditions. In addition, the measuring system has a compact footprint with no need for compressed air.

Interaction With The Cell Controller

The measuring machines in the Starrag cell are fully automated, and the open design that features a large accessibility range from three sides makes automatic loading and unloading by an industrial robot possible. The robot communicates with the machine via an E/A interface or an Ethernet connection.

The machine’s rotary table was modified according to customer demands, with the pneumatic chuck being integrated directly onto the rotary table. The machines automatically return feedback to the cell controller, starts and stops measurements on demand, transmits measurement reports and updates machine status.

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