In the aerospace industry, quality assurance is an essential part of production, particularly when it comes to processes and components. High quality standards are driven by the extremely high technical requirements placed on parts. As a result, there are also special requirements placed on components, not only in terms of comprehensive functional testing before they actually enter use, but also in relation to how components are actually produced. The Steinbeis Transfer Center for Manufacturing Technology & Machine Tools has been working in this area with the Steinbeis Consulting Center for High-Pressure Waterjet Technology to identify different ways to optimize the production of components and develop new technologies.
To safeguard the proper functioning of components used in the aerospace industry, there are extremely tight tolerances, for example, in terms of exact measurements and material structures. Also, components can be extremely expensive, making it essential to include sufficient system safeguards at different stages of the value chain.
Launch vehicle production is marked by intense international competition so there is enormous cost pressure. This makes it more difficult to introduce new production technologies, also because the production techniques that have been used until now for such tried-and-tested parts have been through a significant number of approval procedures. Making changes to production methods during a live program is thus an extremely sensitive issue, requiring the appropriate forms of evidence and quality testing. Also, for the organization making the rocket, the prospect of increasing the metal removal rate from 1 cm3/min to 7 cm3/ min – and potentially even raising it to 29 cm3/min – felt like a total impossibility. But in reality, EADS Space Transportation had already been working in collaboration with the Steinbeis Transfer Center for Manufacturing Technology & Machine Tools and had already achieved such a quantum leap, demonstrating that heavy-duty machining is possible, even with mission-critical components. Because of the interfering contours encountered during the production of combustion chambers, tool clamps need a minimum projection length of 300mm and the side milling cutters must have a diameter of 63mm and a blade thickness of 0.6mm.
The development partners optimized both components to cope with the extreme acceleration in milling speed. At the same time, they conducted tests on dry milling with minimal lubrication. Because the proper functioning of combustion chambers is essentially linked to structural effects resulting from the milling process, the experts also carried out a series of tests to look at this aspect. They found that all crystals (or grains) were clearly being sliced smoothly by the cutting process. Fringes resulting from the process were of an acceptable size and under 10μm. The testing clearly demonstrated that performance improved significantly compared to conventional milling processes used to date – in other words, it would be more economical to mill at between 12,000 and 19,000 rpm. This is equivalent to improving efficiency by a factor of 6-10.
Another example of a development of new production methods comes from testing carried out on ultrasound-assisted milling in the production of engine parts made of materials that are resistant to extremely high temperatures. The Steinbeis experts moved away from previous application areas and decided not to apply the technology to tools made with undefined cutting edges or advanced materials, but instead to use conventionally designed tools. When the team carried out ultrasound drilling with interrupted cuts, they discovered the potential to reduce local milling forces by forming a protective chamfer on the tool, thus not only reducing contiguous temperature but also applying specific compressive stress to bore walls. They also found no weld-ons on the cutting edges resulting from oscillations. Compared to surface treatment without ultrasound, introducing ultrasound to milling in production (finishing, radial feed 0.5mm) resulted in a reduction in force, without tool chatter.
The combined effect of tool rotation and the axial ultrasound movement creates a “peel cut” and there is also a change in the effective direction of the peripheral cutting edges, such that there are phases with and without stress being applied to the tool.
Ultrasound technology is also used for so-called roll hammering, a process applied to surface compaction. Unlike glass bead blasting, which results in random distribution, this uses NC-controlled movement to target the tool directly on a specific area of the part that is being compacted. This allows for the compaction to be varied right up to yield point, depending on variables such as static preload, ultrasound amplitude, track speed, and line spacing.
New types of production techniques can also be found in optimizations made in the production of integral aluminum and titanium parts. How perfectly a part is milled is not just about the specific milling parameters, but also an overall process of adding value: Successful milling is about machine optimization, matching tools to the milling parameters and appropriate programming strategies. This was most aptly demonstrated in the production of integral parts used in airplanes, especially parts that are over 90% milled. Aluminum milling involves high-speed milling machines with tool speeds of 24,000 rpm, whereby milling volumes of up to 3,000 cm3/min are possible. To optimize processes, the experts went through several stages of determining and enhancing machine performance. For example, they geared the high feed rates (9-15 times per minute) to design factors, identifying the limitations of the spindle system and tools, and taking suitable processing strategies into account within the NC programs, as well as key processing parameters. The time needed to produce integral components for the A380 Airbus could be dramatically reduced as a result.
When milling long-fiber reinforced plastics, the special layered structure of the material creates quite different challenges in production. If tools or process parameters are suboptimal, the edges of the materials being processed can become delaminated or result in protruding fibers after milling, which is clearly unacceptable in aviation. To avoid this, tool makers offer a variety of tool designs for drilling and milling composite structures, primarily in order to optimize the quality of the milling process itself. But aside from the design of the tool, the milling strategy applied to the part is central to the success of the milling process. The Steinbeis experts are currently working on a project that involves investigating the drilling process used for an unstable CFRP structure. The aim is to determine the degree of instability that would be acceptable for a specific drill hole quality, depending on certain influences, which can be static or dynamic in nature. The axial feed force (Fz) affects material deformation depending on the material’s flexibility, resulting in a force (Ff ) being exerted along the z-axis based on the equilibrium of forces. As the tool cuts through the final layer, spring tension makes the material recoil and this is conducive to delamination. Ultimately, knowing this provides the means of calculating specific options for reinforcing unstable CFRP structures. The results of such calculations can be evaluated according to the milling strategy, the machining parameters, and the degree of tool wear. The evaluation criteria used for this are: the degree of delamination (or the extent of fiber protrusion) and the exactness (dimensions and shape) of component design.
One alternative method for preparing parts – i.e., for cutting raw parts out of sheet materials typically used in aviation – but also for finishing or refining CFRP structures, is to use abrasive water jet cutting. The two alternative approaches often compete with one another and the experts at the Steinbeis Consulting Center for High-Pressure Waterjet Technology frequently conduct comparative testing on the processes applied to component materials, both in terms of process productivity and quality. They are also currently working on a Web-based monitoring system for use during the process of water jet cutting of different types of CFRP materials. Their aim is to find reliable ways to monitor different parameters relating to delamination and unstable processing and thus make the overall process more reliable.
Prof. Dr.-Ing. Michael Kaufeld is the director of the Steinbeis Transfer Center for Manufacturing Technology & Machine Tools and the Steinbeis Consulting Center for High-Pressure Waterjet Technology at the University of Ulm. Work at both Steinbeis Enterprises revolves around manufacturing technology, tooling machines, tool technology, and quality/production measurement technology.