Lapping head for machining workpieces of hard stone materials *** Description The present invention relates to a lapping head for machining workpieces of hard stone materials, such as granite, porcelain-gres, ceramic material or agglomerate materials made using the Bretonstone technology, obtained by mixing together inert materials such as silica or quartz with organic binders such as polyester resins or inorganic binders such as Portland cement. In order to polish hard stone materials, lapping heads with oscillating segments are conventionally used, namely heads which comprise a body rotating about a vertical axis and having, mounted thereon, a number of arms oscillating about an axis which is horizontal and arranged radially with respect to the rotary vertical axis of the head. The bottom end of each arm has a tool holder on which a tool or abrasive segment is mounted. It should be noted that all the arms have identical dimensions and also the tools have the same dimensions with the result that all the tools are arranged along a circumference at the same distance from the vertical axis of rotation. When the head is driven into rotation about its vertical axis, by means of actuating mechanisms arranged inside the head, each of the arms are caused to swing about its radial axis of oscillation and therefore also the corresponding tool or abrasive segment is caused to swing "which thus maintains a cylindrical cutting profile also during the wear process. The lapping/polishing action occurs as a result of the rotational movement of the abrasive tools about the vertical axis of rotation of the head, while each diamond tool (which has a working surface consisting of a cylindrical portion) swings about its radial axis of oscillation and always ensures a tangential contact of the tool with the workpiece being machined. During their movement, the tools cover a circular ring, leaving necessarily uncovered a circular area in the centre of the head, for the purpose of allowing oscillation of the arms without collision between the adjacent tools. For further clarifications on the type of mechanisms used, reference should be made to Italian patent 1,247,201 filed on 21.3.1991 or European patent 649,706 filed on 17.9.1994, both in the name of Luca Toncelli. The lapping heads are then mounted on machinery known as lapping machines or polishing machines, depending on the lesser or greater finishing requirements of the machined workpieces. Lapping machines comprise a bench with a moving belt on which the material to be lapped rests and a beam positioned above the belt and arranged parallel with the said belt. Various rotary spindles are mounted in an aligned manner along the beam and at the bottom end of each of them a lapping head provided with diamond tools is mounted, as explained above. The beam is moved back and forth transversely wrth respect to the belt while, at the same time, each lapping head is driven into rotation about its vertical axis and each tool swings about its radial axis of oscillation. The heads are pressed toward the workpiece to be machined so that the tools are brought into contact at a given pressure against the said material and, owing to their swinging movement, are able to perform the lapping of hard materials since a high specific pressure is generated along the tangential line of contact, whereby an aggressive action is exerted on the stone material. The back and forth movement of the beam instead allows lapping of the workpiece, usually in the form of slabs, over its entire surface. ώ . Although lapping/polishing technology is now well-established, defects - albeit slight — are nevertheless visible on die polished material thus obtained. In particular, in proximity of the ends or rather die side edges of die polished workpiece, on materials ."which are particularly dark and/or difficult to lap visual defects can be found which consist of shadows, or of light and dark zones or even of small circular grooves caused by the abrasive tools. Any such defects may be visually perceived, in particular by a well-trained eye, so that they have a negative effect on the final quality of the finished product. Basically the product obtained may be of a partly diminished quality due to these defects, particulady in case of some types of material and/ or shades of colour. This problem may be explained by considering that, in order to ensure lapping of the workpiece over its whole width, in the two end positions assumed by the beam, namely in the two points where its movement is reversed, the tools must not only reach die side edge of the slab to be lapped, but must also go beyond said edge in order to ensure a sufficient coverage of the surface being machined. In so doing, however, when the beam reaches the end positions, not all the tools have the whole of their abrasive surface continuously in contact with the workpiece, but some tools are projecting beyond the side edge of the workpiece being machined and therefore do not rest completely on the material. When all the tools engage with the workpiece to be lapped, the pressing force which the head exerts against the workpiece is uniformly counter-balanced by the reaction of the workpiece on the tools. However, when in each instant some tools do not engage with the workpiece along the whole of its surface being machined, then the pressing force of the heads is no longer counter-balanced in an uniform and regular manner. In other words the head tends to "collapse" on the side where the tools do not rest completely on the material to be lapped. Owing to this phenomenon, machining no longer occurs in a uniform manner and consequently the abovementioned defects may arise. Moreover, it should be considered that in the end positions where the beam reverses its movement, it is necessary first of all for the beam to slow down, then to stop, reverse the movement and finally to accelerate. These accelerating and decelerating movements give rise to inertial forces which stress in an unbalanced manner the said heads and therefore negatively affect the lapping/polishing operation. Moreover, during stoppage of the beams, the tools are unable to cover the circular surface portion coaxial with the head. In particular, seizing effects can also take place, namely during deceleration the head can seize on the workpiece, causing a jerky braking action which further worsens the situation. It is therefore desirable to reduce, if not eliminate completely, any defects present on the polished material so that they are not visible to the naked eye, and the quality of the final product which is obtained is not affected. The object of the present invention is therefore to envisage a new head for lapping/polishing workpieces of hard materials or in any case to provide the polishing head with particular contrivances which make the above mentioned surface defects invisible, or nearly so, to the naked eye. This object is achieved by a lapping/polishing head for workpieces of hard stone materials comprising a body rotating about a vertical axis and having tools mounted thereon, each tool swinging about an axis arranged radially with respect to said vertical axis and the said tools being circumferentially equidistant from each other, characterized in that said tools are divided into at least two groups of tools, each group being positioned at a distance from said vertical axis different from that of each other group, the tools of each group being equidistant from said vertical axis and arranged in a circumferentially equidistant manner with respect to each other. In particular, when the tools are six in number, circumferentially equidistant from each other, so as to form an angle of 60° relative to each other, said six tools are divided into two groups, each group consisting of three tools circumferentially equidistant from each other so as to form an angle of 120° relative to each other. According to the invention, the tools of a group are arranged on a circumference which is smaller than the other and outer circumference and this is possible due to the fact that the 120° spacing between tools of a same group prevents them to collide during their swinging movement. It is obvious that in this way the overall working surface of the tools, i.e. the surface that rests on the workpiece being machined, is no longer concentrated on the outer circumferential part so as to define an outer ring, but is distributed over two rings, an outer ring and an inner ring, for which reason the central circular area not c . covered by the tools during rotation of the head is greatly redwced. Thus, in the end positions where the beam reverses its movement, although the tools arranged at a greater distance from the vertical axis of rotation of the head do not rest completely in each instant on the surface of the workpiece being machined, the tools positioned more close to said axis instead always remain in contact with the workpiece. The pressing force of the head exerted on the material is therefore always suitably balanced and in any case sufficient to ensure an optimum operation of the heads, avoiding also the risk of seizure. These and other characteristic features of the present invention will emerge more clearly from the following detailed description provided solely by way of example and with reference to the following accompanying drawings in which: - Figures Ia and Ib show, respectively, cross-sectioned and from below, a conventional lapping head, where it is possible to note the six tools all arranged on a single outer circumference, wherefrom a large, exposed, central circular area is provided for preventing collision between adjacent tools during oscillation; ~ O ~

- Figures 2a, 2b show views, similar to the preceding views, of a lapping head according to the present invention. Hereinbelow only the lapping head according to the invention with reference to Figures 2a, 2b will be described, even if the same reference numerals have been used in Figures Ia, Ib in order to indicate elements which are structurally or functionally equivalent. In Figures 2a, 2b, 10 denotes overall a lapping head for machining workpieces of hard stone materials such as, for example, granite, ceramic materials, gres or agglomerate obtained by means of Bretonstone technology using silica or quartz-based inerts. The lapping head 10 comprises a box-like body 12 with an approximately cylindrical shape rotating about a vertical axis Y and having a plurality of arms 14 mounted on its bottom side. At the bottom end of each arm 14 there is a tool-holder 18 on which a diamond coated tool 20 is mounted. More precisely, the head 10 has six arms 14a,b,c,d,e,f which are mounted on the body 12 so as to be able to swing about respective axes of oscillation 16a,b,c,d,e,f. The bottom side of each arm 14a,b,c,d,e,f carries a tool-holder respectively denoted by 18a,b,c,d,e,f on which a diamond tool 20a,b,c,d,e,f is fixed. The axes of rotation 16a,b,c,d,e,f of the arms 14a,b,c,d,e,f are arranged in a circumferentially equidistant manner, namely spaced by 60° with respect to each other. The box-shaped body 12 houses driving means 12 which, owing to the rotational movement about the axis Y imparted to the lapping head, transmit the swinging movement to each of the six arms 14a,b,c,d,e,f. For further details about the type of driving means used, reference should be made to the above mentioned patents. The six oscillating arms 14a,b,c,d,e,f are of two types so as to form two separate groups: a first group consisting of the arms 14a,c,e and a second group consisting of the arms 14b,d,f. Both groups comprise three arms which are mounted on the head 10 so as to be circumferentially equidistant, forming an angle of 120° relative to each other, and equidistant from the vertical axis of rotation Y. Essentially the arms of the first group alternate with those of the second group. The second group of arms 14b,d,f is such that the respective tool-holders 18b,d,f are positioned at a distance from the vertical axis of rotation Y smaller than the corresponding distance of the tool-holders 18a,c,e of the first group of arms 14a,c,e so that the tools 20b,d,f supported by the second group of oscillating arms 14b,d,f are also situated at a distance from the axis Y smaller than that of the tools 20a,c,e supported by the first group of oscillating arms 14a,c,e. The tools of each group therefore define a radial extension defined by their maximum and minimum distance from the vertical axis Y. Since the difference in the distance of the tool-holders 18a,b,c,d,e,f from the vertical axis Y, between the two groups of arms, is less than the radial length of the tools 20a,b,c,d,e,f, an intermediate circular zone or band is formed on the workpiece which is subject to the action of both groups of tools. In this 'way the area on which the tools rest and polishes the workpiece is increased, ensuring a more uniform machining even when the tools portion at a greater distance from the vertical axis Y projects beyond the side edge of the workpiece. The workpiece is thus polished in an optimum manner and does not show any visible surface defect along its side edges. n s It is obvious that any variation or modification iof the invention which is conceptually equivalent falls within the appended claims. For example, the arrangement of one group of tools could be of such a radial extension that it does not to result in partial overlapping relative to the tools of the other group, in particular the two groups of tools can be adjacent. Moreover the tools, instead of all being of the same radial length, could have a varied length. For example, the tools in each group could be of the same length, but the length of the tools in one group could be other than the length of the tools in the other group. Finally, the lapping/polishing head could have a different number of arms such as, for example, a head with nine arms, and therefore nine tools, circumferentially equidistant from each other so as to form an angle of 40° relative to each other. The nine arms and tools may be subdivided into three groups of even number, each group being arranged at a different distance from the vertical axis Y and in such a way that the tools of each group are circumferentially equidistant from each other at an angle of 120° relative to each other. — 1 —

Alternatively, the nine arms and tools, which are circumferentially equidistant from each other so as to form an angle of 40° relative to each other, could be subdivided into two groups: a first group of three arms and three tools circumferentially equidistant from each other so as to form an angle of 120° relative to each other and arranged at a smaller distance from the vertical axis Y than the second group of six arms and six tools arranged in pairs and circumferentially equidistant from each other so as to form an angle of 120° relative to each other. ***