The present invention refers to a machine for polishing marble and in particular to a plate that holds suitable polishing tools.

As known, the polishing of slabs of marble/Granite/natural stone is carried out on machines made of a base with a conveyor belt that makes the slabs advance and of a bridge able to move transversally to the forward movement, on which a series of spindles are fixed that, by means of suitable operating heads, kept in contact with the material by pneumatic cylinders, provide for polishing the slabs.

In known machines the polishing of slabs is obtained through different tools depending on whether dealing with marble or granite; in the case of marble, rotating metallic plates are used that carry sectors, so-called "Frankfurts", which are fixed on suitable fittings provided on the work surface of the plate; in the case of granite, the rotating polishing heads, on the periphery, carry a series of so-called spatula arms provided with oscillating movement with respect to the head; the so-called "Fickert" sectors with the polishing grains are fixed on such arms.

A typical spindle for such machines, for marble, is illustrated in figure 1. In particular, such a spindle comprises a spindle body 1, which is fixed to the bridge of the machine, inside which a sleeve 2 slides vertically due to the action of pneumatic cylinders 3; inside the sleeve a rotating shaft 4 is housed, supported by means of the bearings 5, and which bears, at the lower end, a bell 6 that has the function of supporting an abrasive-tool holder plate 7.

The connection of the abrasive-tool holder plate with the bell is known to be made by means of a certain variable number of rubber cylinders (pads) 8, which also have the function of transmitting, by means of the abrasive holder plate 7, the movement to the abrasives 9, fixed on the plate by means of "tabs" 10, and of adapting the position of the abrasive-tool holder plate to the material being processed.

The abrasive-tool holder plate adapts to the surface being processed exploiting the elasticity of such rubber pads, which compress more or less during the rotation of the plate; in brief, the pads compress firstly due to the force transmitted by the pneumatic cylinders, and also depending on the inclination undergone by the plate in order to adapt to the slab, are subject to a different elongation that translates into a (positive or negative) delta of the base compression value.

Such a work system, although ensuring acceptable functionality, provides a not entirely satisfactory polishing quality, mainly in the case of slabs that are not perfectly flat and of coloured materials that have the feature of highlighting the so-called "passes", i.e. the processing marks left by the abrasives.

The stress of the rubber pads is rather high, taking into account the force exerted by the pneumatic cylinders (of the order of 200 kg) and the delta consequent to the compression/release cycle to which the pads are subjected during the rotation of the plate, due to its inclination, as stated above.

In such conditions of high stress, the response of the pads to the arrangement variations of the plate is logically reduced, due to the high stress of the pads, which translates into a forcing of the position of the plate .

The purpose of the present invention is to avoid the aforementioned drawbacks, reducing the amount of stresses on the rubber of the pads.

An aspect of the present invention concerns a machine for polishing marble with improved stability having the characteristics of claim 1.

The characteristics of the present invention will become clearer from the following description and from the attached drawings, relating to a non-limiting example embodiment, in which:

• figure 1 illustrates a front view of a spindle of a machine for polishing slabs according to the present invention;

• figures 2a and 2b illustrate such a plate with the pads positioned according to an embodiment of the present invention;

• figures 3a and 3b illustrate such a plate with the pads positioned according to a further embodiment of the present invention.

With reference to the cited figures, the machine for polishing slabs according to the present invention essentially comprises a base with a conveyor belt that makes the slabs advance and a bridge, movable in the direction transversal to the advancement movement of the slabs, on which a series of spindles are fixed that, by means of suitable operating heads, kept in contact with the material by means of pneumatic cylinders, provide for polishing the slabs.

The spindles are kept in contact with the slabs by means of the cited cylinders that allow an up and down movement of the operating head according to the profile of the underlying slab.

Such spindles comprise at least one spindle body 1, which is fixed to the bridge of the machine. Such a body is provided with means for moving along a vertical axis and for rotation about such an axis for a bell 6 that has the function of supporting an abrasive-tool 9 holder plate 7.

Such moving and rotation means preferably comprise a sleeve 2 that slides inside such a spindle body due to the action of pneumatic cylinders 3; a rotating shaft 4 is further housed inside the sleeve, supported by bearings 5, and which bears, at the lower end, such a bell 6.

Such tools 9 are connected to the plate by means of quick fasteners 10, like for example interlocking fasteners with tabs 10.

The connection of the abrasive-tool holder plate with the bell is made by means of a plurality of rubber cylinders (pads) 8 that also have the function of transmitting the movement to the abrasives and of adapting the position of the abrasive-tool holder plate 7 to the material being processed.

According to the present invention a suitable positioning of such pads 8 determines an improvement of the smoothing performances of such a plate and of the machine in general . In particular, the invention consists of positioning a first central rubber pad at the central axis of the sleeve and a plurality of peripheral pads positioned according to various configurations around such a central pad. The central pad, having a different configuration and being more robust than the others, i.e. with higher shore hardness, which has the purpose of almost completely absorbing the force transmitted by the pneumatic cylinders. In such conditions, the force that is absorbed by the peripheral pads, in the case of a flat slab, is theoretically null; the function of said peripheral pads being limited to only allowing the adaptation of the plate to the slab and to bringing the plate itself into the horizontal position when no longer in contact with the slab. Of course, the peripheral pads are still responsible for transmitting the rotation from bell 6 to plate 7.

A preferred arrangement of the pads is illustrated in figures 2 and 3. In particular, figure 2 shows a central pad 81' and nine peripheral pads 82' positioned on a circumference at about 2/3 of the diameter of the plate. The nine pads are organized in equidistant groups of three pads each.

Figure 3 shows a central pad 81'' and twelve peripheral pads 82'' positioned on a circumference at about 2/3 of the diameter of the plate equidistant to each other.

Preferably, the hardness of the central pad is 60 shore, whereas that of the peripheral pads is 40 shore.

According to the present invention the peripheral pads are cylindrical whereas the central one is a hollow cylinder of larger size. The size/hardness combination of the central pad is such as to ensure that the same absorbs practically all of the force of the pneumatic cylinders, as stated above.

Clearly, in such conditions, the adaptation of the plate to the inclination of the slab is much more rapid and effective, not being hindered by overloaded pads; the consequence is a greater polishing quality since there will no longer be the defects described earlier. A possible further configuration could provide the use of pads made of different materials, like for example, but not limited to, polyurethane thermoplastic elastomers or combinations thereof; provided that it is still ensured the criterion that the central pad absorbs practically all of the load, whereas the peripheral pads are exclusively responsible for the function of distributing the load as evenly as possible in the case of a non-flat slab and that of allowing a quick return of their deformation without yielding since the level of stress of the peripheral pads is very little; of course, the peripheral pads are still responsible for the function of transmitting the rotary motion from bell 6 to plate 7.