Toshiba GE Turbine Components (TGTC, Yokohama, Japan) has reduced the time required to inspect and measure steam turbine blades from 280 minutes to 45 minutes through use of the Maxos noncontact measurement system from NVision Inc. (Southlake, TX). The coordinate measuring machine (CMM) TGTC used in the past to inspect blades was not only slow but also unable to access hard-to-reach areas such as dovetail hooks and fillets. The Maxos uses five axes to reach every point on the blade and also generates specific and accurate measurements of critical areas. Resulting measurements are reported instantly and the need for additional manual inspection is eliminated.
“The Maxos optical scanner provides the best possible accuracy, eliminates the need for matte coating and integrates easily with our engineering and production processes,” says Tomio Kubota, president of TGTC. “Our trials also demonstrated that the Maxos is significantly faster than the other systems that we considered. The expertise and professionalism that were evident during this trial gave us the confidence to adopt this new technology.”
In the past, it took longer to inspect blades than to make them. A turbine bucket is comprised of an airfoil and a root. The long blades built at TGTC have mid-span geometry that provides support for the midsection of the airfoil. The complex geometry of these blades means that many cross-sections must be examined. The conventional 3-D CMM used in the past to inspect the blades was unable to reach many points on the root. Also many fillet radii are inaccessible to the CMM or too small to be measured by a touch probe ball.
The Maxos uses a proprietary noncontact probe consisting of a concentrated light that collects individual points at a rate of 100 per second, many times faster than a CMM. Unlike laser or white light fringe scanners, the Maxos can inspect turbine blades without having to apply a matte coating that introduces dimensional inaccuracy.
The Maxos provides accuracy of ±0.0004 inch and a resolution between measured points down to 0.0002 inch on this project. The system can be provided with a resolution as low as 0.0001 inch. The Maxos software is configured with an overall best fit of the measured geometry to allow a part with some error to fit within the overall tolerance envelope of the reference data. Win3DS Blade inspection software is configured to give fast results and different kinds of evaluations on mid-span, fillets, gaps and airfoils. Different best fits are available, including Gauss and Chebyshev.
In this application, the Maxos was configured with a five-axis horizontal arm with three linear axes and two rotary axes. One rotary axis is for the sensor and the other for positioning the blade. The rotary axes are servo controlled and are not indexed. This configuration makes it possible to measure every part of the blade even when it is mounted vertically. Vertical mounting is preferred for the long blades because it prevents them from bending under their own weight. The Maxos does not require highly accurate fixturing of the blade because it scans the roots as an alignment procedure.
TGTC is now looking to implement the system at the other facilities around the world.