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|Restoration and History|
|From January to the end of October 2018, the Old Town Astronomical Clock in Prague underwent a restoration project. The goal of the restoration work was to give the watch its original look and original tuners of the 1960s.
During the Second World War, the astronomical clock was hit by bullets and subsequent fire severely damaged the clock. After the war, the clock was repaired to be fully functional again. More contemporary approaches were used to fix the clock. As a result, the whole mechanism consisted of both historical and modern-day parts. After this year’s renovation, this cultural and historical monument was to return to a state that was both functionally and visually reminiscent of its original version.
Creaform’s Czech distributor SolidVision was contacted by a member of the Czech Horological Society, which was aware of the benefits of 3D scanning technology. The Society tasked SolidVision to scan this historically valuable cultural asset. Repairs had to be accurately documented by 3D scanning and saved for future repairs. During the renovation work, the astronomical clock was stored in a special workshop outside Prague.
The Scanning Process
The project consisted of a highly accurate scan of the clock mechanism in order to generate a CAD model. Each scan has its own specific criteria, which greatly influenced the selection of the appropriate scanning solution. Thanks to its versatility, speed and high precision, the 3D scanner MetraSCAN 750 Elite was chosen for the project.
The scan work was carried out in the studio managed by the administrators of the Old Town Astronomical Clock. The studio was relatively small. In addition, a large part of the room was cluttered with tables on which tools, machines, and components of the clock were stored. Therefore, the first thing to do was to find a place to install the scanning system. When the C-Track optical camera system was positioned and ideally placed to track the position of the MetraSCAN 750 Elite, magnetic reference points were placed on the metallic clock mechanism.
Thanks to the reference points, dynamic positioning was performed; the system transferred points on the scanned clock to the CMM. These avoided inaccuracies caused by possible vibrations or accidental movements of the tripod with the mounted C-TRACK during the scanning process.
The scanning of the outer part of the mechanism was executed very quickly and without the slightest of difficulty. The closer the team got to the middle of the mechanism, the more often the system had to be moved because the scanner had to remain in the field of view of the C-Track. Despite all these challenges, SolidVision, thanks to the MetraSCAN 750 Elite, was able to create a complete scan of the mechanism with the accuracy of up to 0.078 mm. With the same accuracy, other parts that were previously dismantled were scanned. These were three handmade 1100 mm gears, a 640 mm spur gear and two smaller sprockets. Scanning the dismantled parts was easy and to the satisfaction of both SolidVision and the restorers.
The Benefits of 3D Data
With such accurately scanned data, the team was able to collect data in the events that spare parts had to be developed. In addition, the 3D data can now be used anywhere in the world. Scans are stored in various formats for multilingual STL networks. These formats use, for example, 3D printers, reverse engineering programs, and graphics programs to create visualizations and simulations. In the end, the clock’s rich history and design has been documented to preserve this important relic.
Technical Scan Parameters
Used equipment: Creaform MetraSCAN 750 Elite
Accuracy: 0.078 mm
Number of polygons: 16 225 000 triangles
Mechanism scan resolution: 2 mm
Scan resolution of components: 1 mm
Scan time: 5 hours
Note: The orginial article has been published in the Czech Magazine Metrologie 4-18, page 36-37.