QMT Features: April 2012
Laser scanner closes the gap
The new Nikon LC15Dx steps up 3D scanning accuracy to deliver more in-depth insight into turbine blade aerofoil geometry and provide more accurate section and feature extraction.

Metrology is widely used to trace inaccuracies in blade geometry and positioning, which may cause energy conversion loss and untimely blade failure. The measurement of turbine blades used to be primarily the domain of a CMM and a tactile probe, which provide the required accuracy, but at slow or moderate throughput times.

The quality of blades in gas and aerospace turbines is critical. As hot combustion gas expands through the turbine, it spins aerofoil blades to drive a generator that produces electricity. Air-cooled through its internal channels, turbine blades reach high rotational speeds and face temperatures in excess of 1000°C. Turbine blades are designed for optimum aerodynamics and mass centre location, and are made of advanced metal alloy castings to increase strength, resist extreme temperature, and avoid corrosion.

Laser scanners already proved successful for the inspection of stationary gas turbine blades. At Siemens Fossil Power Generation in Berlin, for example, metrology engineers use Nikon Metrology laser scanners to verify the shape of the aerodynamic blade surface and the dimensions of milled planes and flanges, drilled holes and alignment notches. Compared to traditional tactile inspection, laser scanners capture full 3D point data in a shorter time frame, and embed measurement and analysis in an automatic digital inspection process. At the same time, non-contact measurement requires little or no blade pre-alignment, and eliminates probe compensation when scanning freeform blade surfaces.

High scanning accuracy, resolution and speed
For over a decade, Nikon Metrology pioneered laser scanning and gradually sharpened the capabilities of this non-contact measuring technology. The Nikon LC15Dx digital laser scanner is a new milestone, as its accuracy comes close to the accuracy expected when using a CMM and tactile probe. Thanks to new solid state laser scanner technology, an innovative calibration method and high quality Nikon lens, the LC15Dx achieves a probing  accuracy of 2.5µm and a multi-stylus test accuracy of 6µm in tests comparable to EN/10360-2 and -5. Unique thermal stabilization also enables to start measuring immediately after powering the scanner. The LC15Dx offers the accuracy, resolution and speed to successfully tackle the more demanding turbine blade metrology applications.

“The LC15Dx is a significant breakthrough product that allows our customers to realize the measurement possibilities and productivity gains customers have been asking for,” comments Kenji Yoshikawa, chairman and CEO of Nikon Metrology, “Following an intense R&D phase involving engineering from Nikon Japan, the LC15Dx is the first CMM laser scanner to receive the Nikon branding.”

Measuring turbine blades of aero engines
Turbine blade and jet engine manufacturers want to gain a full appreciation of the dimensional quality of the blades  without compromising on cycle times. When the LC15Dx passes over the blade, a laser line is projected onto the surface. The line measures 70,000 points per second at intervals of 22µm (0.0008in), digitizing a complete blade in just a few minutes.

Due to its superior scanning capabilities, the LC15Dx successfully responds to some of the more challenging turbine blade inspection challenges, including twist, leading/trailing edge radius and chord length. The full 3D image of the blade allows far more complete section analysis and feature extraction than possible with a limited number of tactile inspection points.

As the entire part is checked to the design CAD model, any areas of concern are immediately highlighted using colour mapping. Further investigation and analysis is possible using fly-outs, sections and a library of geometric dimensioning and tolerancing (GD&T). This approach offers the capability to discover and tackle issues in a much faster way.

The LC15Dx also supports a greater range and mix of surface materials, finishes, colours and transitions. Nikon Metrology’s unique third-generation Enhanced Scanner Performance (ESP3) technology maintains accuracy, speed and data quality by intelligently adapting the laser settings for each measured point in real-time. Also an advanced software filter neutralizes unwanted reflections while changes in ambient light are absorbed by a high-grade daylight filter. As such turbine blades form different materials or surface finishes can be measured more efficiently without user interaction, manual tuning and part spraying.

One of the reasons that laser scanning firmly reduces the investment payback time is the ease of programming and measuring. Automating a scanning inspection routine requires only the straightforward definition of the scanner travel path and corresponding scanner angles. It is much easier than to program the hundreds of individual touch sensor points for a tactile inspection job. In addition, laser scanning only needs standard holding fixtures, whereas traditional inspection methods demand costly dedicated positioning and fixation tooling.

High-precision CMM applications
The application reach of the LC15Dx stretches way beyond turbine blades. It provides significant benefits for a wide variety of high-precision parts, including freeform surface and small details, semi-rigid parts and demanding materials. The LC15Dx is a valid alternative to a tactile probe for an increasing number of high-precision CMM measuring applications. Regardless whether targeted development, production and reverse engineering, the scanner supports a diversity of production methods (machined, molded, stamped, cast and forged) and material types (metal, plastic, rubber, clay, ceramic and composites). It reliably and accurately captures surfaces, geometric features, profiles and sections.

In some cases a single sensor technology is insufficient for measuring all of the features. The LC15Dx can be combined with tactile probe measurement to form a versatile multi-sensor CMM. Depending on the application, both technologies can be used independently or together in the same inspection program. Fully automatic sensor changing is possible with the addition of an optional change and storage rack that is mounted on the table of the CMM.  Nikon’s customers can rely on Nikon Metrology as a solution provider of fully integrated metrology systems, covering LK CMM, laser scanner, multi-sensor software and point cloud software. The advantage for customers is that they benefit from a single after-sales services organization that delivers true economic value.l
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