Real life in miniature

Advanced metrology techniques and CNC milling technology usually found in the automotive sector were pressed into service when Italian restoration experts decided to create a scale replica of one of the country’s historic buildings.

The Palazzo Madama in Turin, the former home of the Savoy dynasty, had undergone restoration work since the 1980s, with the finishing touches applied in 2006. A feature of the city since the Roman Empire, the building now houses Turin’s Civic Museum of Ancient Art. The sheer intricacy of the architecture and the number of different styles united in the building convinced the museum’s curators that it was worth creating a 1:30-scale miniature version for visitors.

The model, measuring 199 x 220 x 136cm, is on display in the Palazzo Madama itself.

Creating the accurate replica of the intricate, richly embellished exterior was, however, a considerable technical challenge that took more than a year.

The regional council asked Hexagon Metrology to help with the project, which would go on to encompass a range of advanced measurement, modelling and CNC milling techniques before the faithful end result could be put on display.

The original renovation had included a survey of the interior and façade in conjunction with Geomar, meaning that most of the 2D survey data was available to the team working on the 3D replica when the time came.

Surveying the external façade was a challenge in itself. It involved attaching reflectors to the building via a scaffold. The reflected light was captured by a Leica Geosystems Total Station, which supplied the 2D data.

Diego Giachello, the architect in charge of the refurbishment, also used satellite positioning technology to aid the surveying process. ‘We were in a position to locate our measurements of the palace with reference to the GPS coordinates of the building,’ he said. ‘This was a great benefit to the accuracy of the model.’

The team used the 2D data and the existing drawings of the palace to create a 3D model of the structure using AutoCAD and Rhinoceros software.

The next step was to find a technique able to produce the highly accurate and detailed reproduction of the original that the project was aiming for.

Rapid prototyping was considered, but the project team decided that it would not deliver the desired level of accuracy. The option it settled on with Hexagon Metrology involved the use of CNC milling technology of the type more usually associated with the automotive sector. The material chosen was synthetic resin with its compact surface and wood-like characteristics.

The system used to create the model was based on a coordinate measuring machine from the DEA Prima RI series, which can be fitted with a milling head and operates at up to 3,000rpm.

Marco Forneris, special application manager for Hexagon Metrology, said: ‘We extracted the milling paths from the Geomar CAD data using the Auton-CAM software package. Then we were able to start work on the synthetic resin. We precisely milled one part after another using the Prima RI and DigiScan software. In the end, no part deviated from the original by more

than 0.1mm.’ One of the biggest challenges faced by the modelling team was to faithfully reproduce the palace’s front face. The most detailed part, it is embellished with complex reliefs, columns and statues. The team realised that to accurately scan it and produce CAD data would be a huge undertaking and instead decided to use a sculptor to create clay models of these elements at scales of 1:5 and 1:10. This allowed the features to be more accurately reproduced than would be possible using the 1:30 scale of the main model.

The next step involved Hexagon Metrology technicians scanning in the sculptor’s completed models using Romer measuring arms and CimCore. The resulting stereo lithography models were then scaled down to match the 1:30 scale of the main model and the milling paths derived.

Once the complex elements were incorporated, the model was handed over to restoration experts to apply the finishing touches. The milled surfaces were cleaned and components were sealed with protective varnish before the final model was assembled on a steel frame.

According to the project team, the model represents a thing of beauty for visitors and a demonstration of another technical option for accurate modelling.

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