Description

Technical Field

The present invention relates to machine tools or machining centers for machining workpieces, for instance workpieces made of wood, plastic, light metals or the like. Embodiments of the invention relate to machining centers or machine tools designed especially to machine elongated workpieces, such as components of chairs, tables or other pieces of furniture.

State of the Art

To machine through chip removal workpieces of different type, for instance and especially elongated workpieces made of wood or plastic, such as components of pieces of furniture, windows and doors and the like, numerically controlled machine tools or machining centers are generally used, that comprise a structure bearing an operating head with one or more numerically controlled axes and one or more electro-spindles with respective tools. The operating head cooperates with one or more workpiece-holding slides or tables, typically moving between a position where workpieces to be machined, so-called blanks, are loaded and a position where the machined workpieces are unloaded, which is at a safety distance with respect to the working area where the operating head moves.

Several different manual and automatic systems have been developed for loading and unloading the workpieces and for blocking them on the workpiece- holding slide to machine them.

Figures 1A, IB and 1C show a plan view, a front view according to B-B, and a side view according to C-C, respectively, respectively of a prior art machining center. In this case, the machining center, labeled as a whole with number 100, comprises a gantry structure 101, onto which an operating head 103 is arranged. The operating head 103 is carried by a slide 105 movable along a crossbeam 10 IT of the gantry structure 101, which also comprises two columns 101M supporting the crossbeam 101T.

The operating head 103 comprises a plurality of electro-spindles and is provided with a movement according to a numerically controlled axis Y of horizontal translation and a numerically controlled axis Z of vertical translation. The operating head is also provided with a numerically controlled movement according to rotary axes A and C (figure 1C). Associated with the gantry structure 101 is a base 105, along which a workpiece-holding slide 107 moves. The workpiece-holding slide is provided with one or more blocking members 109, each of which is advantageously provided with jaws to clamp a workpiece to be machined.

The base extends horizontally through the gantry structure 101 and is sufficiently long to allow the workpiece-holding slide 107 to translate from a loading position to an unloading position, these two positions being arranged on opposite parts or sides of the gantry structure 101. In figures 1A and IB both the positions are indicated with a continuous line and indicated with 107C (loading position) and 107S (unloading position). The workpiece-holding slide or table 107 and the base 105 are associated with a conveyor or conveyor belt 111. The conveyor belt or conveyor 111 extends horizontally from the loading position 107C towards an area where the workpieces PI to be machined are positioned. The area where the workpieces to be machined PI are positioned is indicated as a whole with number 113. The machined workpieces, indicated with P2, are unloaded in a gathering area 1 15 arranged, with respect to the structure 101, at the opposite side with respect to the rest area 113 of the workpieces to be machined (blank) PI .

The machining center schematically illustrated in figures 1A-1C and briefly described above works as explained below. The workpieces to be machined PI are loaded, generally by means of an automatic system, translating them according to arrow fPl on the conveyor belt or conveyor 111. This latter transfers the workpieces to be machined PI towards the loading position 107C where the workpiece-holding slide 107 is temporarily positioned. An insertion system, not shown, transfers the workpieces to be machined PI according to the arrow fl to insert them one by one into the blocking members 109 of the workpiece-holding slide 107 when this latter is in position 107C. The single workpieces to be machined PI, blocked by means of the blocking members 109, are transferred, with a motion according to arrow X along the base 105 of the workpiece-holding slide 107, to the working area of the operating head 103. The workpieces are machined by means of a coordinated movement of the tools according to the numerically controlled axes X, Y, Z, A, and C, until machining is completed. Once the working cycle has been completed, the workpiece-holding slide 107 is moved with a motion according to the axis X along the base 105 up to the unloading area 115, where the machined workpieces P2 are unloaded manually or automatically by means of unloading members, not shown, of the workpiece-holding slide 107.

Reference 117 indicates a preferably adjustable stop for defining the position that the workpieces to be machined PI shall achieve to arrive in front of the workpiece-holding slide 107 and be inserted in the blocking members 109.

The machine or machining center of figures 1A, IB, 1C has great limits as regards the times necessary for loading and unloading the workpieces. Indeed, once the machined workpiece P2 has been completed, the workpiece-holding table or slide 107 shall translate from the working area of the operating head 103 up to the unloading area 115, the blocking members 109 shall be opened and the machined workpiece P2 shall be unloaded. Once the workpiece-holding slide 107 is free, it shall translated along the base 105 in opposite direction up to the position 107C and the new workpiece to be machined PI shall be translated according to arrow fl from the conveyor 111 up to the workpiece-holding slide 107, blocked by means of the blocking members 109 of the workpiece-holding slide 107 and transferred form the loading area 107C to the working area of the operating head 103. These operations require relatively long times and affect the productivity of the machining center 100.

It is therefore necessary to improve the machining centers or machines to decrease the time required to load and/or unload the workpieces and thus increase the productivity of the machining center.

Summary of the Invention

According to one aspect, the invention provides a machining center for machining elongated pieces, comprising at least one operating head movable according to a plurality of numerically controlled axes and at least one workpiece- holding slide movable between a working area and an area where workpieces to be machined are loaded on the workpiece-holding slide and machined workpieces are unloaded from the workpiece-holding slide, and vice versa. At least one blocking member is provided on the workpiece-holding slide and carried thereby, to block at least one workpiece on the workpiece-holding slide. An insertion system for inserting the workpieces on the blocking member is advantageously provided to load the workpieces to be machined on the workpiece-holding slide. The insertion system for inserting the workpieces to be machined may be advantageously associated with a loading conveyor of the workpieces to be machined extending from a workpiece receiving area towards the area of loading the workpieces to be machined on the workpiece-holding slide and unloading the machined workpieces from the workpiece-holding slide. In the area of loading the workpieces to be machined on the workpiece-holding slide and unloading the machined workpieces from the workpiece-holding slide, the workpieces to be machined are transferred from the loading conveyor to the workpiece-holding slide. A removal system is also advantageously provided for removing the machined pieces from the workpiece- holding slide, especially from the blocking member arranged on said workpiece- holding slide. The removal system can be associated with an unloading conveyor that, in advantageous embodiments, extends from the area where the workpieces to be machined are loaded on the workpiece-holding slide and the machined workpieces are unloaded from the workpiece-holding slide towards a gathering area of the machined workpieces. The machined workpieces are transferred from the workpiece- holding slide to the unloading conveyor in the area where the workpieces to be machined are loaded on the workpiece-holding slide and the machined workpieces are unloaded from the workpiece-holding slide. The insertion system of the workpieces to be machined and the removal system of the machined workpieces are arranged at two different distinct heights. Moreover, advantageously the loading conveyor and the unloading conveyor overlap at least partially, at least in the area where workpieces to be machined are loaded on the workpiece-holding slide and machined workpieces are unloaded from the workpiece-holding slide. In this way it is possible to load a workpiece to be machined by transferring it from the loading conveyor to the workpiece-holding slide while a machined workpiece, just removed from the workpiece-holding slide, is transferred from the unloading conveyor towards the gathering area of the machined workpieces. In this way the unloading and loading operations occur at least partially contemporaneously, thus increasing the productivity of the machining center. According to some embodiments, the blocking member advantageously comprises at least one lower clamping element and at least one upper clamping element, movable with respect to each other to block a workpiece there between. The workpiece-holding slide preferably comprises at least two pairs of upper clamping element and lower clamping elements to block elongated workpieces. Advantageously, the distance between the two pair of clamping elements can be adjusted, for instance by providing adjustment guides on the workpiece- holding slide.

Both the lower clamping element and the upper clamping element may be advantageously movable in a vertical direction with respect to the workpiece-holding slide. As it will be better explained below with reference to one embodiment, this allows to handle workpieces at different (loading and unloading) heights and, at the same time, to define the position of the workpiece to be machined with respect to the operating head in a precise and easily repeatable way. In fact, the lower clamping element can be used as a reference stop or reference surface for the workpieces. The upper clamping element forms a closing jaw, whose stroke varies according to the vertical dimension of the workpiece to be machined. The upper element may be coated with an elastically yielding material to protect the workpiece, avoiding damages thereof during machining.

In advantageous embodiments the lower clamping element is vertically movable to be placed alternatively at the height of the insertion system of the workpieces to be machined and at the height of the removal system of the machined workpieces. The insertion system of the workpieces to be machined is preferably at a greater height than the height of the removal system of the machined workpieces.

The positions of the lower clamping elements with respect to the workpiece- holding slide can be advantageously adjusted for each lower clamping element independently of the others. Similarly, also the positions of the upper clamping elements with respect to the workpiece-holding slide can be advantageously adjusted for each upper clamping element independently of the others. In some embodiments, pairs of respectively upper and lower clamping elements, are carried by a single support that can be adjusted horizontally along the workpiece-holding slide. The reciprocal distance between at least two pairs of clamping elements can be adjusted, preferably in a direction orthogonal to the movement direction of the workpiece- holding slide, so as to adapt to the longitudinal dimension of the workpieces to be machined, that may be elongated workpieces directed orthogonally to the movement direction of the workpiece-holding slides.

Further advantageous features and embodiments are described below and in the attached claims, forming an integral part of the present description.

Brief description of the drawings

The invention shall be better understood by following the description and accompanying drawing, which shows non-limiting practical embodiments of the invention. More in particular in the drawing, wherein equal or equivalent parts are indicated with the same reference numbers:

figures 1A, IB, and 1C, already described above, show a prior art machining center;

figure 2 is a plan view of a machining center or machine tool in one embodiment of the invention;

figure 3 is a view according to III-III of figure 2;

figure 4 shows a view analogous to that of figure 3 with the workpiece- holding slide in a different position;

figure 5 is a view according to V-V of figure 3;

figure 6 is a plan view of a machining center or machine tool according to the invention in a further embodiment;

figure 7 is a view according to VII-VII of figure 6;

figure 8 shows a view similar to that of figure 7 with the workpiece-holding slide in a different position;

figure 9 is a view according to IX-IX of figure 7;

figure 10 is a view similar to the plan view of figure 6, in a different operating mode of the machining center.

Detailed description of embodiments of the invention

The detailed description below of example embodiments is made with reference to the attached drawings. The same reference numbers in different drawings identify the equal or similar elements. Furthermore, the drawings are not necessarily to scale. The detailed description below does not limit the invention. The scope of the present invention is defined by the attached claims.

In the description, the reference to "one embodiment" or "the embodiment" or "some embodiments" means that a particular feature, structure or element described with reference to an embodiment is comprised in at least one embodiment of the described object. The sentences "in one embodiment" or "in the embodiment" or "in some embodiments" in the description do not therefore necessarily refer to the same embodiment or embodiments. The particular features, structures or elements can be furthermore combined in any adequate way in one or more embodiments. Figures 2 to 5 show a first embodiment of a machining center or machine tool, indicated as a whole with number 1, for machining workpieces made of wood, aluminum, plastic or the like. The machining center is preferably designed to machine elongated workpieces, such as components of pieces of furniture, chairs or the like in a particularly efficient way.

The machining center or machine tool 1 may comprise a support structure 3 bearing an operating head 5. The support structure 3 can have any design; it can be for instance a gantry structure as shown in figures 1A, IB, and 1C. As illustrated in the drawing, in other embodiments the bearing structure 3 can be a column structure, advantageously with movable column. The structure 3 may have for instance a column 4 carried by a base 6 onto which guides 7 are provided, along which the column 4 moves according to a numerically controlled axis of translation X. The movement according to X (as well as the other numerically controlled translation movements) may be imparted by means of an electronically controlled electric motor and a threaded bar, or by means of any other suitable system or mechanism.

In some embodiments the column 4 carries the operating head 5 so that this latter can move according to a vertically oriented numerically controlled axis of translation Z. The column 4 may carry for instance guides 9 along which a slide 11 carrying the operating head 5 translates.

Moreover, the operating head 5 may be provided with a numerically controlled movement according to a first rotary axis C, for instance vertical, and according to a further rotary axis A, for instance horizontal. The operating head 5 may carry a plurality of electro-spindles 5A. Each electro-spindle 5A may be provided with a respective tool U. In the illustrated example, the operating head 5 is a crosswise head with four electro-spindles carrying four tools. In other embodiments the operating head 5 may be provided with a different numbers of electro-spindles, and thus of tools. A different number of numerically controlled axes and/or a different spatial arrangement of said axes may also be provided. .

In some embodiments the machining center 1 comprises a further base 13, onto which guides 15 for a workpiece-holding slide or table 17 are arranged. In some embodiments the base 13 is separated from the base 6, and the two bases are individually applied to the floor, as illustrated in the exemplary embodiment of figures 2 to 5. In other embodiments the base 13 and the base 6 may be directly connected to each other to form a single monolithic structure.

The workpiece-holding slide 17 is movable along the base 13, for instance through guides 15, according to a numerically controlled axis of translation Y.

In some embodiments the workpiece-holding slide 17 has a blocking member or a plurality of blocking members indicated as a whole with number 19. Herein, reference will be made to a single blocking member, being understood that this term can indicate a set of blocking members and being also understood that in some embodiments more blocking members can be provided on the workpiece-holding slide 17.

The blocking member indicated as a whole with number 19 may be for instance formed by more blocking systems 19, each of which may have an upper clamping element 21 and a lower clamping element 23. Advantageously, each pair of upper clamping element and lower clamping element 21, 23 may be carried by a support 25, which can be advantageously adjusted in position along the workpiece- holding slide 17. To this end the workpiece-holding slide 17 may be for instance provided with adjustment guides 27. Advantageously, the adjusting movement may be orthogonal to the numerically controlled axis Y.

The movement of the single supports 25 may allow adjusting the position and the mutual distance of the pairs of clamping elements 21, 23 so as to adapt the configuration of the blocking member indicated as a whole with number 19 to the longitudinal dimension of the workpieces to be machined.

In some embodiments both the upper clamping element 21 and the lower clamping element 24 are movable vertically to block the workpieces to be machined and to perform the loading and unloading operations, as better described below.

The workpiece-holding slide 17 can move according to the direction defined by the numerically controlled axis of translation Y, both to perform relative motions between the workpiece and the tools U carried by the operating head 5, and to move the workpieces between a loading and unloading area and the working area of the operating head 5.

In greater detail, in figure 3 a broken line indicates a possible position taken by the workpiece-holding slide 17 during the machining of a workpiece, generically indicated with P, blocked by means of the blocking member 19 on the workpiece- holding slide 17. A solid line indicates the position of the workpiece-holding slide 17 in an area where the workpieces to be machined are loaded on the workpiece-holding slide 17 and the machined workpiece are unloaded from the workpiece-holding slide. In figures 2 to 5 this loading and unloading area is indicated as a whole with number 29.

In the loading and unloading area 29 two conveyors converge, for instance in the form of conveyor belts, roller conveyors or the like. The conveyors are schematically indicated with 31 and 33 respectively. The conveyor 31 is a loading conveyor of the workpieces to be machined (blanks) that are indicated with PI. The conveyor 33 is an unloading conveyor of the machined workpieces that are indicated with P2.

As shown in particular in figures 3, 4, and 5 the conveyors 31 and 33 are arranged at different heights. More in particular, in the illustrated embodiment the loading conveyor 31 is at a height HI with respect to a floor Pv, while the unloading conveyor 33 is at a height H2 with respect to the floor Pv, H2 being lower than HI . The opposite arrangement is also possible, wherein the loading conveyor is at a lower height with respect to the unloading conveyor. In the illustrated example the conveyors 31 and 33 are arranged substantially horizontally. In other, embodiments the conveyors 31 and 33 may be designed so as to have ascents and descents. The entrance of the conveyor 31 may be for instance at the same height as the exit of the conveyor 33. In this case the conveyor 31 may have an ascending segment or portion and the conveyor 33 may have a descending segment or portion. However, in any case there will be an area where the two conveyors 31 and 33 are arranged at different heights, substantially overlapping each other. In this area the conveyors have preferably a rectilinear extension, due to reasons that will be clear from the description below of the operation of the machining center.

Basically, the loading conveyor and the unloading conveyor 31, 33 overlap at least partially, at least in the area 29 where workpieces to be machined are loaded on the workpiece-holding slide and machined workpieces are unloaded from the workpiece-holding slide so as to transfer single workpieces to be machined PI in front of the workpiece-holding slide 17 and to pick up machined workpieces from the workpiece-holding slide 17 to transfer them towards a removal area. An insertion system is associated with the loading conveyor 31 for inserting the workpieces PI to be machined in or on the blocking member 19 carried by the workpiece-holding slide 17. In the figures, the insertion system for inserting the workpieces PI to be machined is schematically represented as cylinder-piston actuators 35 carried by a fixed structure 37, indicated only schematically. The cylinder-piston actuators 35 may be fixed to a movable crossbeam 36 pushing the workpieces PI to be machined removing them from the loading conveyor 31. The actuators 35 transfer the workpieces PI to be machined, carried by the conveyor 31 , from this latter onto the workpiece-holding slide 17, and especially inside the blocking member 1 between pairs of clamping elements 21, 23 transferring them with a movement according to the arrow f35 (see in particular figure 2).

A removal system, schematically labeled 37 in figures 3 and 4, is associated with the unloading conveyor 33. In this exemplary embodiment the removal system 37 is below the insertion system 35, accordingly to the overlapped arrangement of the conveyors 31 and 33.

Advantageously, associated with the loading conveyor there may be a stop 39 (figures 2 and 5), which may be movable and/or adjustable, to define the stop point of each workpiece to be machined PI fed by the loading conveyor 31 towards the loading and unloading area 29.

In some embodiments, the loading conveyor 31 extends from a receiving area

41 of the workpieces PI to be machined towards the area 29 where workpieces to be machined are loaded and machined workpieces are unloaded. The unloading conveyor 33 extends from the area 29 where workpieces to be machined are loaded and machined workpieces are unloaded towards a gathering area, schematically indicated with 43, of the machined workpieces P2.

In some embodiments, in the receiving area 41 of the workpieces PI to be machined and in the gathering area 43 of the machined workpieces P2, systems may be provided for mechanically moving the workpieces. In the receiving area 41 of the workpieces to be machined systems may be provided for translating single workpieces PI to be machined, these systems feeding single workpieces PI to be machined onto the loading conveyor 31.. To this end flexible elements 45 may be for instance provided, such as chains or the like, with pushers 47 acting on the single workpieces PI to be machined, moving them forwards according to the arrow f47 (figure 2) towards the loading conveyor 31.

In some embodiments, in the gathering area 43 of the machined workpieces removal systems may be provided for removing the machined workpieces P2 and transferring them for instance onto a rest surface 43 A. In the embodiment schematically illustrated in figure 2, thrust actuators 49 are for instance provided, pushing single machined workpieces P2 outside the conveyor 33 on the surface 43 A. Moving systems different than those schematically indicated may be also provided to move the workpieces PI to be machined onto the loading conveyor 31 and the machined workpieces P2 from the unloading conveyor 33.

In some embodiments the workpieces PI to be machined, or blanks, may be loaded on the conveyor 31 manually. In some embodiments the machined workpieces P2 may be unloaded manually.

Thanks to the machining center briefly described above with reference to figures 2 to 5 it is possible to perform loading, machining, and unloading cycles as described below. In figures 3 a broken line illustrates a position where the workpiece- holding slide 17 is arranged during a machining cycle. The workpiece P blocked by means of the lower clamping elements 21 and the lower clamping elements 23 may be approached by the tools U of the operating head 5. The numerically controlled axes X, Y, Z, A, C allow the relative movements between tools and workpiece to machine it. As the axis Y is numerically controlled, the position of the workpiece- holding slide 17 indicated with a broken line in figure 3 is neither fixed nor unique, as the workpiece-holding slide 17 moves along the axis Y in a coordinated way with the movements of the tools U along the numerically controlled axes X, Z, A, C to machine a workpiece according to a generic working cycle.

Once the workpiece P (P2) has been completely machined, the workpiece- holding slide can translate according to the numerically controlled axis Y along the base 13 to move towards the loading and unloading area 29. A solid line shows in figure 3 the position of the workpiece-holding slide 17 in the loading and unloading area 29 taken during the initial step of the unloading cycle of the machined workpiece P2.

To unload the machined workpiece P2, the lower clamping element 23 moves downwards. In advantageous embodiments the lower clamping element 23 may move to the minimum achievable height, for instance defined by a stop of a cylinder-piston actuator carried by the respective support 25, whose piston rods are labeled 26. When the lower clamping element 23 is in the lower position, it is substantially at the same height as the upper branch of the conveyor 33. The removal system 37 for removing the machined workpieces P2 may transfer the machined workpiece P2 from the blocking member 19 towards the conveyor 33. To this end it is possible to provide that, with respect to the position indicated in figure 3 with a solid line, the workpiece-holding slide 17 moves further forwards towards the unloading conveyor, until achieving the position illustrated in figure 4.

It should be understood that the removal system of the machined workpieces may be also carried by the workpiece-holding slide 17, instead of being arranged in a fixed position in the loading and unloading area 29. A movable stop may be for instance provided, carried by the workpiece-holding slide 17 that makes the machined workpiece P2 move shortly away from the blocking member 19. On the workpiece-holding slide 17 a stop, not shown, may be for instance provided, movable in horizontal direction and designed analogously to what described in DE102011084540.2.

Once the machined workpiece P2 has been transferred onto the unloading conveyor 33 as shown in figure 4, the lower clamping element 23 may be raised up to nearly the height of the upper branch of the loading conveyor 31. In the meanwhile, on the conveyor 31 a new workpiece PI to be machined has been placed, that has been moved up to the stop 39 (figures 2 and 5). The insertion system comprising the actuators 35 and the crossbeam 36 pushes the new workpiece PI to be machined transversally to the longitudinal direction of the loading conveyor 31 , to transfer it on the lower clamping elements 23.

In this step, the upper clamping elements 21 may be raised with respect to a final clamping position. The upper position of the lower clamping element 23 during this step may be the maximum rise position, defined by the stop of the cylinder- piston actuator (or other adequate actuator) whose piston rods 26 are shown. The position of the lower clamping elements 23 can be therefore repeated precisely at every machining cycle. Therefore, the lower clamping elements 23 may advantageously constitute reference stops of the workpiece to be machined PI . In other words, the reference height at which each workpiece PI to be machined is positioned to perform the subsequent workings, and at which these workings refer, is the position defined by the lower clamping elements 23.

The workpiece PI to be machined may be completely clamped by lowering the upper clamping element 21 of each pair 19 A, so that the workpiece PI to be machined is firmly blocked on the workpiece-holding slide 17. Now, this latter can translate according to the axis Y to bring the workpiece PI to be machined in the working area of the operating head 5.

Whilst the new workpiece PI to be machined is loaded on the workpiece- holding slide 17 as described above, the machined workpiece P2 transferred from the workpiece-holding slide 17 to the lower unloading conveyor 33 may be conveyed by means of the lower unloading conveyor 33 towards the gathering area 43 of the machined workpieces P2 where, automatically or manually, the single workpieces P2 may be collected for instance on the collection table 43A or in any other adequate manner.

It is clearly apparent from the description above that thanks to the new arrangement of the loading and unloading systems and of the corresponding conveyors, the unload of the machined workpieces from the workpiece-holding slide 17 and the load of the new workpieces to be machined on the workpiece-holding slide 17 are accelerated and performed at least partially in a timely overlapping manner. The machined workpiece P2 shall be simply translated for a short stroke to free the lower clamping elements 23 without the need of transferring it by means of the workpiece-holding slide 17 towards the gathering area 43 of the machined workpieces. Once this short operation of transferring the machined workpiece P2 on the unloading conveyor 33 has been performed, the new workpiece to be machined PI can be loaded on the workpiece-holding slide 17 substantially timely overlapping the operation of moving the machined workpiece P2 away and the operation of loading the new workpiece to be machined PL

The use of movable lower clamping elements 23 and movable upper clamping elements 21 on the one hand allows unloading and loading the workpieces by means of the lower clamping elements 23 at the two different heights at which the loading conveyor 31 and the unloading conveyor 33 are arranged. On the other hand, it is also possible to define a reference position of the workpiece PI to be machined that is fixed and repeatable, independently of the vertical dimension of the workpiece PI to be machined. The mobility of the upper clamping element 21 allows the blocking member 19 immediately to adapt to workpieces PI to be machined of any height within the operating range of the machine practically given by the stroke of the upper clamping element 21 in vertical direction.

As the reference stop in vertical direction of the workpieces PI to be machined is defined by the lower clamping elements 23, the upper clamping elements 21 may have a yielding coating, to avoid damages to the workpieces clamped by means of these elements. Deformation of the coating of the upper clamping elements 21 does not alter the position of the workpiece PI, which is anyway always referred to a stop formed by the lower clamping elements 23, which at every working cycle are always in the same, easily repeatable position. The lower clamping elements 23 are substantially rigid, and therefore the compression force acting on the clamped workpieces does not alter the lower reference surface.

As the position of the lower clamping elements 23 may be fixed and unchangeable with respect to the vertical thickness of the workpiece PI to be machined thanks to the mobility of the upper clamping elements 21, it is possible to move the lower clamping elements or jaws 23 between a position of maximum fall and a position of maximum rise, these positions being fixed and independent of the shape and vertical dimension of the workpiece to be machined PI. These positions are determined according to the height HI, H2 of the loading conveyor 31 and unloading conveyor 33, respectively.

The machining center 1 of figures 2 to 5 comprises only one single workpiece-holding slide 17 and one operating head 5. In other embodiments more than one operating head 5 may be provided to perform more complex working cycles in shorter times on single workpieces PI to be machined, making the two or more operating heads 5 cooperate with the single workpiece-holding slide 17.

In other embodiments more than one workpiece-holding slide 17 may be provided, cooperating with one or more operating heads 5. Figures 6 to 9 show for instance a machining center with two workpiece-holding slides 17 to machine two workpieces at the same time. In the illustrated embodiment two operating heads 5 are also provided, each of which working with one of the two workpiece-holding slides 17 so as to machine in parallel. The working cycles may be equal to or different from one another. In figures 6 to 9, the same reference numbers indicate the same or corresponding parts to those illustrated in figures 2 to 5. Equal or corresponding parts of the two workpiece-holding slides, of the two operating heads and the corresponding bearing structures, as well as of the insertion system and of the removal system of the workpieces with respect to the workpiece-holding slides are indicated with the same reference numbers for the two workpiece-holding slides.

The embodiment of the machining center illustrated in figures 6 to 9 therefore provides for an area 29 for loading the workpieces PI to be machined and unloading the machined workpieces P2, extending for an extension comprising a double arrangement of bases 13 and corresponding workpiece-holding slides 17.

The loading conveyor 31 and unloading conveyor 33 are still at different height, indicated again with HI and H2, and overlap each other in the loading and unloading area 29. As in this area there are two bases 13 and two workpiece-holding slides 17, the length of the segment where the loading conveyor 31 and the unloading conveyor 33 overlap each other is greater than in the embodiment of figures 2 to 6, as it is particularly visible by comparing figures 5 and 9.

Along the loading conveyor 31 two stops are advantageously provided, indicated with 39A and 39B, instead of a single stop 39 as shown in the embodiment of figures 2 to 6.

The stops 39A and 39B define the position where the workpieces to be machined PI shall stop to be transferred from the loading conveyor 31 to the respective workpiece-holding slides 17. To allow workpieces PI to be machined to move from the receiving area 41 to the loading and unloading area 29 in correspondence of the base 13 farther from the receiving area 41 of the workpieces PI to be machined, the stop 39A is movable according to the double arrow f39 (figure 6). The stop 3 B downstream with respect to the forward direction of the workpieces to be machined PI may be fixed.

In general, also more than two bases 13 and respective workpiece-holding slides 17 may be provided, with an equivalent number of stops 39 distributed along the extension of the loading conveyor 33; all these stops, excluding, if necessary, the last one, shall be movable to allow the workpieces PI to be machined to move towards the position farthest from the receiving area 41. Each section of the machining center 1 illustrated in figures 6 to 9 where there is one of the workpiece-holding slides 17 operates as described above with reference to the embodiments of figures 2 to 5. Therefore, the operation of the machining center 1 will not be described again. Figures 7 and 8 show the sequence of movements of the blocking member 19 and especially of the upper clamping elements 21 and the lower clamping elements 23, as well as of the insertion systems for inserting the workpieces to be machined and the removal systems for removing the machined workpieces to unload the machined workpiece P2 from one of the two workpiece-holding slide 17 and load the workpiece PI to be machined on one of the two workpiece-holding slides 17.

The machining center illustrated in figures 6 to 9 may operate in swinging cycle and/or using the two operating heads 5 to simultaneously machine a workpiece PI carried by one of the two workpiece-holding slides. This operating mode is schematically represented in the plan view of figure 10.

The embodiments described above and illustrated in the drawings have been explained in detail as examples of embodiment of the invention. It will be clearly apparent to those skilled in the art that modifications, variants, additions and omissions are possible, without however departing from the principles, the scope of the concept and the teachings of the present invention as defined in the attached claims. The scope of the invention shall be therefore determined exclusively based upon the widest interpretation of the attached claims, wherein these modifications, variants, additions and omissions are included within this scope. The terms "comprising" "to comprise" and the like do not exclude the presence of further elements or steps in addition to those specifically listed in a claim. The term "a" or "an" before an element, means or feature of a claim does not exclude the presence of a plurality of these elements, means or features. If a claim of a device claims a plurality of "means", some or all these "means" can be embodied by a single component, member or structure. The enunciation of given elements, features or means in distinct depending claims does not exclude the possibility of combining said elements, features or means together. Any reference numerals in the appended claims are provided to facilitate reading of the claims with reference to the description and to the drawing, and do not limit the scope of protection represented by the claims.