Automated marble refining
A highly sophisticated custom-made machine from Böhm AG in Thuringia is the first of its kind worldwide to enable the fully automated refinement of marble. The innovative system uses machine vision from STEMMER IMAGING to guarantee the reliable removal of undesired structures in the stone reducing its value.
Marble is one of the most decorative construction materials in the world and highly valued as a high-class finish in buildings such as hotels, airports or upscale residences. No marble slab is identical to another, as inclusions, cracks or so-called veins shape the appearance of the naturally formed stone and always create a unique look. Depending on the number and characteristics of such flaws, marble is divided into three quality classes, which generate different prices on the market.
Special requirements apply to highly decorative buildings. The marble slabs should be as uniform as possible in colour and free of impurities. How close the stone comes to the ideal, however, only becomes apparent when the large blocks excavated in the quarry are sawn into slices. In terms of colour, slabs from one block are usually quite similar.
However, veins caused by sedimentation during the formation process can considerably spoil the appearance due to their sometimes strongly deviating colours and thus significantly lower the value of individual marble slabs.
32 metres to perfect marbling
With the BMM (Böhm Marble Machine) introduced in autumn 2019, Böhm AG, based in Zella-Mehlis, Thuringia, has developed a solution which enables the fully automated finishing of marble for the first time.
The machine is offered in two sizes for processing the two globally established standard sizes of marble raw material: For tile manufacturing, marble is cut to a size of 120 cm x 120 cm with a thickness of 1 cm to 3 cm, while slices with dimensions of 325 cm x 185 cm and a thickness of 2 cm to 3 cm serve as the basis for other marble products.
The two BMM variants used for refining these two slab formats come in lengths of 14 and 32 metres, respectively. Depending on the degree of perfection, the larger version refines up to 1000 m² of natural stone surface in 12 hours, while the smaller one manages up to 400 m² in 12 hours.
Thus, the two systems significantly exceed both the speed and the precision of conventional marble refiners.
The process is the same for both sizes and comprises a total of six steps. After the marble slabs are inserted, they are scanned in the first module by a high-precision, self-learning machine vision system. Based on the scanning results and the subsequent image evaluation, a laser is controlled via a CNC program in the second module to vaporise the detected value-reducing structures. The third module provides for thorough cleansing before a high-performance printer retouches the lasered areas in the fourth work step.
It uses individually adapted colour mixtures of six basic colours to reproduce the colour tone of the currently processed marble as well as the veins near the vaporised areas.
"After the fully automated filling with transparent sealing material and a final polishing process on the treated surfaces, even experts will not be able to tell the difference between the natural stone and the artificial restructuring," Hellmuth emphasises.
Machine Vision is the key
The inspection results obtained by the machine vision module used in the very beginning are the basis for all subsequent steps performed on the BMM. Precise positioning of the imperfections is an essential prerequisite for the laser treatment, which should only remove the structures that actually reduce the value.
In order to minimize visual differences between the laser-treated and the natural areas, the colour blending applied must match the colour surrounding the lasered structures as closely as possible. It is important that structures and veins crossing a retouched area are visible without any discontinuity.
Here, again, the image processing quality with regard to the recognised colours and structures is decisive for the quality of the outcome. The sealing and polishing of the marble slabs at the end of the process is also based on the position data of the imperfections detected in the first module.
Managing Shareholder and Executive Board member of Böhm AG
„The company's experts saved us a lot of time during the extensive tests carried out during the evaluation of the vision solution in their in-house laboratory. They put together a recommendation for the required components that proved to be the optimum solution. In addition, we were able to greatly benefit from STEMMER IMAGING's expertise during the subsequent tests and development support.“
The image acquisition module is based on two trilinear RGB line scan bars from Mitsubishi Electric with widths of 600 mm and 900 mm respectively and built-in lighting.
A Teledyne DALSA Xtium CameraLink frame grabber is used to transfer the image data to a customized PC, where the partial images captured by the two line scan bars are combined and evaluated with the help of STEMMER IMAGING's machine vision programming library Common Vision Blox (CVB).
Hellmuth continues: "This solution is ideally suited to identify inclusions, veins or cracks in the marble slabs with an accuracy of 40 µm, as well as to analyse the colours and structures surrounding the areas to be mended."
Felix Hellmuth highlights the importance of the machine vision software used: "This software is self-learning in order to constantly improve the quality of image recognition and evaluation. We put a lot of effort into its programming – the results are quite convincing, so the hard work was worth it."
About 200 to 300 images are required to train a new type of marble, which are later used by the AI-based software to automatically create suggestions for processing the slabs. Due to the manual adjustments of these suggestions, the software continuous to learn during operation thus enabling an ongoing improvement of the processes as well as an adaptation to the individual taste of the user or the customer.
A worldwide unique solution
The BMM is the only one of its kind in the world and enables considerably more cost-effective marble refining than the manual retouching of poor-quality marble slabs which is still frequently used these days. Hellmuth points out: "The system is completely automated and can be run 24/7. This means that quarry operators can now add cutting and refining to their services and expand their value add."
Felix Hellmuth lists other possible applications such as the imprinting of artificial elements like imitation fossils, gilded inlays, arbitrary pictures or even company logos, which is often requested by hotels in particular.
Apart from that, however, Hellmuth has more ideas for the future use of BMMs: "Theoretically, we can also process other materials besides marble, such as artificial stones, quartz, concrete or wood using a similar concept."
He can also imagine offering the machine vision module separately, for example for thickness control of plate-shaped materials. "STEMMER IMAGING’s machine vision solution has proven to be highly reliable and powerful in our BMM. At the same time, it’s flexible enough to be used for other conditions and materials."
Your contact at STEMMER IMAGING
Do you have any questions about this interesting solution? Please get in touch with our sales contact person
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