Six-legged revolution

A new machine tool could save millions for industry. David Fowler explains how

At the end of last month a group of industrialists and researchers applauded the long awaited unveiling of the Variax Hexacenter machine tool at Nottingham University’s Responsive Manufacturing Centre.

Evaluation of the revolutionary six-legged, effectively five-axis machine tool forms a key part of the Rapid Response Aerospace manufacture project, a university/industry research collaboration being carried out under the Innovative Manufacturing Initiative.

The project brings together the Responsive Manufacturing Centre with three divisions of British Aerospace, Rolls-Royce and Shorts as well as Hexacenter designer Giddings & Lewis. Cliff Fowkes, RRAM project manager and chief manufacturing technologist at British Aerospace Dynamics, says the Hexacenter is merely ‘the most glamorous’ aspect of the project.

Equally important, he says, the project is looking at ‘the CAE tools surrounding the Hexacenter’, including simulation and scheduling (looking at techniques beyond MRP). In addition the project covers research into flexible fixturing.

It is looking at how the techniques could apply in aerospace and beyond. BAe Dynamics is interested for its own sake and also because, as a prime contractor, it needs to understand what level of capability it can reasonably expect from its subcontractors.

A key area for the research is how small yet intricate parts can be machined to high tolerances more economically, a keen area of interest in Fowkes’ business. ‘In dynamics, we make geometrically complex parts,’ he says. Tolerances are crucial because in dynamics great emphasis has always been placed on interchangeability of components. This is not the case to the same extent in aircraft manufacture, where, for example, it is still common for a wing skin to be drilled and riveted to the airframe in situ, so that every one is different.

As far as the Hexacenter is concerned, Fowkes says that if it ‘fulfils some of its promise’ then it will be able to remove some of the stages of machining, clamping and repositioning a component. Reducing the number of machining operations from five to two or three would produce ‘significant savings’. Complementary work on fixturing systems is being undertaken at Nottingham’s department of manufacturing engineering to reduce set up times.

‘In aerospace there is incredible investment in jigs and fixtures,’ says Fowkes. If fixturing systems could be made more universal, rather than dedicated to specific components, millions of pounds could be saved.

The university’s Professor Nabil Gindy, says the Hexacenter offers several potential advantages over conventional orthogonal machines (with X, Y, and Z axes at right angles). Essentially, the machining spindle is mounted on a platform supported above the workpiece on six extending legs.

‘The way it is structured allows it to move faster, compared with conventional machine tools where you need to move heavy lumps of metal at great speed, and inertia becomes a limiting factor.

‘There are less sources of error than in a conventional machine. All you need to worry about is controlling the height of the six legs. In an orthogonal machine errors in the different axes can be cumulative.’

Finally, ‘the rigidity of the triangulated structure allows you to do deeper cuts, take more load, and move more accurately while cutting than a conventional machine’.

Whether or not these advantages are realised in practice remains to be seen. Many believe designs more closely related to orthogonal machines will be able to achieve similar levels of accuracy (Technology News, page 36).

In principle, hexapod type machines are fairly simple mechanically and could be produced at relatively low cost if accepted by industry.

Control is an issue: an advanced software system is needed to control the six legs simultaneously, with a sophisticated laser feedback system (produced by Renishaw) to measure accurately the length of each leg.

Proving took longer than expected, but Gindy is not surprised. ‘It is like any new development. There was a lot of software to be tested.’

Applications are likely in the die and mould industry to produce complex, sculpted shapes with a good surface finish needing no separate polishing. The machine could also be used for rapid prototyping in the final material.

‘Part of the work we do is aimed at making a tightly integrated CADCAM system in which you can go from the CAM file to simulation straight to the machine as soon as the design is finished,’ says Gindy.

Fowkes hopes one outcome of the project will be to take a step towards eliminating quality inspection – a view some will see as heretical. But he says inspection is ‘a non-value added process. The product is no better after inspection than before. If we can understand the manufacturing process to a high enough degree we could remove inspection.’

The Hexacenter is ‘very rigid, very accurate and very repeatable’, he says. ‘It ought to be able to machine parts with a high confidence level of meeting the required tolerance.’

He admits that ‘the expectations of customers and regulatory bodies would have to be changed’. A high degree of inspection is required in aerospace manufacture under current rules. Fowkes’ view is that ‘in engineering we are used to inspection. But there are some things you can’t measure.’

For example, Royal Ordnance makes rocket motors used once only. Obviously these cannot be tested. ‘Instead their people put energy into process control. If you can control the ingredients and the process, the outcome will be right.’

{{Rapid Response Aerospace Manufacture

PartnersBritish Aerospace AerostructuresBritish Aerospace DefenceBritish Aerospace DynamicsGiddings & LewisRolls-RoyceShort BrothersNottingham University Responsive Manufacturing Centre

SuppliersDeneb RoboticsKistler InstrumentsRandom ComputingRenishawParametric Technologies}}