Transforming turbine blade inspection

Aircraft Engineering and Aerospace Technology

ISSN: 0002-2667

Article publication date: 1 June 2000



(2000), "Transforming turbine blade inspection", Aircraft Engineering and Aerospace Technology, Vol. 72 No. 3.



Emerald Group Publishing Limited

Copyright © 2000, MCB UP Limited

Transforming turbine blade inspection

Keywords Mitutoyo, Turbines, Inspection

Turbine blades are one of the most complex three-dimensional forms found in engineering, which makes their inspection extremely difficult. Doncasters Deritend uses the investment casting method to manufacture blades for gas turbines that are destined for power generation OEMs around the world. Following the purchase of a Mitutoyo Euro C Apex 9166 CNC co-ordinate measuring machine (CMM), the company now has no fewer than five CMMs in use. These are operated with several of Mitutoyo's software packages to give what the company describes as exceptionally repeatable results, with statistical data also being made available for the production engineers and customers.

Blades are manufactured in a range of sizes from 75mm to 1m long - the longest investment cast vane in the world. Although CMMs are also used within the factory for inspecting the largest sizes, the new Euro C Apex 9166 machine is used in the small products cell for blades up to 0.5m long. Four of the five CMMs are installed in the finishing shop, only a few metres away from the equipment used to finish, polish and non-destructively test the blades. Indeed, the same multi-skilled operators carry out the finishing operations and use the CMMs to check the blades.

The first CMM was installed in Deritend around ten years ago and it is now common for customers to insist that inspections must be carried out by CMM rather than by the traditional method of using shutter gauges. "CMMs are more accurate and they eliminate the possibility of operator error, giving totally repeatable results", according to Paul Atkinson, the site director. He adds: "Customers can now be given printouts of the data or we e-mail them to them, and the same data are also being fed back to our wax shop where the patterns are made."

A separate CMM in the wax shop is used to inspect the wax patterns and by comparing the data for the wax and for the castings, it is reported to be possible to make fine adjustments to the mould tooling to correct any repeating errors. Because of the nature of the investment casting process, it is sometimes necessary to build-in compensation in this way and the CMMs allow this feedback loop to be closed very quickly, thanks to the large volumes of data that can be generated and analysed.

Another advantage of having plenty of data is that the number of points measured can sometimes be reduced. If Mitutoyo's Statpak software indicates that some dimensions have never changed throughout production, Deritend and its customer can agree to drop the dimension from the inspection routine. This reduces the inspection and lead times and therefore cuts costs.

Because of the complex shape of the turbine blades, it is important to be able to display the data in a way that can be readily understood by the people who use them.

Mitutoyo has developed Autoplot specifically in response to requests by Deritend and another leading UK manufacture of gas turbine components. The software shows the blade profile and highlights points where the blade is outside the permitted tolerance bands. "Autoplot is absolutely perfect for use on the shopfloor", comments Colin Sankey, a senior engineer. He adds: "We have been pleased with all of Mitutoyo's software packages and they are always quick to respond to our requests for information or advice. As for the CMMs themselves, I think the fact that we now have five of them speaks for itself!"

Mitutoyo's Euro C Apex 9166 CMM is said to be robust enough to be used on the shopfloor and its ease of use makes it suitable for use by multi-skilled machine operators. But when the Euro C is used in conjunction with the Geopak software for automated inspection, and the Statpak and Autoplot packages for data analysis, it becomes an extremely versatile, cost-effective inspection system for complex three-dimensional forms.

Details available from: Mitutoyo (UK) Limited. Tel: +44 (0) 1264 353123; Fax: +44 (0) 1264 354883; E-mail: