energy

Adaptive manufacturing of turbine blades

Starrag develops a flexible manufacturing system for the individual series machining of turbine blades

A state-of-the-art flexible manufacturing system from Starrag performs the finishing of precision-forged titanium turbine blades. The Swiss machine and plant manufacturer applied the expertise it has gained over its many years of experience in design and in key details to reliably produce up to 40,000 blades per year. For example, an individual milling program is generated for each blade due to small component differences arising from the process.

Starrag AG, which is based in Rorschacherberg, has been handling turn-key projects for the manufacture of turbine blades and structural components for over 20 years. Around 60 automated multi-machine systems have been created during this time, providing a wealth of technological experience. A large number of customers currently benefit from this wealth of experience, as there is a clear trend towards flexible manufacturing systems (FMS) worldwide. Head of Sales Rainer Hungerbühler explains: “While in the past we had a maximum of two such projects in progress at any time, we currently have four projects that will be delivered to Asia and America. The aerospace and energy industry is increasingly taking advantage of the benefits offered by this type of automated series production, in terms of both quality and efficiency, in the manufacture of key components such as turbine blades.”

The finishing of compressor blades for use in aircraft engines is a particularly impressive project. The titanium blanks for machining are precision forged. While the accuracy of the two blade surfaces (suction and pressure side) mean that no post-machining is required, the edges still need a specific radius. The transitions to each turbine blade must also take place without any noticeable offsets or milling marks.

Forging – even when performed with this high level of precision – is a process in which variations between components occur. While minimal deviations within the blade surface have a negligible effect on the end application, contour differences pose considerable challenges with regard to the milling of the edges and transitions.

Adaptive set-up ensures a consistently high blade quality

To ensure a high level of availability, the set-up of the system was also carefully planned. The minimal contour differences arising from the forging of blanks mean that Starrag still needs to rely on human assistance. “Given that the entire process is otherwise virtually fully automated, this was not an easy decision”, explains Patrik Rutishauser, Head of Application Technology. “But the calculations showed that our operator-assisted set-up is the most efficient solution for this system.” The reason for this is that a productive milling process requires secure clamping with specified forces. Although the deviations from blade to blade are only small, if the device is configured automatically, some blanks may not rest exactly on the support points. During fixing, stresses may arise in the component, leading to deformation upon release after milling and making the blade unusable.

Therefore, Starrag developed an adaptive device that adapts to the specific blade blank. Restrictions on the permitted component tolerances were also considered as an alternative. All blade blanks that do not comply with these tolerances would have to be removed in advance. Automation with a fixed device would then be feasible. However, with forged titanium blanks, the discarded quantity would be too great to ultimately achieve an acceptable result.

The basic procedure is the same for all blades. Each blade is machined in two clamping positions, meaning that two different fixtures are required. In the first fixture, the blank is clamped using a clamping principle developed by Starrag. This leaves the edges and transitional areas on both sides free for machining. The second fixture includes a blade clamping system that allows the base and head of the blade to be milled. The necessary deburring takes place after the milling process in the machine. After the original marking is removed during the second milling process, a new 2D code is added to each blade with a marking needle. 

A further fixture is needed for the final optical measurement process during which over 50 features of each individual blade are checked.

To ensure set-up is as efficient as possible, during full operation of the system all three fixtures for a blade type are available concurrently at the set-up point. Once machining is complete, the operator removes the blade from the measurement fixture, changes over the other two, and places a new blank in fixture one.


Machining expertise

Application engineer Patrik Rutishauser highlights other valuable features of the system: “Starrag’s expertise lies not only in the machines, but also in the machining process. Our CAM software is used to generate the adaptive milling programs and our solid carbide milling tools ensure optimum cutting performance.”

In order to handle the high demand for tools in the four machining centres, which operate around the clock, Starrag also provides the linked manufacturing system with two tool grinding machines plus auxiliary equipment, the capacity of which is planned from the cell controller. They ensure that the final grinding of specific tools can take place on site. As with all required tools, these tools are measured, shrunk in the holder and made available to the machines as required.

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