The invention concerns a sanding method for substantially bidimensional objects and the sanding machine suitable for the application of this method.

As already known, the sanding machines for substantially bidimensional objects, for example wooden panels, but also for other objects made of different materials, are mainly constituted by a frame that supports a conveyor belt suitable for feeding the pieces to be processed, which is arranged horizontally and according to the longitudinal axis of the machine.

One or more sanding units rest on this conveyor belt, said sanding units being provided with an abrasive tape that rotates around a series of rollers arranged with parallel axes and transversally with respect to the longitudinal axis of the machine.

Each one of said sanding units is provided with a plurality of pressing units positioned side by side and independent of one another, each one pressing on a section of the rotating tape, so that the tape exerts. a sanding action on the piece passing between the lower part of the tape and the conveyor belt.

A sanding machine carries out a good sanding process when the piece is sanded uniformly.

To obtain a uniform sanding result it is important that the machine may exert on the sanding tape a pressure that is inversely proportional to the thickness of the piece to be processed with the same type of surface to be sanded.

It can be easily understood, in fact, that the thinner is the piece to be sanded, the greater will be the force necessary to deform the tape, since this will be forced to extend more in order to come in contact with the surface to be sanded.

Consequently, the pressure exerted on the piece by each pressing unit must also be greater.

According to the patent EP 0 484 303, in order to ensure that each pressing unit can exert the force necessary to uniformly sand a given piece with a given thickness, the piece to be sanded must pass under detecting and control means suitable for detecting its thickness and shape. Each one of said sensors supplies to a control panel equipped with a processing unit a voltage value that is proportional to the development of the thickness of the pieces being

processed and in turn this control means is capable of defining in each one of the solenoids associated with each pressing unit a specific current value proportional to the voltage generated by the sensor.

In practice, as the thickness varies, the current carried by the solenoid associated with each pressing unit varies accordingly.

Obviously, with reduced thicknesses compared to a given reference standard, the current must be increased in such a way as to increase the thrusting force of said pressing units, since, as already explained, part of the force applied to each pressing unit is necessary to further deform the abrasive tape, in such a way as to make it come in contact with the surface of the piece to be processed.

It is sufficiently clear that this method, though being rigorous and precise for detecting the thickness of every part of the piece. and then supplying the suitable force to each pressing unit, requires that each pressing unit be governed by a an electromagnet carrying a significant current that increases as the thickness of the piece to be processed decreases.

Besides the high amount of energy expended, this also involves considerable heating of the pressing units and therefore of the sanding units.

Furthermore, the management of voltage in each sensor and of current in each pressing unit associated with it requires the use of a rather complex and expensive control system.

The American patent US 4,601, 134 concerns the construction of a sanding machine provided with a plurality of pressing units, wherein each one of said pressing units is provided with contact and pressure means capable of exerting a force that can individually and continuously vary from zero to a given predetermined value depending on the information previously transmitted by a system that detects the piece thickness.

Also in this case the force transmitted to each pressing unit is supplied by electromagnets carrying a current that is proportional to the pressure force that must be exerted by the pressing unit itself.

Therefore, also according to the invention mentioned above, the structure of the machine is complex, since it is necessary to manage each electromagnet of each pressing unit, even with different electric characteristics. Furthermore, the energy consumption is considerable, due to the high current that must be supplied to each electromagnet, since the current used is responsible both for

the shifting of the pressing unit towards its operating position,, that is, in contact with the piece, to supply a force capable of deforming the sanding tape, and for the further shifting that generates the force necessary for the actual sanding process.

The aim of the invention is the embodiment of a sanding method and relevant machine that overcome the drawbacks described above.

First of all the aim is to separate the force necessary to deform the tape from the force necessary to exert the pressure required to obtain the desired degree of sanding.

Another aim is that the means exerting the pressure to obtain the deformation of the tape should be different from the means used to carry out the sanding process.

More particularly, the aim is that the force necessary to deform the tape should not depend on the current circulating in the electromagnet and therefore that no energy should substantially be expended to obtain this deformation.

A further aim of the invention is the embodiment of a simple and economic device that controls all the pressing units involved in the sanding process of a given piece having given shape and thickness, wherein the pressing units in operation are fed by on-off devices, so that it is not necessary to define a different behaviour between a pressing unit and the other pressing units involved in the process.

The aims mentioned above and others that will be highlighted in greater detail below have been achieved through the embodiment of a sanding method for objects substantially developed according to two main dimensions, whose thickness varies within certain limits, said sanding process being carried out with the aid of a sanding machine comprising a conveyor belt and at least one sanding unit provided with at least one abrasive tape arranged transversally with respect to the direction of advance of the objects to be sanded, said at least one tape being provided with a plurality of pressing units positioned side by side and independent of one another that rest against said at least one tape, said method being characterized in that it includes the following steps: - detecting, upstream the sanding unit, the presence and shape of each object to be sanded by means of sensors connected to on/off devices; - electrically connecting each sensor to each pressing unit by means of a control unit;

-applying to the rod of each pressing unit operated by the on/off signal of a first force constituted by the magnetic attraction force of at least one permanent magnet associated with said rod, said at least one permanent magnet constituting with said rod the moving core of at least one electromagnet and interacting with a mass of a soft metal belonging to said at least one electromagnet, said force being variable according to the variation of the distance between said permanent magnet and said mass of soft metal ; - applying to each rod of said each pressing unit a second force obtained by feeding with constant current said at least one electromagnet having a ferromagnetic core housing said rod, said second force being the same for each rod of each pressing unit and depending on the degree of sanding desired for the objects to be processed.

The invention concerns also the sanding machine suitable for the application of the method described above.

To advantage, according to the method object of the invention the force necessary to make each pressing unit rest on the piece to be sanded is exerted by a permanent magnet associated with the rod of the pressing unit, which is attracted by the metal mass belonging to a non-excited electromagnet and therefore without using electric energy to activate the coil of the electromagnet associated with each pressing unit.

Any difference in thickness between two pieces is compensated for by the increasing attraction force of the permanent magnet with respect to the metal mass. More precisely, so long as the pressing unit resting on the abrasive tape doesn't reach the piece surface, the permanent magnet continues to move towards the metal mass of the electromagnet with increasing force, since, as already known, the attraction force increases as the distance between the magnet and the metal mass decreases.

Therefore, owing to the use of a permanent magnet a variable force is available, which depends on the thickness of the piece and so is perfectly capable of satisfying any need to further deform the abrasive tape.

Successively, a second force is added, which is delivered by the electromagnet whose coils are fed by a current that is proportional to the degree of sanding desired, but does not depend on the thickness of the piece.

It is important to consider that in all the pressing units operated by the on-off

device the electromagnet carries the same current.

The simple structure of the device that feeds the electromagnets is clear, in fact it is an on-off device in which the activated electromagnets are fed with the same voltage.

To obtain a different degree of sanding, for example to carry out a deeper sanding, it will be sufficient to increase the voltage and therefore the current in all the electromagnets activated by the on-off signals.

Further characteristics and details of the invention will be highlighted in the description of a favourite application of the sanding method and of the machine object of the invention illustrated in the attached drawings, wherein: - Fig. 1 diagramatically shows the pressing unit of a sanding unit of the machine object of the invention when at rest; - Fig. 2 shows the pressing unit resting with the abrasive tape against the piece before carrying out the sanding action; - Fig. 3 is a schematic view of the sanding unit; - Fig. 4 diagramatically shows the pressing unit against the piece to be sanded; - Fig. 5 is a schematic view of the sanding machine object of the invention; Fig. 6 shows the rod and the moving core of a pressing unit associated with a special embodiment of the electromagnet according to the invention, when the pressing unit is at rest; - Fig. 7 shows the core of Fig. 6 shifted downwards when the pressing unit is in contact with the piece to be sanded; - Fig. 8 shows the electromagnet of Fig. 6 and the moving core during the sanding process; - Fig. 9 shows a diagram illustrating the development of the attraction force of the permanent magnet according to the shifting of the moving core; - Fig. 10 shows a variant of the electromagnet associated with each pressing unit when at rest; - Fig. 11 shows the electromagnet of Fig. 10 when the pressing unit is in contact with the piece to be processed; - Fig. 12 shows the electromagnet of Fig. 10 during the sanding operation.

With reference to the figures mentioned above and in particular to Fig. 3, it can be observed that in this schematic view the sanding machine object of the invention comprises a conveyor belt 1 on which the pieces to be sanded 2 are

laid, said pieces successively passing under the sanding unit, indicated as a whole by 3.

Said sanding unit comprises a series of rollers 31,32 and 33 arranged with transversal axis with respect to the longitudinal axis of the conveyor belt, and said rollers set rotating the abrasive tape 4 tensioned between them.

The sanding unit 3 is provided with a series of pressing units 5 that are also aligned according to a transversal-line orthogonal to the longitudinal axis of the conveyor belt 1.

Before the pressing units 5 the sanding machine, diagramatically shown in Fig. 5, is provided with a series of sensors 6 of known type, for example mechanical feeler pins or optical reflexion sensors, all suitable for detecting the presence and the shape of the piece 2 that will be sanded ; The sensors 6 are connected to corresponding pressing units 5, so that only the pressing units whose position coincides with that of the piece to be sanded will be activated, while the others will remain inoperative, as described below.

The sensors 6 send out an on-off signal, therefore when the piece to be processed 2 reaches the correct position the signal of each sensor is transferred by the control unit 7 to the corresponding pressing unit 5 that will be operated to sand the piece.

Each one of the pressing units 5, as can be observed in Fig. 1, is provided with a pressing element 51 that is in contact with the abrasive tape 4 and exerts its pressure on the piece to be sanded through said abrasive tape.

Said pressing element 51 is operated by a rod 52 made of a preferably non- magnetic material, such as stainless steel, which is provided with at least one permanent magnet arranged in such a way that it interacts with the polar expansions of at least one electromagnet, thus constituting its moving core.

In the application of the invention shown in Fig. 1 and visible in greater detail in Fig. 6, the rod 52 is associated with two permanent magnets 53 and 54 centrally separated by an non-magnetic material, for example aluminium discs indicated by 55, and at the ends by further discs made of non-magnetic or insulating material, indicated by 56 and 57.

The permanent magnets 53 and 54 have their equal polarities facing each other, for example the ends 531 and 541 that are two north polarities. The permanent magnets 53 and 54 carried by the rod 52 slide between the polar expansions of two electromagnets, a lower one indicated by 60 and an upper

one indicated by 61. Said electromagnets, geometrically. and functionally connected to each other, have a flat geometry and each one of them is provided with a pair of coils, indicated by 605 and 602 for the lower magnet 60 and by 610 and 611 for the upper magnet 61, while the polar expansions of the lower electromagnet 60 are indicated by 603 and 604.

The upper electromagnet 61 and the lower electromagnet 60 have magnetic circuits that are separated through the interposition of a flat element 620 made of non-magnetic material.

The polar expansions of the upper electromagnet 61 are indicated by 612 and 613.

In rest conditions, as shown in Fig. 1, that is, before the piece 2 reaches the sanding position under the pressing unit 5, the pressing unit remains in a position of neutral equilibrium, since the north poles 531 of the permanent magnet 53 and 541 of the permanent magnet 54 are attracted by the metal mass constituted by the polar expansions 612 and 613 of the upper electromagnet 61.

More precisely, owing to the attraction force exerted by the magnet 53, the rod 52 tends to move upwards, while owing to the equal attraction force exerted by the permanent magnet 54 in the opposite direction, the rod 52 tends to move downwards, and the result is that the position reached is the equilibrium position illustrated in Fig. 6 and corresponding to the position illustrated in Fig. 1.

When, as shown in Fig. 2 and Fig. 3, the piece to be sanded 2 is positioned under the pressing unit 5 of the sanding unit 3, the corresponding sensor 6 enables the on/off signal and the control unit 7 sends a current pulse to the coils 605 and 602 of the lower electromagnet 60. In this way, according to Fig. 7, the permanent magnet 53 is attracted downwards until the pressing unit 51 (see Fig. 2) rests-with interposed abrasive tape 4-on the surface 21 of the piece to be sanded 2.

When the pulse of the current that passes through the coils of the electromagnet 60 ends, the rod 52 and therefore the pressing unit 51 remain in the position reached. In fact, even though the action of the current ceases, the action of the attraction generated by the south polarity of the permanent magnet 53 towards the cylindrical protrusion 600 belonging to the metal core of the electromagnet 60 continues.

It is also clear that this contact of the pressing unit 51 with the surface 21 of the piece to be processed 2 is reached even if the thickness of the piece 2 varies to a certain extent, for example of 1-2 mm. Therefore, if the. thickness is reduced, the rod 52 continues its downward stroke and for a given length there is no contact between the pressing unit 51 and the surface 21. This takes place while the mutual attraction force between the permanent magnet 53 and the metal mass 600 increases, since the distance between the magnetic mass 600 and the permanent magnet decreases.

This is particularly evident when observing the diagram of Fig. 9, in which the x-axis corresponds to the attraction force exerted by the permanent magnet expressed in kg and the y-axis corresponds to the shifts of the rod with respect to the upper mechanical retainer.

As shown in this Figure, with current 1=0 in the lower coil, that is, after the coil has been transiently excited, the pressing unit 51 moves downwards, attracted by a force that with a shift of 11 mm with respect to the upper mechanical retainer is equal to 0.400 kg, while if the piece is thinner and therefore the lower stroke increases, reaching for example 12 mm, the attraction force of the permanent magnet 53 on the metal mass 600 is equal to 0.500 kg.

It can thus be observed that a variable force for the deformation of the tape is available, with no need to activate the current circulation in the coil of the electromagnet 60.

It is therefore possible to carry out a system with automatic adjustment that can exert a deforming force on the tape as required, with no need to use complex control and piloting devices for this purpose.

As soon as the actual sanding operation begins, the pressing unit 51 obviously cannot simply rest on the piece, but must exert a pressure that is proportional to the degree of sanding to be obtained. It is at this point that a current depending on the degree of sanding to be obtained is fed to both the electromagnet 60 and the electromagnet 61, obviously in opposite directions, so that both electromagnets produce an action owing to which the permanent magnets 53 and 54 are pushed downwards.

In this way, as can be observed in Fig. 8, the block of non-magnetic material 56 approaches and almost touches the surface 601 of the cylindrical protrusion 600. It is evident, in fact, that the action of the two electromagnets is such as to increase the pressure force and add it to the force already produced by the

permanent magnets 53 and 54 with no current circulating in the coils.

In Fig. 9 it is possible to observe that the coils may be fed with a current amounting, for example, to 30%, 60% or 90% of the maximum rated current that can be supplied, and this can be made according to the degree of sanding to be obtained.

In the case of a current amounting to 30% of the rated current, it can be observed that with a rod stroke of 11 mm the electromagnet exerts a force equal to 1 kg on the moving core of the rod, said force being the sum of the first force exerted by the permanent magnet and of the second force due to the action of the active electromagnets 60 and 61 on the permanent magnet.

Since the difference in thickness is absorbed, as explained above, by the variable force exerted by the permanent magnets and not directly by the current circulating in the coils of the electromagnets, it can be easily understood that to obtain a uniform sanding result it is sufficient to feed all the coils excited by the sensors 6 in the same way, that is, with the same circulating current.

In practice, this can be obtained by simply feeding with the same voltage in parallel all the coils excited by the on/off signal, that is, all the coils involved in the processing of the piece.

This power supply system is obviously simple and does not require any special equipment, device or control device, which instead, as already pointed out, in the prior state of the art are rather complex and therefore very expensive.

At the end of the sanding process the control unit 7 sends to the coils of the electromagnets 60 and 61 an impulse current with opposite directions, so that the permanent magnets 53 and 54 are attracted upwards and assume the position of equilibrium shown in Fig. 6.

A variant application of the invention is illustrated in Figures 10,11 and 12.

According to this variant, the rod 52 is associated with three permanent magnets 81,82 and 83 that are substantially equidistant and separated by non-magnetic material constituted by a series of cylindrical pads 84 and 85, as well as by small terminal cylinders of the moving core indicated by 86 and 87, respectively, also made of non-magnetic material. The permanent magnets, together with the discs of non-magnetic material, constitute the moving core that slides on a cylindrical housing formed by the polar ends of two electromagnets indicated by 70 and 71, which in this case are cylindrical.

Said electromagnets are provided with coils 72 and 73, respectively, positioned inside the cylindrical metal core.

In rest conditions, the permanent magnets 82 and 83, which are arranged with opposite polarities, stabilize in the median plane of each one of the two polar expansions of the electromagnet 71, as shown in Fig. 10.

Owing to the attraction exerted by the soft iron core belonging to the electromagnet 71, the assembly constituted by the rod 52, the permanent magnets and the non-magnetic discs connected with it is balanced. Obviously, also in this case the lower part of the rod 52 is connected to the pressing unit 51, not shown.

To position the rod 52, and therefore also the pressing unit 51, so that it rests against the piece, also in this case the control unit 7 sends an impulse current to the coil 72, so that the two permanent magnets 82 and 83, as well as the permanent magnet 81, are attracted downwards and the rod 52 assumes the position illustrated in Fig. 11.

In this figure it can be observed that the non-magnetic material 86 has come much nearer to the end of stroke represented by the surface 701 of the cylindrical protrusion 700.

When the action of the current ends, the rod 52 remains in the position reached, since the attraction force of the permanent magnet 81 towards the cylindrical protrusion 700 is equal to the reaction force due to the deformation of the abrasive tape 4 that rests on the piece to be processed 2.

In Fig. 12 the power supply to the electromagnet 70 and therefore the passage of current in the coil 72 are such that the permanent magnet 81 and also the permanent magnet 82 are further attracted downwards and so the disc of non- magnetic material 86 almost reaches the end-of-stroke limit.

This obviously depends on the thickness of the piece.

Obviously, the amount of current carried by the coil 72 will depend on the type of sanding to be obtained.

It is clear that to obtain a deep sanding a high current must be used, so that the rod 52 is attracted downwards even more strongly and effectively.

At the end of the sanding process the control unit 7 sends to the coil 73 of the electromagnet 71 an impulse current with opposite direction, so that the whole rod is attracted upwards and reaches the position shown in Fig. 10.

The permanent magnets 82 and 83, once the current pulse has ended, remain in the position of equilibrium reached before.

The exhaustive description given above shows that the sanding method and the sanding machine object of the invention are extremely effective and that the sanding machine is simplified, especially in the electric parts controlling the electromagnets that in turn control the pressing units, since the aim has been achieved to supply the machine with a variable force that actually is supplied by the nature of the materials used, that is, a force due to the fact that the permanent magnets are attracted by the soft iron, said force being inversely proportional to the attraction distance between permanent magnet and metal mass.

This variable magnetic attraction force is the force that has been used to vary the force to be exerted on the pressing unit to make it rest against the material to be sanded, and this also in case of significant thickness differences.

Obviously, the shape and arrangement of the permanent magnets associated with the rod and the pressing element may vary, as well as their size, and also the combination and alternation of permanent magnet and non-magnetic material may vary, and in any case all these variants will be protected by the invention.

Therefore, with reference to the above description and the attached drawings, the following claims are put forth.