Finding the cut above

Boeing and Sheffield University have teamed up to form a research centre that pools the experience and knowhow of the world’s top aerospace names – and could reshape aircraft design. David Fowler reports.

Sheffield’s Advanced Manufacturing Research Centre, a partnership between Sheffield University and Boeing, is set to have a profound impact on hi-tech manufacturing in the UK.

Collaborating with key UK companies in the aerospace field, the centre is conducting research aimed at dramatically improving the effectiveness of the aerospace supply chain. Its work in areas such as the machining of difficult materials could influence the design and manufacture of future aircraft, while one of the centre’s explicit aims is to form the nucleus of a cluster of world-class aerospace firms and to help regenerate Sheffield as a renowned manufacturing centre.

The list of collaborators that have supplied help, in cash and in kind as well as their expertise, ranges from IT companies such as PTC to machine tool and cutting tool specialists such as Cincinnati, Sandvik and Technicut. From Boeing’s point of view, assisting UK suppliers or potential suppliers to be more effective feeds into its stated aim of improving its own performance as a company through a global network of cutting-edge research centres. Launched in 2001 with an initial investment of £15m in regional development funding, the centre is already producing genuine improvements.

One of the key areas of research for the centre, and for aerospace in general, is machining hard materials. ‘Aerospace companies want to push designs to make them lighter, faster and more efficient. A lot of that will be achieved through better performing materials. But almost by definition they are more difficult to cut,’ said Professor Keith Ridgway of the university’s mechanical engineering department and research director of the AMRC.

The materials in question include titanium, composites, nickel-based alloys such as Inconel and heat-resistant superalloys. Titanium, for example, normally has to be machined much more slowly than the aluminium alloys that were the mainstay of the industry in the past.

One of the key techniques being used by the centre is the identification of ‘sweet spots’ or optimum machine conditions for cutting a given material. These are a function of both the machine tool and the material, and allow cutting depth and speed to be increased and overall performance to be improved without reducing the quality of the part or tool life. The AMRC achieves this using acoustic harmonic analysis to identify the machine’s modes of vibration.

Adrian Allen, the centre’s commercial director, said: ‘The machine and workpiece form part of a system including the structure of the machine, the spindle, the chuck, the tool, the workpiece and its fixture, supported on the bed which is on bearings feeding back into the structure. All those parts have a natural frequency of vibration, of which one is dominant. When you push a tool to the maximum it vibrates, leading to chatter, or a poor surface finish.’

AMRC researchers can identify the dominant frequency. By running the spindle at a speed at which the impact frequency of the cutting tool matches the dominant frequency the problem is alleviated. Going a step further, if the dominant vibration can be damped or modified, the cutting speed can be increased until a new limiting frequency is reached.In this way performance constraints can successively be removed, resulting in greatly enhanced rates of material removal, or ‘surface generation’ as Ridgway puts it, since much aerospace machining these days involves machining large surfaces accurately.’Our core expertise is that we have a lot of experience in interpreting the data to identify the optimum conditions,’ he said. ‘Typically we can reduce cutting times by a third.’

However, it is important to be sure that higher cutting rates are not having a detrimental effect on quality or accuracy and are not setting up defects or stresses in the materials’ internal structure, which could affect its fatigue behaviour, for example. AMRC conducts extensive physical testing to measure residual stress and look at the surfaceand microstructure, grain deformation and fatigue characteristics of the component.

By definition this is time consuming – the centre has some fatigue tests that will be ongoing for two years – so researchers are also working on simulation techniques to model the material behaviour. Results from the physical work are being used to build up a database.

‘We’re building the model, taking the data and asking: does it fit the model?’ said Ridgway.

Graphical and virtual reality techniques are being developed at the same time, so that it will be possible to simulate the cutting operations on computer and look at a representation of the material’s internal structure at the end of the process.Ridgway said the centre is ‘a good way down the line’ to completing the model and building the database, and that it should be complete in two to three years. The idea will be to identify machining operations that would damage the component without actually having to make it and test it. ‘Our strength will lie in identifying 80 per cent of the operations you can’t do,’ he said.

Ridgway stresses the importance of the collaborative nature of the projects. Industry partners pay a membership fee to participate. ‘We don’t send them a report every six months. We have our staff at their place and they send their people to ours, and we look at the problem together. We’ve never tried to sell the perception that we have all the expertise – our partners include some of the best companies in the world and we rely on their expertise,’ he said.

Projects fall into three categories. ‘Generic projects’ are of interest to all the partners and are paid for from general funds. AMRC owns any intellectual property developed. They are essentially applied research projects with repayment potential, and are expected to yield results within two to three years. A typical generic project is investigating strategies for milling titanium pockets.

‘With a generic project we expect the work to come to fruition in a case study which demonstrates a benefit in a partner company,’ said Ridgway. For example, an early project on machining a titanium rib has reduced the cutting time by a factor of six.’Specific projects’ are undertaken with one partner, which funds the work. Ownership of intellectual property rights can be negotiated. The company has a specific project with Sandvik to look at cutting tool wear characteristics.

‘Innovative projects’ are undertaken with a number of partners, with funding split between the companies and the research councils. This type tends to be more blue-sky or long-term research. In one such project the centre is working with tool manufacturer Dormer on new drills for novel materials, with part funding from the EPSRC.

All projects have to be approved by the board and as part of the approval process project sponsors have to set out the benefits to AMRC, the general partnership, the region and other stakeholders. Estimated financial benefits for the participating firms have to be quantified.

At present the centre has 31 generic, six specific and 14 innovative projects. Benefits of the research will not be limited to aerospace, since many of the participating companies are also part of the supply chain for Formula One and for medical technology.

Because much of the centre’s work is concerned with finding constraints in machining, working out how to get round them and generally extracting better performance from the machine tools, Ridgway hopes the work will influence the design of future machine tools.

More than that, it could influence the design of aerospace components themselves. ‘The trend in aerospace is towards big, monolithic parts requiring less assembly. If you can show you can machine bigger shapes or generate larger surfaces more quickly, then there will be more opportunities for designers to do that,’ said Ridgway.