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Adaptive manufacturing of turbine blades

Starrag develops flexible manufacturing systems 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, 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.

Highlight: adaptive milling

The Swiss plant developers opted to use LX 051 machines for this purpose. This series of machines has been developed by Starrag for the highly precise, simultaneous five-axis machining of turbine blades. To achieve an optimum transition for each blade, Starrag opted for adaptive milling. This means that the milling process is individually adjusted for each blank. First, a measurement process is performed in the machine, during which the shape of each blank is scanned. Based on this data, an individual NC milling program is calculated for each blank and is then immediately executed. The result is a tailored transition between edge radius and blade surface.

Günter Leitold, who is responsible for the automation of flexible manufacturing systems at Starrag, explains: "This adaptive milling is one of the system's highlights. It is important that the measurement process is performed very quickly to ensure high productivity levels are maintained. This is achieved by using the latest scanning technology available on the market." The engineer points out that Starrag acts as a general contractor for this type of manufacturing system, and therefore has overall responsibility: "Our core competence lies in the field of machine tools, tools, equipment and CAM systems. We also have extensive knowledge of the configuration of such systems. What's more, it is important that, for other components, we only work with leading companies that can achieve the high level of performance and availability that is required."

Redundant design ensures high availability

The reliability of the system was a key consideration during the planning stage. Ultimately, in the final configuration, up to 40,000 turbine blades will be produced per year. The designers therefore opted for a fully redundant setup. The FMS consists of two lines with identical configurations, each comprising two loading stations, two LX 051 machining centres with scanning device, one washing system, a marking needle and a measuring cell. A six-axis robot transports the components and also supplies the machines of the second line if needed.
The entire system is controlled by a cell controller with a PPC system, and this controller also enables various emergency strategies. This ensures that the system can operate continuously for six days a week in three-shift operation. Günter Leitold adds: "Even maintenance can be carried out during operation by stopping individual cell parts and using the redundancy. The unit in question is removed from cell controller operation with the click of a mouse and is then available for maintenance work or program tests."

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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.

Fully automated:

Milled twice, deburred, washed, marked and checked

The system has a total of four set-up points. Each point is equipped with a control PC that communicates with the cell controller. The blanks are already marked with a unique 2D code upon delivery. This code is recorded by the reading device upon set-up and is then transmitted to the cell controller. Head of Sales Rainer Hungerbühler highlights the versatility of the FMS: "The system is designed for seven different turbine blade variants. The blade production sequence is specified by the cell controller using different priorities."

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 device, 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 device includes a blade clamping system that allow 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 device 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 devices for a blade type are available at the set-up point. Once machining is complete, the operator removes the blade from the measurement device, changes over the other two, and places a new blank in the device.

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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.

Many years of experience in the systems segment

The Starrag Group, a global technology leader that manufactures high-precision machine tools under the brands Berthiez, Bumotec, Dörries, Droop+Rein, Heckert, Scharmann, SIP, Starrag, TTL and WMW, has extensive expertise in the systems segment across a range of industries. Starrag, based at its Swiss headquarters in Rorschacherberg, specialises in the Aerospace and Energy sector. It not only provides specialised machine concepts but has also been handling flexible manufacturing systems for over 20 years. This includes the entire project planning stage and engineering as well as the delivery and assembly of all system modules including machines, tools, fixtures and automation. User training, production support and service are of course also included.

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