The present invention refers to a machine for polishing/ flattening slabs of natural stone or marble or granite or the like in order to obtain smooth and flat slabs, able to be used for example to product polished and/or smoothed stone material in general.

In particular, such machines comprise a base with a conveyor belt that makes the slabs advance from a bridge, movable in the direction transversal to the forward motion of the slabs, on which a series of spindles are fixed that, by means of suitable operating heads, kept in contact with the material through pneumatic cylinders, provide for the polishing of the slabs. The spindles are positioned on the bridge parallel to one another transversal to the movement of the slabs.

Such starting slabs to be polished can be produced with various methods and machinery; these range from diamond frames, grit frames, giant-disc machines or multi-wire machines.

Depending on the production method used and the type of material being processed, the quality of the slabs can vary in terms of the shape, the surface finish and the dimensional tolerances.

The finished product, in outlet from the polishers, can consist of slabs calibrated in thickness with predetermined tolerances, or simply polished slabs for which reason the tolerance of the thickness is not important .

In machines of the known type, the polishing of the slabs is obtained through different tools depending on whether it is marble or granite; in the case of marble metallic plates are used that carry the so-called "Frankfurt" sectors, which are fixed onto suitable couplings provided on the work surface of the plate; in the case of granite, the polishing heads rotate and carry on the periphery a series of so-called wiper plates provided with oscillating motion with respect to the head; the so-called "Fickert" sectors with the polishing grains are fixed onto such plates.

When calibration is required, the polishing operation must be preceded by a calibration operation through a machine configured and equipped with spindles controlled in height, so as to ensure, at the outlet of the machine, a given thickness, with the successive action of many calibrating spindles in sequence. The calibration tools generally (but not exclusively) consist of planetary diamond plates or tangential diamond-discs for granite, whereas for marble there can again be planetary plates or diamond-plates with a continuous crown.

In the case in which only polishing of the surface of the slab is required, the operation of preparing for polishing must be preceded by an operation that provides for eliminating possible surface defects from the slabs, such as "ridges" but particularly crests and valleys of the surface of the slab and of preparing a flat surface without any regard to the final thickness; such a purpose is normally achieved by using, as first heads, (normally two or three) spindles driven by pneumatic cylinders that, by means of FrankFurt sectors on which sintered diamond is applied instead of polish, "clean" the surface of the slab removing a layer of material and thus preparing it for the subsequent action of the actual polishes.

The effectiveness of such action is not, however, always ensured particularly in the case in which the surface of the slabs has dimensional defects, so-called "ridges", i.e. variation of the thickness in the transversal direction of the slab, since, in this case, the action of the spindles will tend to reproduce a cleaner plate but roughly with the same original dimensional defects since the pneumatic cylinders only ensure an abrasive action but without dimensional references .

The purpose of the invention is to equip existing pneumatic spindles with a device that ensures a mechanical downward end-stroke so as to limit the amount of material removed, ensured by a movable mechanical end-stroke and, finally, ensure that the slab has a flat surface.

An aspect of the present invention concerns a machine for polishing/ flattening slabs of natural stone or marble or granite or the like, provided with a control mechanism of the position for a flattening spindle of such a machine having the characteristics of claim 1. A further aspect of the present invention concerns a method for polishing/ flattening slabs of natural stone or marble or granite or the like having the characteristics of claim 8. The characteristics of the present invention will become clearer from the following description and from the attached drawings, relative to a non-limiting embodiment, in which:

figure 1 illustrates a perspective view of a spindle of a machine for polishing slabs to which the control mechanism of the position of the flattening spindle according to the present invention is applied;

figure 2 illustrates the same mechanism of figure 1 in a side view;

figure 3 illustrates the means for adjusting the stop position of the spindle according to the present invention .

With reference to the quoted figures the machine for polishing slabs to which the control mechanism of the position of the flattening spindle according to the present invention is applied essentially comprises a base with a conveyor belt that makes the slabs advance from a bridge, movable in the transversal direction to the forward motion of the slabs, on which a series of spindles 1 are fixed that, through suitable operating heads 2, kept in contact with the material by pneumatic cylinders, provide for polishing the slabs. The spindles are positioned parallel to one another transversally to the movement of the slabs.

The spindles are kept in contact with the slabs by the aforementioned cylinders that allow an up and down movement of the operating head depending on the distance at which the underlying slab is located.

The mechanism according to the present invention comprises means for stopping the vertical movement of the spindle in a lower stop position and means for adjusting the lower stop position of the spindle itself .

The stop means comprise a gripper 3 actuated by a first pneumatic cylinder 31 that locks on a stem 4 of the pneumatic cylinder 6 for controlling the spindle 1 limiting the downward stroke thereof by means of an abutment 5 that defines such a stop position.

The means for adjusting the stop position comprise a second pneumatic cylinder 7, which keeps the gripper, when not operative, at a certain predetermined height. When the gripper is closed around the stem of the spindle, the cylinder immediately retracts freeing the movement of the gripper together with the spindle towards the stop position.

Such a cylinder 7 also comprises a washer 71 for manually adjusting the port L that constitutes the size of the overstroke of the gripper, i.e. the minimum thickness to be removed on the surface of the slab.

Such an overstroke value or port L is normally comprised between 0 and 5 mm; in the case of standard materials a reference value is 0.2-0.3 mm.

A type of operation of the device could be the following :

The spindle lowers onto the material being processed controlled by the devices of the polishing machine; then it performs one or two passes on the material itself, identifying, through the position control of the spindle (cylinder) and suitable sensors, the lowest position .

On such a position, the stop means, by means of the closing of the gripper, define the stop position, the second cylinder 7 of the adjustment means will retract and the flattening head 2 can perform a sort of flattening up to the minimum height defined by the abutment 5.

If the difference in thickness encountered by the head is excessive for the removal ability of the spindle, the head itself will have the possibility of lifting up, since controlled by a pneumatic cylinder that allows elasticity of the position, therefore without carrying out the requested flattening but avoiding "embedding"; the flattening will be completed by the successive head (or heads), which will operate on a "prepared" surface.

The flattening/polishing process that is carried out by the machine according to the present invention is substantially as follows.

• performing at least one first pass with the spindle and the operating head in contact with the surface of the slab,

• detecting, during such first processing, the lowest point or minimum height of the spindle,

• at the end of this pass, making the gripper 3 intervene, at the minimum height, and retracting the cylinder 7 that ensures that an overstroke beyond the minimum height identified is reached · performing further processing of the spindle on the slab with the operating head in such a position. If the difference in thickness encountered by the head is excessive for the removal ability of the spindle, the head/spindle group lifts automatically since it is actuated pneumatically, thus avoiding jamming whilst still continuing to remove material based on its own actual capability. The steps of the method described above are also applicable to a calibration or calibration/polishing machine, in which the vertical position of the spindles is ensured by mechanical abutments controlled by screws or analogous systems; the spindles being kept in contact with said abutment by the pneumatic cylinder. The profile of the slab being processed is detected in inlet to the machine and the position of the mechanical abutments is made so as to optimise the removal capability of the heads; if the processing requires a greater removal capability than the possibility of a head, it would automatically raise since it is kept on the mechanical abutment by a pneumatic cylinder, going back into the correct position as soon as the processing conditions (for example reduction of the belt advancing speed) allow it. The extra removal would thus be divided among the successive heads.