RICERCA ITALIANA     

Technologies to machine and to control the surfaces in natural stone

Università degli Studi di Cassino

Abstract

In the last forty years the stone material has been used in a continuous way and it has seen only short periods whereas its employment has decreased. This means that stone employment has faced an increase of demand to which an increase of the offer has corresponded. This offer increase has been due to new countries whereas stone has started to be extracted, to the intensification of the extraction by the just operating quarries and to the discovery of new quarries. From 1999 the increase of granite and marble use has ever stopped, despite it has faced the competition of alternative materials; it has been very bigger than the expansion of the whole international economy. The rate of production increase has been of about 6.8% and the exchange rate has achieved 8.3%. The achievement of this results has been possible thank to the development of technology, to the increase in speed of the transport and to the re-discovery of marble and stone by the designer and builder.
Stone field and its surrounding has seen an increase in extraction and use of about sixty times in the ninth century, while the world population has increase of fifth times. The world stone production has overcome 100 millions of cubic metres with a turnover higher than 20 milliards of euro.
To strengthen and increase the Italian position in stone field it is necessary to create a net of technological and scientific competencies that support the innovation of the manufacturing processes and systems through the understanding and the optimization of the existing technologies and the introduction of innovative ones.
The firms operate actually in an empirical way according to indications received by the tool supplier or to the knowledge reached in many years of experience. This approach defines a solution that may be very far from the optimum in terms of tool choice or of process parameters definition, of discards decrease causing a reduced environmental impact, of improvement of the worker conditions.
The sector is interested to research and development activities facing the technological problems by a scientific point of view and giving solutions really usable. In this way Italy could achieve a stable leadership position in the field of natural stone and manufacturing technologies for stone; it may significantly influence the Italian occupation.
Therefore, this project aims to study the surface machining technologies, such as polishing and AWJ surface machining, that together the cutting technologies studied in the previous project, are considered the most promising in stone field. This project aims to develop analytical models that correlates the cutting strength, the tool wear and the cutting quality to the properties of the stone, to the process parameters and to the wheel characteristics (both diamond and binder).
The results will be achieved through the exploitation of synergies existing on the national territory that constitute a net of competencies. To improve the competitivities of the firms, the results will be discussed with firms during project development; they will be transferred on an internet website too. <<<

Principal Investigator

Luigi CARRINO Università degli Studi di CASSINO

Research Objectives

The general purpose of the project is to exploit the competences of national network constituted during the previous financed project COFIN 01 (entitled Stone Manufacturing technologies) by Universities and firms operating in stone manufacturing field. In this previous project studies on cutting of natural stone by traditional technology using diamond mill, disk or bead and by abrasive waterjet technology have been faced. Particularly, the aim of the present project, that goes on with the activities started in COFIN 01, looks at surface machining of natural stones through traditional and innovative technologies.
Moreover this project aims to develop procedures for natural stones products management from a quality viewpoint, satisfying the new norms related to building products.
The surface machining is constituted by the set of processes on surfaces of a natural stone to give them aesthetic properties. Surface machining is fundamental to exploit stone product's expressive properties, such colour, texture, roughness, patterns and all the possible aesthetic attributes of a natural rock can be valorized, modified or minimized using the proper processes. Moreover, stone machining may improve its technical properties, like weather- or absorption-resistance. However, choosing a surface finishing depends on a set of variables, the most important of which are the final colour and, obviously, its related machining cost. In the range of superficial processing have a fundamental rule polishing process.
Polishing is the most important machining process to carry out a good surface finishing. It allows to obtain smooth and bright surfaces. Polishing process is different for marble and granite.
Polishing of marble is a chemical and abrasive process; in fact it is carried out by abrasive wheels constituted by oxalic acid and aceto-acetyl; it is employed in the final stage of the process.
By reducing water quantity in the final step of the polishing process, stone overheating is caused that bring a patina, due to the interaction of the mentioned substances with constituents of the stone, to form on stone surface.
In particular, the project aims to develop a set of models in order to put into relationship polishing parameters, diamond tool and stone properties with the cutting force and with the abrasive wheel wear.
These models will be employed to polishing optimization that means to reduce processing costs and/or times obtained acting upon the factors like cutting force, tools properties and vibrations.
A second theme of the present project is to examine carefully the application of abrasive water jet to natural stone, moving from the study of cutting, deepened in the last financed project COFIN 01, to that of surface machining by abrasive water Jet. The objective is to understand the technical and economical advantages of AWJ surface machining with respect to traditional mechanical bush-hammering or blasting. This study will allow to enrich with new results the comparison technical and economical between traditional and abrasive water jet technology, undertaken in the previous COFIN 01 yet.
A third theme of the present project is to manage the products in natural stone according to quality requirements of the current norms of building products. In compliance with current norms, referring to directive 89/106/CEE, decrees that building materials are traceable through marks and indelible codes apposition. To do this the present project wants to study 2 technologies that allow a fixed marking: abrasive water jet and laser. The purpose is to study abrasive water jet and laser applicability to natural stones marking. Models will be developed to reproduce the interaction between energetic bundle and processing material in order to design and, then, implement a prototypal equipment to mark stone. This equipment should be transportable and cheap. The results achieved through these 2 marking techniques will be compared by considering legibility requirements connected to a human read of the code or an automatic read by artificial vision techniques.
In fact, this project wants to study artificial vision technique application to control the quality of surfaces in natural stone. The surface machined through the technologies investigated in the present project (polishing and AWJ) will be studied. The code impressed by permanent techniques investigated in the present project will be discussed too . The objective is to design and implement a prototypal system based upon artificial vision for in-process surface processing control, as traditional as innovative. This control involves both quality product control and, indirectly, machining process and diamond tool wear control. The same system will be used to read the code marked on the product through abrasive water jet and laser technologies and, then, for automatically starting the process that the product must pass through and/or for valuing precision in reproduction of code identified on the natural stone product.
Project proposal wants to exploit the results and the competences acquired in the previous financed project COFIN 01, and puts itself at support of innovation efforts and development in natural stones machining area.
Project objective is to study the following technologies: polishing, surface machining through abrasive water jet, incision through abrasive water jet and laser. Moreover, the project wants to deepen the possibility to apply artificial vision techniques to quality control of natural stone products. Cutting forces and energy, diamond tools, wear progression will be studied in order to optimize surface machining processes.
All the experimental phases of the project will be carried out on two kinds of stone, Perlato Coreno and White Carrara. <<<

First Results

To have the set of tools and stones needed to develop the activities of the project, the test procedures and the values of the process parameters to model all the processes involved in the following stages.Expected results stage 2.1: Experimental equipment and model of the polishing forces for the two considered stones that will be function of both the process parameters and the used wheels.

Expected results stage 2.2: a tool life model and a methodology of validation by means of experimental tests.

Expected results stage 2.3: a semi-empirical relationship between the tool wear and the cutting forces.Expected results stage 3.1: Determination of the energetic material removal model in function of the process parameters for AWJ surface machining.

Expected results stage 3.2: Determination of the energetic material removal model in function of the process parameters for AWJ incision. An equipment to mark by AWJ small areas on rough surface of a product in natural stone.

Expected results stage 3.3: an equipment to mark by laser small areas on the rough surface of a product in natural stone.

Expected results stage 3.4: Selection and set-up of testing methods for the micro and macro mechanical properties of specimens provided by the project partners for the main processes considered in this project. Evaluation of the applicability of tests in the laboratory and possible extension to production, to acceptance inspection and for the evaluation of new materials.Expected results stage 4: A single versatile or more prototypes of an artificial vision system. Selection and set-up of testing methods for the micro and macro mechanical properties of specimens provided by the project partners for the main processes considered in this project. Evaluation of the applicability of tests in the laboratory and possible extension to production, to acceptance inspection and for the evaluation of new materials.Expected results: models of technical-economical optimization of the machining process of natural stoneExpected results: an internet portal of technological innovation in the field of natural stone. Survey of actions with a similar nature that have been just carried out in order to underline the benefits and the lacks. Evaluation of the efficacy of the internet portal by the analysis of the access statistics and/or exam of a case study. <<<

Timescale

24 months

National and international background

Surface finishing is fundamental to exploit a stone product's expressive potential: colour, texture, roughness, patterns and all the possible esthetic attributes of a natural rock can be valorized, modified or minimized using the right treatments. Stone machining may improve its technical properties, like weather- or absorption-resistance. However, choosing a surface finishing always depends on a series of variables, the most important of which are the final colour and, obviously, its related cost.
Traditional manufacturing of natural stones uses diamond tools. The process occurs through chip removal due to the interaction between diamond grits and stone. Interaction between tool and stone generates process forces that depend by the properties of stone, of the tool and by the process parameters. Force and energy of cutting are very important parameters in studying and optimisation of cutting process. They have a direct influence on tool wear, on process temperature and on surface quality.
Very few researches exist in the literature about stone cutting. Jerro shows a mathematical approach to define and derive theoretical chipping geometries [1]. He empirically investigated the relationship between tangential cutting force and chip thickness. Brach et al. studied the problem to convert dynamometer reading of specific cutting energy into consumed power [2]. Asche et al. showed empirical studies on the influence of process parameters on tool wear [3]. Konstanty presented a theoretical model of natural stone sawing by means of diamond impregnated tools for both circular and frame sawing [4]. Pai et al. collects and observes chip samples under a scanning microscope and puts them into relationship with the specific grinding energy [5].
Several are the studies about diamond tools wear utilised in natural stones cutting.
Based on the results obtained by Luo [6] and Webb [7, 8], Tonshoff [9] and Brook [10] performed an analyses on the removal process with diamond tools. Hypotheses formulated by Brook concerning the interaction mechanisms between grains and stone material allowed to develop a model [11, 12] that has been proven to be in good agreement with the experimental results obtained by Luo [6]. In order to utilise that model it is necessary to know the maximum wear velocities of the matrix and diamonds, which in turn are functions of tool characteristics and the properties of the worked material. As far as this latter is concerned, Brook suggests the Schmidt hardness [13]. During subsequent investigations and based on the results obtained in [14], the model has been calibrated utilising the wear trends detected on single diamond grains [15]. The analysis of the wear trend of the grains, though highly time consuming, has been revealed to be a good tool of investigation aimed to understand the influences of the tool and the machined material characteristics.
In the same time only a work has been found in the literature on the optimization of stone cutting. Li et al. showed a study on the optimization of granite cutting by diamond disk [6]. They experimentally calculated the values of the process parameters that allow to decrease the cutting cost, once fixed the cutting power and the tool wear as constraints.
The modelling of the polishing process by abrasive wheel has not faced yet. The works of the literature on stone polishing are empirical; they showed methods to measure the quality of the polished surface [7] or the temperature at the interface between stone and abrasive wheel by means of thermocouples [8]. Moreover, economic considerations have been developed on polishing of stone by Gerard in [9].
High pressure Waterjet finds application in natural stones processing technology context.
High pressure Waterjet cutting is one of the most innovative and promising techniques appeared in the mechanical field in the latest years to solve cutting situations related to a very large range of materials. In the pure waterjet cutting (WJ), the high velocity fluid is responsible for the material removal, while, with the abrasive waterjet (AWJ), the water stream has the only function of transferring its momentum to the abrasive particles, which abrasive and erosive action causes the material removal. In the field of natural stones, the high pressure waterjet has been recently used also in surface machining. The use of rough surface is currently spreading in construction for the production of internal and external floors and for walls' coating. The application of this machining on marble and ornamental stones in general but, mainly, on granite and similar stones, has currently reached a percentage of 25-30% of the whole field of surface machining. Another kind of application of WJ/AWJ technology in the field of natural stones is Marking and Incision, that is currently a very interesting subject for the relationship with the tracing problems of materials in a quality production system. At the moment natural stone products are marked by means of paint applied to the surface using metallic masks. AWJ marking of natural stones is obtained producing a groove on the smooth surface of a depth depending by the jet energy. However, the AWJ incision is currently not easy to apply on inhomogeneous materials such as marbles and granites.
Some works exist in literature about surface processing through AWJ technology. Monno et al. report a study about erosion due to some abrasive grains on the stone [20]. Bortolossi et al. show a relationship between process parameters and surface processing features obtained through AWJ[21]. Carrino et al. report a study about surface processing through AWJ [22]. Natural stone AWJ cutting literature is richer than that of surface processing [23,26]. In this works have been faced experimental studies about natural stones AWJ cutting.
As about laser technology, laser is used in ornamental stone field in order to clean monuments and make artistic engravings. Laser makes it possible to remove an altered surface layer and soil deposits, by reducing undesired effects on the layer beneath the surface. Since it is fairly non-invasive, it may clean surfaces even on extremely fragile products, whereas traditional mechanical techniques cause damage. Moreover, laser is used to mark materials by creating a series of micro-holes on a polished surface, which generate a unique visual contrast impossible to achieve in any other way. This effect is very evident on very fine-grain black granite, since the micro-holes create different tones of grey in dependence by their depth. Difficulty to mark light and no-polished surfaces, and the studies about the interaction between laser beam and natural stone in order to model the mechanism of groove marking are the subjects of interest of the present project. In fact, different laser cutting models have been formulated but none have ever been applied to the cutting of natural stones. In Buinting [27], Beyer [28], Petring [29] e Schulz [30], shape and dimension of laser cutting groove were estimated by means of a power balance between surface absorption, material melting, material heating and material conduction. This model assumed a constant kerf width equal to the beam diameter, semi-circular cutting front contour, and constant angle of inclination for the cutting front, which was experimentally determined. The beam absorption as a function of cutting front orientation was studied by Petring [31]. The reaction between a coaxial gas jet and the cutting front was studied through a two-dimensional mass balance and energy balance at the molten layer [32]. This model assumed diffusion of liquid particles as the main mode of material removal. The dynamic effects of striation formation in laser cutting was also studied in Schuocher [33], by taking into account the oscillation of the kerf width over time. Heat transfer models for evaporative laser grooving were developed in Chryssolouris [34], Modest [35]. The groove depth and shape were estimated as a function of beam power, diameter and scanning velocity through energy balance for the incident laser beam, conduction and phase change. In Modest [35], the area of the beam/material interaction is divided into three region: region prior to beam interaction, evaporation region, and established groove region. In Chryssolouris [36], the shape of the erosion front was dependent on the incident energy density and can be divided into low, medium and high energy density solutions. Cai and Sheng [37] presents the development of a generalized model describing the mechanisms which influence laser evaporative and fusion cutting. A unique work of the literature deals with cutting of slate by using a CO2 laser [38]. A 1.5 kW CO2 laser was used to perform different experiments in which, the influence of some process parameters (average power, assist gas pressure, cutting speed) on the geometry and quality of the cut was studied. The results obtained show that the CO2 laser is a feasible tool for a successful cutting of the considered of slate. Recently, Fraunhofer Institute Werkstuff und Stahltlchnik has studied the possibility to cut on natural stone micro-holes that result no visible and no feeling by hand. In this way the polished surface of natural stone is made anti-slipping, even if it does not lose its aesthetic properties, such as brightness and colour tone. The researches of this Institute have developed a transportable equipment that uses collimated laser spot with a diameter of about 100 micron to makes micro-holes on tiles constituting the floor of a room.
Artificial vision helds a foundamental roule in quality control.
Artificial vision has appeared first in robotics in the seventies and has been increasingly used in industrial robotics and automation over the last two decades. The first massive application has been developed for the electronic industry. With the rapid evolution of the Information Technology these systems have become very attractive for an increasing number of industrial applications and are now a strategic technology in production. Among the sensors used in industry, artificial vision systems are the most general-purpose and flexible tools and can successfully automate one of the most powerful human sensorial capabilities.
The Department of Mechanical, Nuclear and Production Engineering (DIMNP) has long and diversified experience in the industrial application of artificial vision, carried out at the Industrial Vision Lab. An overview has been presented as Technical Report at the Scientific and Technical Committee Assembly (STC "A") of Cirp [39].
At the DIMNP a pioneering study has been carried out, for the realisation of an artificial vision prototype for the automatic sorting of tiles, using a statistical and a neural based approach [40]. This study has been then extended to other materials such as granite [41] and to defect detection [42].
Artificial vision techniques have been applied to a variety of process for automation and control purposes. Some are reported here.
- Three dimensional vision assisted robot operations require the implementation of sophisticated models of the geometric configuration and of the behaviour of the different system components (cameras, lenses, etc.). In [43] two new localisation methods have been proposed with two radically different approached: the first one using a single camera and artificial neural network, the second one, developed in co-operation with the Stanford University, for stereo matching problem.
- A neural based stereo calibration and three dimensional localisation method has been developed and applied and extended to the case of three cameras [44]. Such system operates in real time and is able to retrieve the position in space of the observed object.
- With reference to assembly, an integrated force/vision system has been proposed for the peg-in-hole problem.
- A new method has been proposed to increase the three dimensional pose estimation of parts moving on a priori known trajectories.
- A simple acquisition method of two-dimensional profiles from wood prototypes for the numerical control programming on machine tools is described.
- Within the National SPI Framework, an on-line welding control system has been developed at the DIMNP to be exploited in an automatic production cell.
The analysis of the state of the art on machining of surfaces in natural stone that has been previously described has put into evidence the complete lack of models to optimise surface machining, both traditional and non conventional. However, the importance of the stone field inside the national and international market prevent not to solve many technological problems of the field in order to increase firm competitiveness. The national and local economy has emphasized the importance of the stone sector by creating the Industrial Districts of Stone at Coreno and Carrara, that identifies the firms placed in this geographical area has fundamental for the regional and, then, national economy. <<<