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Stream of consciousness

technical & practice - The simplicity of the Princess Diana memorial fountain masks the complexity of the coordination of CAD/CAM technology

Andrew Morton's third book about Princess Diana, Diana: a Pursuit of Love, was published one month ago in readiness for the upswing of publicity surrounding the unveiling of the Diana, Princess of Wales Memorial Fountain. On the day of the unveiling in Hyde Park on 6 July, emotions were high and the tabloids conveyed their joy at seeing the warring Spencers and Windsors seated together in harmonious reverie. The 'queen of people's' hearts' evidently still bringing people together through a socially inclusive piece of water-feature artwork.

Indeed, much has been written about the symbolic representation between the torrid water movement and the Princess's 'turbulent life', as Andrew Morton described it; between the eddy currents and the 'battered this, battered that', as Diana herself described it; or between the ebbs and flows and the 'bingeing and vomiting', as Martin Bashir described it.

The design by Gustafson Porter has previously been described in the AJ in the course of its arduous journey from competition win to physical realisation. Suffice to say that it comprises an elevated circular water cascade which, as designer Kathryn Gustafson says, represents the many facets of the Princess's life.

'She was so inclusive, ' she says, 'we wanted it to be a place you felt you were part of.' Some unkind reviewers have described it as a storm drain, suggesting that the only thing moving about the sculpture is the water.

Not so. The story of its manufacture and construction is a stirring tale of ingenuity, skill and technical excellence. The project involved the transportation of huge blocks of granite around the country and abroad to facilitate each local part of the production chain. Besides, the idea that this is nothing more than a glorified storm drain was given lie by the fact that it overflowed and had to be closed on the second day of its official opening after torrential rains.

Hew it The 210m circumference fountain has been constructed from granite mined from the De Lank Quarry in Bodmin, Cornwall, chosen for its hard-wearing qualities and its lack of porosity.

This type of stone is silver-grey and has been used on projects as diverse as Beachy Head lighthouse and Karl Marx's gravestone. Unfortunately, as the stonemasons testify, the hardness of the material also meant that it is a bugger to work.

The basis of the design was that 545 individual shaped pieces - finished to ¦5mm tolerances - would be delivered to site and fitted together into a smooth, tightly jointed jigsaw. Great slabs of the hewn granite, ranging in weight from 250kg to 1.5 tonnes, were transported to Northern Ireland to be cut into shape by a family firm of masons based in Kilkeel, to the south of Belfast. Masonry techniques have moved on since Marx's day and now the job involves computer-driven sawing, routing and modelling technology, but the complexity of this project necessitated new unified protocols between designer, modeller and manufacturer - effectively introducing automated car design techniques and product manufacture technology to the project.

The use of computer technology helped to reduce the programme to just a 32-week cutting schedule, but this foreshortened time frame seems to have been assisted by the fact that stonemason McConnell & Sons worked a continuous 24-hour shift.

CAD confessions Surface Development & Engineering (SDE) engineered the scheme proposal on screen using ICEM Surf, an advanced, 'extremely expensive' modelling, analysis and visualisation software package used to play with the shape. It is usually associated with the automotive industry as 'very few cars, these days have any straight lines, ' says John Gould of three-dimensional modelling company Texxus.

The benefits of this tried-and-tested modelling software facilitated the manipulation and development of this flexible form.

SDE worked with the architect to develop the neckless shape of the fountain. This design was then passed over to Texxus to 'superimpose the surface textures'. Texxus coordinated the computer input and developed the surface treatment to come up with a surface modelling technique that used Bezier parametric surface patches to generate accurate representations of flowing forms. Its system, which it is not divulging details of, allows designers to create and mould forms in a much more flexible and free-form way than is generally available in solid computer models. Using Bezier rendering - favoured by Texxus in preference to the other industry standard 'NURBS' (non-uniform rational B-spline), preferred by Rhinoceros - geometric three-dimensional shapes, as well as complex undulating skins, can be created.

Bezier curves are generally rendered as polygons, but because the polygons are generated dynamically, the model becomes progressively smoother with increased computer speeds. Although Bezier is fully NURBS-compatible, Texxus insists that Bezier is more controllable than NURBS-based systems.

What Texxus has achieved with this project is the development of a computer system that coordinates the spatial and realisation requirements of designers with the rigours and tolerances of the manufacturing industry. This completed visualisation was sent over to Ireland via Vero International's VISI-Series CAD/ CAM software, used in the milling and tooling industry.

Cutting-edge technology McConnell & Sons geared up to receive 520 tonnes of stone. Two CNC (computer numerical control) milling machines were used to maintain the project on its tight programme, with milling times for individual sections of up to 70 hours for the more complex blocks. Effectively, during the manufacturing process, the granite slabs were milled direct from computer programmes.

The Texxus three-dimensional visualisations were converted back to solid models. Vero's 'conversational software', and the fact that the milling machinery could move on all axes, meant that traditional ball end cutters and end mills, for example, were not needed and the operational commands were read as sawing operations.

Each slab is different and has been engineered to suit the laid foundations and the extant topography of the site.

Not only that, each piece has to abut the other to maintain the top surface slope and hence the flow of water. At McConnell & Son's workshop, the large open area adjoining its production facilities allowed it to cold-assemble key sections of the fountain to ensure that it worked as planned.

This is undoubtedly a case study in prefabricated construction excellence and hints at the possibilities of using technology imaginatively to assist speed and accuracy on other sites. However, the inevitable problems of real, as opposed to virtual, sites still blight construction projects.

The discovery of Roman remains at the original designated site of the memorial fountain, necessitating its relocation to its current location, possibly added a further £600,000 to the project. As Princess Diana might have said, a good site survey might have avoided an unnecessary minefield.

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