FIELD OF THE INVENTION
The present invention relates to a bending press or “press brake” having tables with controlled deformation.
BACKGROUND OF THE INVENTION
Bending presses are machine tools of a type that is itself well known. As shown in accompanying FIG. 1, the machine tool comprises a lower table 12 and an upper table 14 that is movable relative to the lower table 12. Usually, the lower table 12 is stationary and the upper table 14 is suitable for being moved towards the lower table 12 under drive from actuators V1 and V2 that act on the ends 14a and 14b of the upper table 14. Usually, the lower table 12 has its free edge 12a fitted with fastener means 16 for fastening bending matrices 18. In the same way, the edge 14c of the upper table 14 is fitted with fastener means 20 for fastening bending punches 22.
A metal sheet or lamination F is placed on the bending matrices 18 of the lower table 12. The sheet F may be of a length that varies widely depending on the circumstances. Under drive from the pistons of the actuators V1 and V2, the punches 22 mounted on the upper table 14 move towards the sheet placed on the matrices of the lower table. As soon as the punch 22 comes into contact with the sheet F, force begins to increase within the sheet as the punch penetrates therein, initially in the elastic range and subsequently in the plastic range, thereby enabling the sheet to be bent permanently.
Because the force is applied to the upper table by the actuators V1 and V2 acting on the ends of the table, the linear load distributed between the two ends of the tables corresponds to the upper table being deformed along a line in the form of a concave arc with deformation maximas close to the midplane of the table. This means that, for bending purposes, at the end of bending, the central portions of the punches have penetrated into the sheet less than have the end portions. If bending were to be performed on a matrix that, itself, were to remain perfectly straight during bending, then the result would be that a workpiece would be obtained having a bend angle that was wider in its central portion than at its ends. Such a result is naturally unacceptable.
In order to remedy that drawback, various solutions have been proposed for the purpose of controlling these deformations at the edges of the tables by using various means in order to obtain a bend that is substantially identical over the entire length of the bent workpiece.
Conventionally, these solutions involve providing slots, such as the slots 24 and 26 shown in FIG. 1, that are formed in the lower table 12 symmetrically about the midplane P′P of the press. These slots 24, 26 then define a central zone 28 of the lower table 12 that is slot-free and that presents a length b, each of the two slots 24 and 26 being of length a.
With slots 24 and 26 of conventional type, i.e. that leave between them a slot-free portion 28 of length b, substantially parallel deformations are obtained for the edges of the upper and lower tables 14 and 12. This ensures that proper bending is achieved. Nevertheless, this result is obtained only when the metal lamination or sheet F for bending has a length that is substantially equal to the total length of the lower or upper tables 12 or 14. In contrast, when the length of the sheet F is less than the total length of the lower or upper table 12 or 14, both of the deformations of the lower and upper tables 12 and 14 are concave.
In addition to the difficulty of proposing a bending press that is suitable for enabling the metal lamination or sheet F for bending to be deformed substantially uniformly over the entire length of said lamination or sheet F, regardless of whether its length is short compared with the length of the tables 12, 14 of the press or, on the contrary, is equal to the length of the tables 12, 14 of the press, there exists an additional difficulty related to the top edges 24″, 26″ of the slots 24, 26 deforming while the bending force of the moving table 14 is being applied to the stationary table 12, and said force is being taken up on the bottom edges 24′, 26′ of the slots 24, 26.
OBJECT AND SUMMARY OF THE INVENTION
An object of the present invention is to remedy these two problems by proposing to dispose at least one stopper in each of the slots 24, 26, which stopper is made up of two elements or wedges that have first surfaces fastened to respective ones of the edges of the slots, and second surfaces adapted for localized mutual contact substantially in the centers of the wedges, in a manner such as to ensure excellent transmission of the bending force from the top edges of the slots to the bottom edges of the slots.
The invention thus provides a press brake for bending at least one metal sheet, said press brake comprising:
- an upper table having a bottom edge carrying first bending tools, and a lower table having a top edge carrying second bending tools, the two tables being movable relative to each other to exert a bending force on the sheet;
said press brake presenting a vertical midplane, one of said tables having, through its entire thickness, two slots disposed symmetrically about the midplane, each slot having a first edge and a second edge, and an open first end opening out in a side edge of the table, as well as a closed end;
wherein:
at least one stopper is disposed in each slot, each stopper comprising a first wedge having a first end secured to the first slot edge and a second end forming a first surface, and a second wedge having a first end secured to the second slot edge and a second end forming a first surface; and
the first surface of at least one of the first and second wedges has a central portion that is domed or protuberant relative to the other portions of said surface so that the contact between the first and the second wedges is established essentially over said central portion.
The expression “secured to the first/second edge” is used to mean that the wedge in question is connected to the first or second edge, it being understood that said wedge may be movable relative to said edge of the slot.
Other characteristics of the press brake of the invention are indicated below:
- advantageously, the first surface of the first wedge and the first surface of the second wedge both have respective central portions that are domed or protuberant relative to the other portions of said first surfaces;
- in an embodiment, the first surface of the first wedge and/or the first surface of the second wedge is a convex surface;
- in an embodiment of the invention, the first surface of one of the wedges presents a concave surface while the first surface of the other wedge presents a convex surface;
- in an embodiment of the invention, the first surface of at least the first wedge and/or of the second wedge is a spherical surface portion;
- advantageously, at least in the zone of the stoppers, the slots present constant height so that the first edge and the second edge are parallel, in the absence of bending force for bending the metal sheet F;
- advantageously, the first surfaces of the wedges are inclined relative to the parallel edges of the slots;
- preferably, the first surfaces of the wedges are inclined at a slope lying in the range 1% to 40% and preferably in the range 5% to 10%, relative to an axis or a plane that is parallel to the edges of the slots;
- in an embodiment of the invention, the first surface of at least the first wedge or of the second wedge presents a plurality of inclined plane peripheral portions connecting to the central portion;
- the central portion presents a height or protrusion lying in the range 0.05 millimeters (mm) to 0.25 mm relative to the other portions of the first surface for a wedge of length substantially equal to 80 mm;
- advantageously, the wedges are mounted on supports connected to respective ones of the edges of the slots; at least one of the supports is movable laterally, i.e. along an axis parallel to the parallel edges of the slot on which it is mounted;
- in a possibility offered by the invention, the two wedges are offset relative to each other laterally, i.e. along an axis parallel to the parallel edges of the slot;
- in a possibility offered by the invention, in the absence of bending force for bending the metal sheet F, the first and second wedges present clearance between them; and
- advantageously, the press brake of the invention has a plurality of stoppers disposed in respective ones of the slots symmetrically about the midplane P′P.
BRIEF DESCRIPTION OF THE DRAWINGS
Other characteristics and advantages of the invention appear more clearly on reading the following description of preferred embodiments of the invention given by way of non-limiting example. The description refers to the accompanying drawings, in which:
FIG. 1 shows a press brake having two slots situated on respective sides of the midplane P′P and extending from opposite sides;
FIG. 2 is a diagrammatic view showing an embodiment of a stopper that is made up of two wedges of the invention, one of the wedges being fastened to the top edge of a slot 24 or 26, and the other being fastened to the bottom edge of the slot 24 or 26;
FIG. 3 is a diagrammatic view of two stoppers provided with a motor-driven control system for controlling the value of the clearance associated with the stoppers;
FIGS. 4 to 7 are diagrammatic views of embodiments of a stopper of the invention, made up of two wedges that are initially in mutual contact;
FIG. 8 is a side view of a stopper wedge of the invention;
FIGS. 9 to 12 show various embodiments of the first surface or contact surface of a stopper wedge of the invention;
FIG. 13 diagrammatically shows the force fields or pressure fields passing through the wedges of a stopper of the invention, when a bending force is applied to the metal sheet F causing contact and a force F0 between the wedges;
FIG. 14 is a perspective view of a wedge whose first surface or contact surface is made up of three portions; and
FIG. 15 is a side view in section of two wedges, one below the other, that are identical to the wedge of FIG. 14.
MORE DETAILED DESCRIPTION
FIG. 2 is a section view of two wedges 28, 29 of a stopper 27 of the invention. The two wedges 28, 29 have respective contact first surfaces 28′, 29′ that, prior to application of a bending force F0, present clearance j between them, as also shown in FIGS. 5 and 15.
Each of the wedges 28, 29 is mounted on a respective support 40, 41 connected to a respective edge 24′, 26′ or 24″, 26″ of a respective one of the slots 24 and 26. The function of each wedge 28, 28′, 29, 29′/stopper 27 is to control the extent to which the edges 24′, 24″ and 26′, 26″ of each slot 24, 26 move towards each other when the bending force is applied. By controlling the extent to which the edges 24′, 24″ and 26′, 26″ of the slot 24 or 26 move towards each other, it is possible to control the deformation of the top edge 24″, 26″ of the slot 24, 26, and therefore the deformation of the top edge 12a of the lower table 12.
At least one of the supports 40 or 41, and optionally both of the supports 40 and 41 is/are mounted to move laterally, i.e. along an axis parallel to the parallel edges 24′, 26′ and 24″, 26″ of the slots 24, 26 on which it is mounted. In the example shown to illustrate the invention in FIGS. 2 and 3, only the supports 40 are suitable for being moved by means of the set of actuators 60, the supports 41 being fastened to the slot edges 24″, 26″ of the lower table 12. Said supports 40 are moved by means of a set of actuators 60, shown in FIG. 3, which set is connected via link arms 61 to the moving supports 10. The set of actuators 60 is controlled by a remote control unit (not shown in the accompanying figures). The motor-driven movement of the supports 40, and thus of the wedges 29, makes it possible to adjust the position of at least one of the wedges 29 relative to the wedge 28, with a view to applying the bending force F0. This adjustment defines the value of the clearance j, it being understood that the initial adjustment of the relative positions of the wedges 28, 29 (before the bending force F0) is applied may also not make provision for any clearance, so that the wedges 28, 29 are in abutment with each other. The clearance j between the wedges 28, 29 or the relative position of the wedges 28, 29 can be adjusted to within one hundredth of a millimeter by means of the set of actuators 60.
Naturally, it is possible to make provision for the set of actuators 60, or for a distinct drive mechanism, also to enable the supports 41 and thus the wedges 28 to move. By way of example, FIG. 8 shows a wedge 28, 29 in which tapped orifices 71, 72, 73 can be seen (the orifices 71, 72 are seen end-on, while the orifice 73, situated in an adjacent face, is shown in dashed lines in the thickness of the wedge), the orifices being designed to enable the wedge 28 or 29 to be fastened to a moving support 40 or to a stationary support 41 by conventional mechanical means, such as a screw or a threaded rod.
In order to allow only limited lateral movement, the supports 40 are provided with slots or holes 62 that extend linearly to define the axes along which the moving supports 40 move, inside which slots guide pins 63 are disposed that are adapted to fit said slots 62. The movement of the supports 40 and of the wedges 29 is ideally parallel to the edges 24′, 26′ of the slots 24, 26. It should be noted that, advantageously, the edges 24′, 24″, and 26′, 26″ of each of the slots 24, 26 are parallel, at least at the wedges 28, 29/stoppers 27.
The first surface 28′, 29′ of each of the wedges 28, 29 advantageously has an inclination relative to the axes or to the planes of the parallel edges 24′, 26′ and 24″, 26″ of the slots 24, 26. This inclination of the first surface 28′, 29′ of each of the wedges 28, 29 lies in the range 1% to 30%, as a function of the material forming the wedges 28, 29, or more exactly of the coefficient of friction of the materials used to constitute the contact surface 28′, 29′ of each of the wedges 28, 29. Thus, by way of example, it can be noted that the inclination of each of the wedges 28, 29 shown in FIGS. 2 and 3 lies in the range 2% to 10% while said inclination lies in the range 10% to 30% for the wedges 28, 29 shown, for example, in FIG. 6, 13, or 14. It should also be noted that the inclinations of the wedges 28, 29 coming into abutment with each other may be the same or slightly different.
In an essential aspect of the invention, at least one of the first surfaces, or contact surfaces, 28′, 29′ of the respective wedges 28, 29 is provided with a domed or protuberant central portion 30, 31 so that the contact between the first and the second wedges 28 and 29 is established essentially over said central portion 30 or 31.
This domed or protuberant central portion 30, 31 may be of various shapes and may be present on one of the two wedges 28 or 29 only, or on both of the wedges 28, 29. In addition, as explained below through the various embodiments, due to the shape of the domed or protuberant portion 30, 31 of each of the first surfaces 28′, 29′ of the wedges 28, 29, the contact between the wedges 28, 29 may consist of contact at a point or substantially at a point, of contact along a line, or of contact over an area.
In FIG. 4, each of the wedges 28 and 29 has a domed or protuberant central portion 30, 31, which portions form the contact zone of the two wedges 28, 29. In this example, the bottom wedge 29 is situated closer to the open end 26a of the slot 26 than is the top wedge 28 so that there is a slight lateral offset between the two wedges 28, 29. In this example, the first surface 28′, 29′ of each of the two wedges 28, 29 consists of a spherical surface, but the vertex S of the spherical surface of each of the two wedges 28, 29 is not situated exactly in the center of the first surface 28′ or 29′. That is why the two wedges 28, 29 are offset slightly laterally relative to each other so that the contact between the two wedges, initially and/or while the bending force F0 for bending the sheet F is being applied, makes contact possible at the central portions 30, 31. Naturally, this manner of arranging the wedges 28, 29 relative to each other is a function of the spherical surfaces of the two first surfaces 28′, 29′ of the wedges 28, 29, but it is also a function of the flexing of the top portion 12c of the lower table 12, and thus of the extent to which the top wedge 28 itself moves.
In general, it should be noted that, by convention, the vertex of the domed or protuberant central portion 30, 31 is considered relative to a plane P0 joining two opposite edges 80, 81 of the wedge 28 or 29, which plane P0 corresponds to the inclination of the wedge 28, 29. The vertex S is the point of the domed or protuberant central portion 30, 31 that is at the furthest distance (“protrusion”) from the plane P0. This plane P0 is shown in FIGS. 14 and 15 that show a last embodiment of the invention. As can be seen in FIG. 14, the plane P0 is the plane joining the two opposite edges 80, 81. The vertex S of the domed or protuberant central portion 30, 31 may be located at one end of said portion, as shown in FIG. 14. The maximum height of the vertex S is referenced h. In FIG. 15, it can be noted that substantially the entire domed or protuberant central portion 30, 31 is at the height h relative to the plane P0. It can also be understood that, due to the inclination of the first surfaces 28′, 29′ of the wedges 28, 29, the vertex S of the protuberant portion 30, 31 does not necessarily coincide with the point of the first surface 28′, 29′ that is furthest away from the slot edge 26′, 26″ to which the wedge 28, 29 is fastened.
FIG. 5 shows a variant of FIG. 4. In this example, the bottom wedge 29 is situated further away from the open end 26a than is the top wedge 28, so that, once again, there is a slight lateral offset between the two wedges 28 and 29, but the offset is the reverse of the lateral offset of the wedges 28, 29 shown in FIG. 4. In addition, in the initial state, in the absence of any force exerted by the actuators V1, V2, the first surfaces 28′, 29′ present clearance j between them. In the embodiment of FIG. 5, as in the embodiment of FIG. 4, the arrangement of the domed central portion 30, 31 of each of the first surfaces 28′, 29′ of each of the wedges 28, 29 and the relative lateral offset of the two wedges 28, 29 are chosen so that, while the bending force is being applied to the metal sheet F, the two wedges 28, 29 are in contact over their respective domed or protuberant central portions 30, 31.
FIGS. 6 and 7 show the bending force F0 for bending the metal sheet F that essentially causes the top portion 12c of the lower table 12 to flex so that the top edge 26″ of the slot 26 moves closer to the bottom edge 26′ of the same slot 26.
Once again, in these two FIGS. 6 and 7, the wedges 28 and 29 are offset laterally relative to each other. In the embodiment of FIG. 6, only wedge 29 has a first surface 29′ with a domed or protuberant central portion 31, e.g. a spherical surface or a plane protuberant surface. The contact between the two wedges 28, 29 takes place, for the wedge 29, over its domed or protuberant portion 31. It should be noted that, when only one of the two wedges 28 or 29 has a domed or protuberant central portion 30, 31, it is advantageous for the wedge that is provided with the central portion 31 to be the bottom wedge 29 that is fastened to the bottom edge 26′ of the slot 26. The wedges 28, 29 with their protuberant central portions 30 and/or 31 and their relative lateral offset are designed to compensate for the non-parallelism of the edges 24′, 24″ and 26′, 26″ of the slots 24 and 26.
The embodiment of FIG. 7 is analogous to the embodiment shown in FIG. 4, but FIG. 7 shows the top portion 12c of the lower table flexing while the bending force F0 is being applied, and said force being transmitted to the lower table 12.
In addition to the tapped orifices 71, 72, and 73 serving to fasten the wedge 28 or 29 to a support 40, 41, FIG. 8 shows a first surface 28′, 29′ seen from the side, on which surface it is difficult to see with the naked eye that there is a domed or protuberant central portion 30, 31. This is because the first surface 28′, 29′ of the wedge 28, 29 is a spherical surface that has a radius of curvature that is extremely large relative to the length of the wedge 28, 29. By way of example, the wedge 28, 29 of FIG. 8 has a length lying in the range 60 mm to 80 mm and the radius of curvature of each of the first surfaces 28′, 29′ lies in the range 7000 mm (or 7 meters) to 9000 mm (or 9 meters). The protrusion, i.e. the maximum height of the central portion 30, 31 relative to the plane surface of the first surface, which plane surface is defined by the straight line joining the opposite ends 80, 81 of the first surface 28′, 29′, lies approximately in the range 0.05 mm to 0.25 mm. The domed or protuberant portion 30, 31 thus has a maximum height or protrusion lying in the range 0.05% of the length (longest surface dimension) of the first surface 28′, 29′ to 0.4% of said length (when the inclination of the first surface is not too large, it can be considered, by approximation, that the length of each of the wedges is equal to the length of its first surface), said maximum height or protrusion preferably lying in the range 0.1% of the length of the first surface 28′, 29′ of the wedge 28, 29 to 0.3% of said length. It can be understood that the difference in height of the domed or protuberant central portion 30, 31 is often not perceptible with the naked eye, and that the accompanying figures intentionally magnify the central portion 30, 31 for reasons of simplification and of understanding.
FIG. 9 shows a wedge 28, 29 having a first surface 28′, 29′ that is substantially cylindrical and dished. In this example, the first surface 28′, 29′ of the wedge 28, 29 is provided with a dome constituting the domed or protuberant central portion 30, 31 of the wedge 28, 29. This dome may consist of a spherical surface that is protuberant relative to the first surface 28′, 29′ presenting a section of substantially cylindrical shape.
The wedge 28, 29 shown in FIG. 10 has an inclined cylindrical first surface 28′, 29′. The center O of the sphere, of which the first surface 28′, 29′ of the wedge 28 or 29 forms a portion, is offset relative to the vertical V starting from the center C of the first surface 28′, 29′ (this vertical V intersects the bottom plane formed by the first end secured to the first edge of the slot 24′ or 26′). In this example, the centre C of the first surface 28′ or 29′ of the wedge 28 or 29 constitutes the point of contact with the first surface 28′ or 29′ of the other wedge 28 or 29. Thus, when the first surface 28′ or 29′ has a spherical surface, the contact with the first surface 28′, 29′ of the other wedge 28 or 29, regardless of the shape thereof, is contact at a point or substantially at a point. The area of contact 28′, 29′ between the two wedges 28, 29 is thus very small and, taking account of the manufacturing tolerances for the wedges 28, 29, and of the materials used, it represents about 1 square millimeter (mm2). In general manner, this point or substantially point contact between the two wedges 28, 29 may optionally take place at the center C of each of the first surfaces 28′, 29′ of the wedges 28, 29 but, as explained above, the contact between the two wedges 28, 29 depends on their respective inclinations and on their relative offset, as well as on the movement of the top wedge 28 while the bending force F0 for bending the metal sheet F is being applied.
FIG. 11 shows a variant embodiment of the first surface 28′, 29′ of the wedge 28, 29. In this embodiment, the central portion 30, 31 consists of a plane surface. This central portion 30, 31 is in the form of a rectangular or square surface representing in the range 5% of the total area of the first surface 28′, 29′ to 25% of said total area, and preferably in the range 10% of said total area to 15% thereof. In this example, the first surface 28′, 29′ of the wedge 28, 29 has four inclined plane peripheral portions 33, 34, 35, and 36 extending from respective ones of the four edges of the first surface 28′, 29′ to the central portion 30, 31. In this example in which the central portion 30, 31 is a plane surface, the contact with the first surface 28′, 29′ of the other wedge 28, 29, presenting a plane contact surface (optionally the first surface 28′, 29′ of said other wedge 28, 29 is identical to the first surface of the wedge 28, 29 shown in FIG. 11), is area contact between the two wedges 28 and 29.
FIGS. 14 and 15 also show an embodiment in which the contact between the first two surfaces 28′, 29′ of the two wedges 28, 29 is area contact. As shown in these figures, the first contact surface 28′, 29′ is substantially an inclined surface, i.e. the opposite edges 80, 81 have different heights. In addition, the first surface 28′, 29′ of the wedges 28, 29 has three successive segments 40, 30 or 31, and 42, extending over the entire width of the wedge 28, 29, and each having a different inclination; the inclinations of the segments 40, 30/31, and 42 increasing going from the segment 40 to the segment 42. The intermediate or central segment constitutes the central or protuberant portion 30, 31. Relative to the plane or the axis 50 joining the opposite edges 80, 81 of the wedge 28, 29, the central portion 30, 31 presents a maximum height h of about 0.1 mm. As can be seen in FIG. 15, the two wedges 28, 29 that are offset slightly relative to each other are identical but the orientations of their respective first surfaces 28′, 29′ are opposite so that only the central portions 30, 31 face each other and are substantially parallel. Because of the offset between the two wedges 28, 29, only a portion of each of the central portions 30, 31 comes into contact, of the area contact type, with the respective portion of the other central portion. It should be noted that, in this example, the two wedges 28, 29 present clearance j in the initial state.
FIG. 12 shows the third possible mode of contact between the two wedges 28, 29, namely linear contact, the first two modes being point or substantially point contact, and area contact. In this example that is chosen to illustrate this third type of contact, only the wedge 28 has a protuberant central portion 31. In this example, the first surfaces 28′, 29′ of the wedges 28, 29 are cylindrical surfaces, but while the first surface 28′ lies on the inside of a cylinder so that the first surface 28′ is protuberant relative to the plane/axis joining the opposite edges of the wedge 28, the first surface 29′ lies on the outside of a cylinder so that the first surface 29′ forms a recess relative to the plane/axis joining the opposite edges 80, 81.
In addition, the center O1 of the cylinder on which the first surface 28′ lies is closer to said surface 28′ than the center O2 of the cylinder on which the first surface 29′ lies. Thus, the radius of the cylinder of which the first surface 28′ forms a portion is smaller than the radius of the cylinder of which the first surface 29′ forms a portion. That is why only the vertex of the central portion 30 of the first surface 28′ comes into contact over the entire width of the first surface 29′ of the wedge 29, so that the contact between the two wedges 28, 29 is linear contact.
FIG. 13 shows the force lines that are exerted while the bending force F0 for bending the metal sheet F is being applied. The force lines converge or are concentrated from the first end of the wedge 28 that is fastened to the edge 26″ of the slot 26 towards the protuberant central portion 30 of the first surface 28′ of the wedge 28 that is in contact with the protuberant central portion 31 of the first surface 29′ of the wedge 29; these force lines then spread out over the entire width of the wedge 29. In this example, the first surfaces 28′, 29′ of the wedges 28, 29 present spherical or cylindrical surfaces so that the contact is respectively point contact, substantially point contact, or linear contact. The wedges 28, 29 may be made of hardened steel while the lower table 12 may be made of mild steel, thereby, in the absence of plastic deformation, making it possible for stress to be high between the wedges 28, 29 but low between the wedges 28, 29 and the lower table 12.
Naturally, the contact between the first surfaces 28′, 29′ that is described as being point contact or linear contact is the first contact during or at the beginning of application of the force F0 because, after this point or linear contact, the pressure from the top wedge 28 on the bottom wedge 29 is such that the first surfaces 28′, 29′ of the wedges 28, 29 enter at least an elastic deformation stage so that a contact zone that is larger is reached. While the force F0, e.g. a force of 200 kilonewtons (kN), is being applied, the contact zone is preferably approximately in the range 20% of the total area of each of the first surfaces 28′, 29′ of the wedges 28, 29 to 50% of said total area.
In accordance with a characteristic of the invention, the domed or protuberant central portion 30, 31 may include the center C of the first surface 28′, 29′ as the center of said central portion 30, 31 so that the protrusion of the domed portion 30, 31 coincides with the geometrical center of the first surface 28′, 29′ of the wedge 28, 29, but it is also possible to make provision for said domed or protuberant central portion 30, 31 to be offset slightly relative to the center C of the first surface 28′, 29′: such an embodiment is, for example, shown in FIG. 7 in which the domed or protuberant central portion 31 is offset slightly relative to the center C of the first surface 28′, 29′ of each of the wedges 28, 29, so that the protrusion or maximum height of the central portion 30, 31 does not coincide exactly with the geometrical center C of the first surface 28′, 29′ of each of the wedges 28, 29. This offset or this eccentricity of the protrusion of the domed portion 30, 31 relative to the center C of each of the first surfaces 28′, 29′ is relatively small and, by way of example, lies in the range 2 mm to 10 mm for a wedge of length 80 mm. This eccentricity or offset of the protrusion of the domed or protuberant central portion 30, 31 relative to the center C of the first surface 28′, 29′ may thus lie in the range 0% of the length of the wedge 28, 29 to 40% of the length thereof.
In a possible embodiment of the invention, the wedges 28, 29 are identical, i.e. their dimensions are mutually equal and their first surfaces 28′, 29′ are mutually identical, both in shape and in size.
However, it is also quite possible, as described for the various accompanying figures, for the two wedges 28, 29 not to be identical, i.e. essentially for their first surfaces 28′, 29′ not to be the same, and optionally for only one of the first surfaces 28′, 29′ of said wedges 28, 29 to have a domed or protuberant central portion 30, 31.
Patent Application
20100229619
