Gloss and Modelling Studies of Stone Polishing Using Linear Polishing Machines with Rotating Heads

The ornamental stone industry has always played an important role in the world economy. Polishing the slabs to increase their gloss is important to enhance the beauty and richness of these natural materials. Many industrial polishing machines rely on a rotating head’s movement along zigzag trajectories (controlled by belt, transverse and rotational speeds), to erode the surface as stochastically as possible (to avoid scratches and other visual defects caused by paths that are too symmetrical). Optimizing (and automatizing) these three speeds together therefore represents a significant gain for the industry, in time, energy, and quality of product. In this work we show that this optimization can be accomplished by fulfilling these conditions: (1) the displacement of the polishing head after a single zigzag movement should be smaller than its diameter; and (2) the displacement of the polishing head after a single rotation should be smaller than its radius. To assess the validity of these two conditions, we studied the polishing activity using experiments based on gloss measurements of the polished stone and computer simulations based on the geometric contacts between the tool and the stone. We concluded that: (i) a clear correlation can be established between experimental and simulation data; (ii) the two displacement parameters represent an effective way to control the quality and efficiency of the polishing process; and (iii) there is a limit to the gloss acquired through polishing processes, thus polishing above a given threshold decreases the efficiency without increasing the quality. The correspondence between experimental and modelling results opens the door to further optimizations of these polishing processes in the future.