Machine for processing bars and corresponding processing method

Abstract

A bar processing machine suitable for processing at least one bar (B) having a mainly horizontal extension comprises at least two support sliders (11), at least one of which is reciprocally movable towards / away from the other along a movement axis (Z), and each of which has a processing surface (12), on which is mounted at least one bending unit (13) suitable for bending at least a longitudinal portion of the bar (B), and at least a first bending unit (14), the first bending unit (14) comprising a first contrast element (17) having a first bending radius (R1) and a first bending element (18) selectively rotatable around the first contrast element (17) to bend the bar (B) around the first contrast element (17).

Description

[Technical field]

[0001] The present invention relates to a bar processing machine suitable for bending and / or curving bars or generally products having an oblong shape such as rods having a circular, polygonal or flat cross-sectional shape, round pieces, square pieces, etc. By way of example only, the bar processing machine can bend and / or curve rods supplied in bar form or in coil form or coming from a center for cutting straight bars. [Background technology]

[0002] It is known that in sectors such as architecture and road construction, it is necessary to reinforce concrete structures using bars made of metallic materials such as iron, steel or other alloys with suitable properties, resulting in so-called "reinforced" structures.

[0003] These bars, more commonly known as reinforcing rods, i.e. rods used to reinforce concrete, often need to be pre-formed, i.e. pre-bent or pre-curved, in order to achieve the shape and profile of the concrete structure they have to contribute to reinforcing. Curving here generally means a deformation of the entire or partial longitudinal extension of the bar, while bending here generally means a deformation localized in a particular zone of the bar.

[0004] The reinforcing rods can have round sections or round sections with ridges and ribs that can facilitate fastening with concrete, and once properly shaped, they can be connected together in known manner to create the desired reinforced structure.

[0005] For this purpose, machines for processing bars are known which are suitable for bending and / or curving bars such as rods, round pieces, square pieces etc., with a circular, polygonal or flat cross-sectional shape. A machine of this type is described, for example, in DE 10 20 033 336 A1. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2017 / 033145 Summary of the Invention [Problem to be solved by the invention]

[0007] Patent document 1 discloses a combined machine for bending and curving rods. The combined machine comprises a support base configured to support and guide along a movement axis two movable sliders, each provided with a processing surface. The processing surfaces are provided with a bending unit and a curving unit, respectively, suitable for bending and curving at least a portion of one or more processed rods.

[0008] In particular, the bending unit comprises a contrast roller and a bending roller between which, in use, the bar is passed so that a bend is formed in one or more portions of the bar having a bending radius corresponding to the radius of the contrast roller.

[0009] This composite machine is very efficient since it can use a bending unit in combination with a curving unit, but it is still capable of bending a bar with a single radius of curvature corresponding to the radius of the contrast roller equipped at that time, whereas in this particular field it is often necessary to bend the same bar with different bending radii in order to fulfil the abovementioned construction requirements, and therefore the machine must be equipped each time with a bending unit suitable to obtain such bending radii.

[0010] This naturally entails a certain waste of time and a certain slowness in the production of bars with different bending radii, since the machine needs to be continually re-equipped with different bending units or the bending operations need to be performed on different machines, and since the bars need to be moved from one machine to another. Moreover, the operator often needs to perform a partial operation on a batch of bars, re-equip the machine, perform a further partial operation on the whole batch of bars or on parts of it, etc., until the bending operation is finished.

[0011] Furthermore, continuous refitting of the machine involves the possibility of assembly errors of the bending units which, if installed incorrectly, can lead to precision errors in the bending operation and safety hazards for the operators who would be required to intervene at the machine more frequently.

[0012] There is therefore a need to achieve a machine for processing bars that is able to overcome at least one of the drawbacks of the state of the art. One object of the invention is to provide a machine for processing bars which makes it possible to carry out bending operations characterized by different bending radii without the need for continuous refitting of the machine.

[0013] Another object of the invention is to provide a machine for processing bars which makes it possible to simplify the execution of different bending operations carried out on the same bar. A further object of the invention is to provide a machine for processing bars which makes it possible to reduce the time required to carry out the different bending operations. [Means for solving the problem]

[0014] Applicant has conceived, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other objects and advantages. The invention is set forth and characterized in the independent claims, while the dependent claims describe further characteristics of the invention or variants of the main inventive idea.

[0015] The embodiments described herein relate to a machine for processing bars. The bar processing machine is suitable for processing at least one bar having a predominantly horizontal development.

[0016] The machine comprises at least two support sliders, at least one of which is reciprocally movable towards / away from the other along an axis of movement. The support sliders each have a working surface on which are arranged at least one first bending unit and at least one bending unit suitable for bending at least a longitudinal portion of the bar.

[0017] The first bending unit comprises a first contrast element having a first radius of curvature and a first bending element selectively rotatable about the first contrast element to bend the bar about the first contrast element.

[0018] According to one aspect of the invention, at least one of the support sliders comprises at least a second bending unit on the working surface. The second bending unit comprises a second contrast element and a second bending element selectively rotatable about the second contrast element to bend the bar about the second contrast element.

[0019] The second contrast element has a curvature radius that is different than the first curvature radius of the first curvature element. Advantageously, in this way it is possible to perform multiple bends on the same bar in the same processing machine without having to perform the multiple bends on different processing machines and therefore without having to move the bar or re-equip the machine with a bending unit different from the one already installed.

[0020] The invention also relates to a method for processing at least one bar on a processing machine, in particular a method for providing at least one curving operation on at least one bar and for providing said bar with two or more curving operations having different radii of curvature.

[0021] These and other aspects, features, and advantages of the present disclosure will become better understood with reference to the following description, drawings, and appended claims. The drawings, which are incorporated in and form a part of this specification, illustrate several embodiments of the invention and, together with the description, are intended to explain the principles of the present disclosure.

[0022] The various aspects and features described herein may be applied individually where possible. These and other characteristics of the invention will become apparent from the following description of some embodiments, given by way of non-limiting example with reference to the attached drawings, in which: [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 shows diagrammatically a machine for processing bars according to the invention. [Diagram 2] FIG. 2 shows the machine of FIG. 1 during a bar bending operation. [Diagram 3] FIG. 3 shows the machine of FIG. 1 during a bar bending operation. [Figure 4] FIG. 4 shows the machine of FIG. 1 during a bar bending operation. [Diagram 5] FIG. 5 shows the machine of FIG. 1 during a bar bending operation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] For ease of understanding, the same reference numbers have been used, where possible, to identify identical common elements in the drawings, it being understood that elements and features of one embodiment may be beneficially incorporated in other embodiments without further specification.

[0025] Reference will now be made in detail to various embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided as an illustration of the invention and is not to be construed as a limitation thereof. For example, features illustrated or described as part of one embodiment may be employed on or in conjunction with other embodiments to produce further embodiments. It is understood that the invention is intended to include all such modifications and variations.

[0026] Before describing these embodiments, it should also be made clear that the present description is not limited in its application to the details of construction and arrangement of components as set forth in the following description using the accompanying drawings. The present description is capable of providing other embodiments and of being obtained or carried out in various other ways. It should also be made clear that the phraseology and terminology used herein are for the purpose of description only and should not be regarded as limiting.

[0027] The embodiment described using the accompanying drawings relates to a machine for processing bars, generally indicated by reference numeral 10 in the accompanying drawings. The machine 10 is suitable for processing at least one bar B having a mainly oblong development.

[0028] The bar B can be fed directly to the machine 10 in bar form or in coil form which is straightened and cut to size, or it can come from a center for cutting straight bars.

[0029] Depending on the end product to be obtained, the bars B fed to the machine 10 may also be of different types, for example different diameters, materials, surface treatments or others. According to some embodiments, the machine 10 comprises at least two support sliders 11, at least one of which is reciprocally movable towards / away from the other along a movement axis Z, each of which is provided with a working surface 12.

[0030] On the working surface 12, at least a bending unit 13 suitable for bending at least a longitudinal part of the bar B and at least a first bending unit 14 are provided. The first bending unit 14 comprises a first contrast element 17 with a first bending radius R1 and a first bending element 18.

[0031] The first contrast element 17 comprises at least a shaped portion having an arched profile characterized by the bend radius R1 shown in FIG. The first bending element 18 can be selectively rotated about the first contrast element 17 to cause the bar B to bend thereabout.

[0032] According to one aspect of the invention, at least one of the support sliders 11 comprises at least a second bending unit 15,16 on the working surface 12, comprising a second contrast element 19,27 and a second bending element 20,28.

[0033] The second contrast element 19, 27 also includes at least one shaped portion having an arched profile characterized by a bend radius R2, R3 as shown in Figures 3 and 4, respectively.

[0034] The second bending elements 20, 28 can be selectively rotated about the second contrast elements 19, 27 to cause bending of the bar B thereabout. The second contrast elements 19, 27 have bending radii R2, R3 shown in Figures 3 and 4, respectively, which differ from the bending radius R1 of the first contrast element 17 shown in Figure 2, so as to bend the bar B with a different bending radius.

[0035] This solution allows the operator to pre-equip the machine 10 with a number of contrast elements and a number of bending elements of suitable sizes for bending the bar B without interrupting the work, for example for replacement of the contrast element or elements or the bending element or elements.

[0036] Furthermore, this solution makes it possible to perform bending operations with different bending radii, for example having a first bending radius R1 and a second bending radius R2, on the same bar B, in addition to the curving operation, without the need to move or remove the bar B from the machine 10.

[0037] Although reference is made in this specification to the processing of a single bar B, it will be apparent that the machine 10 is capable of processing several bars B simultaneously. According to the embodiment shown in Figs. 1 to 5, the support sliders 11 are arranged facing each other and can slide on the guides 21 parallel to each other with respect to the movement axis Z as described above.

[0038] In particular, the support sliders 11 are independently movable along a movement axis Z in relation to the particular task to be performed on the bar B. The guide 21 may have a length in the direction of the movement axis Z that is compatible with the maximum length of the bar B that can be processed by the machine 10, in which case the support sliders 11 may be at both ends of the guide 21.

[0039] The machine 10 also comprises holding means 22 arranged between the support sliders 11 and configured to support and hold the bar B during processing of the bar B. The holding means 22 prevent undesired bending of the bar B in its central portion, which tends to deform under its own weight.

[0040] As described above, the holding means 22 is configured to act on the bar B on a surface that is flush with the processing surface 12, and prevents deformation of the bar B due to interference with the processing surface 12. Furthermore, there may be one or more holding means 22, depending on the length of the bar B that must be supported.

[0041] According to the embodiment shown in Fig. 1, the holding means 22 comprises at least one central holding means 22, i.e. the central holding means 22 is configured to hold a central portion of the bar B. In particular, the central holding means 22 can be moved and positioned in a first direction perpendicular to the movement axis Z and in a second direction coinciding with or parallel to the movement axis Z.

[0042] There are preferably at least three retaining means, so that two lateral retaining means 22 are arranged on either side of a central retaining means 22, these two lateral retaining means 22 being preferably symmetrical with respect to the central retaining means 22.

[0043] Advantageously, the movement of the central holding means 22 in said second direction enables the central holding means 22 to always support the bar B in its central part, regardless of the mutual distance between the support sliders 11 .

[0044] The lateral holding means 22 can only move in a direction parallel to a first direction perpendicular to the movement axis Z. The lateral holding means 22 is complementary to the central holding means 22 when the bar B is very long, i.e. when the support sliders 11 are very far apart, for example at both ends of the guide 21.

[0045] The holding means 22 are configured to move parallel to a first direction by means of slide guides 23. The central holding means 22 is also configured to move in a second direction by means of a transport device 24 movable on slide guides 25.

[0046] If in operation it is necessary to support the bar B with a number of holding means 22, these must be aligned in a transverse direction perpendicular to the axis of movement Z so as not to cause transverse stresses and deformations in the bar B.

[0047] In some embodiments, the holding means 22 may be of the "vice" type, i.e. with two symmetrical jaws movable relative to a first direction, the jaws defining a variable passage gap between them for holding and releasing the bar B.

[0048] When the bar B is fed to the machine 10 in a known manner, it may be supported by the holding means 22 and may rest partially on the working surface 12 of the support slider 11 . According to the embodiment shown in FIG. 1 for example, each working surface 12 comprises a curving unit 13, a first bending unit 14 and a second bending unit 15 as described above.

[0049] Hereinafter, in this specification, curving means that the bar B undergoes a continuous deformation along its extended longitudinal portion or over its entire length. During the curving operation, the curving unit 13 is driven to deform the bar B while one or both of the support sliders 11 move in the direction of the movement axis Z.

[0050] Hereinafter, bending means that the bar B undergoes a deformation localized only in the zone acted upon by the bending unit selected for the operation, for example the first bending unit 14 in Fig. 2. During the bending operation, while the support slider 11 is at rest, one of the bending units 14, 15 is operated to deform the bar B.

[0051] According to the embodiment shown in Figures 1 to 5, a third bending unit 16 is also provided on each work surface 12, the third bending unit 16 comprising a third contrast element 27 and a third bending element 28 selectively rotatable about the third contrast element 27 so as to bend the bar B around it.

[0052] The third contrast element 27 has a different size than the first contrast element 17 and the second contrast element 19 in order to bend the bar B at a third bending radius R3 in FIG. 4 which is different from the first bending radius R1 in FIG. 2 and the second bending radius R2 in FIG. 3.

[0053] In a possible embodiment not shown, it may be provided to install a larger number of bending units on the working surface 12, in relation to the space present on the working surface 12 in any case, more than three, for example up to seven.

[0054] In the embodiment described with reference to Figs. 1 to 5, the bending unit 13 and the bending units 14, 15 are installed in priority positions to facilitate the work and prevent unnecessary interference.

[0055] In particular, the bending unit 13 can be located on the working surface 12 in an eccentric position towards a first side 30 of the working surface 12 perpendicular to the movement axis Z. The bending units 14, 15 can be located on the working surface 12 in an eccentric position towards a second side 31 opposite to the first side 30, even if different positioning possibilities are not excluded.

[0056] According to a possible solution not shown, the bending units 14, 15 can be installed on the supporting slider 11 in a non-mirrored position, i.e. at different positions. According to a possible solution, the drive of the bending unit 13 and at least the bending units 14, 15 may be independent or may be performed by the same motor unit.

[0057] According to one embodiment of the invention shown in FIG. 1, at least one support slider 11 provides a single drive means arranged to drive at least the bending units 14, 15 simultaneously or sequentially.

[0058] The drive means includes a motor member (not shown) provided with a pinion 32 engaging a toothed crown 33 . A toothed crown 33 is associated with the first bending unit 14 and allows rotation of at least the first bending element 18 relative to the first contrast element 17 .

[0059] The first bending unit 14 is kinematically connected to the second bending unit 15 and, if present as in the case shown in Figures 1 to 5, to the third bending unit 16 by gears which are indicated diagrammatically by "dotted and dashed lines" and which determine the simultaneous rotation of the bending units 14, 15, 16.

[0060] According to some embodiments, at least some of the bending units 13 and / or flexion units 14 , 15 may be arranged recessed with respect to the working surface 12 of the respective support slider 11 .

[0061] In particular, the curvature unit 13 and the bending units 14, 15 are movable between a state in which they protrude from the machining surface 12 and can act on the bar B, and a position in which they are retracted from the machining surface 12, positioned below the machining surface 12 and do not interfere with the bar B.

[0062] According to a first example shown in FIG. 2, while the bar B is bent by the bending unit 14, the latter is in the above-mentioned extended position, while the second bending unit 15, the third bending unit 16 and the curvature unit 13 are in the above-mentioned retracted positions.

[0063] According to a second example shown in FIG. 3, during bending of the bar B by at least one bending unit, the second bending unit 15 is in the above-mentioned extended position, and the first bending unit 14, the third bending unit 16 and the curvature unit 13 are in the above-mentioned retracted positions.

[0064] According to a third example shown in FIG. 4, during bending of the bar B by at least one bending unit, the third bending unit 16 is in the above-mentioned extended position, and the first bending unit 14, the second bending unit 15 and the curvature unit 13 are in the above-mentioned retracted positions.

[0065] According to a fourth example shown in FIG. 5, during bending of the bar B by at least one bending unit, the bending unit 13 is in the above-mentioned extended position, and the first bending unit 14, the second bending unit 15 and the third bending unit 16 are in the above-mentioned retracted positions.

[0066] According to some embodiments shown in Figures 1 to 5, the curving unit 13 comprises two contrast rollers 35 arranged opposite each other, at least one of the contrast rollers 35 advantageously provided with positioning means 36 configured to enable selective movement away from / towards the other contrast roller 35 in order to adjust the gap between them with respect to the bar B.

[0067] The curving unit 13 can act on the bar B in a known manner. In the case shown in Fig. 5, given here as an example, the curving unit 13 also comprises a curving roller 37 which can be arranged in relation to the contrast roller 35 and which can be configured to exert a desired deformation on the bar B.

[0068] The position of the curved roller 37 relative to the contrast roller 35 determines the curvature of the bar B. At least the bending units 14, 15 are configured to bend at least one end of the bar B and thus perform bending operations at the head and / or tail, but can also work at intermediate positions of the bar B.

[0069] If one of the bending units 14, 15 described above is interposed at an intermediate position of the bar B, it can be interposed after at least part of the curvature has been obtained or before the curving operation is started.

[0070] According to a possible solution, the bending units 14, 15, 16 may be of the type shown in figures 1 to 5 with pin-shaped contrast elements 17, 19, 27 and similarly shaped bending elements 18, 20, .

[0071] According to a possible modified embodiment, the bending units 14, 15, 16 may be provided with hoe-shaped contrast elements 17, 19, 27. Each working surface 12 of the support slider 11 is also provided with clamping means 26 suitable for holding at least a part of the bar B during the bending operation. In this way, the stresses acting on the bar B during the bending operation are limited to the part of the bar B that is being bent.

[0072] The clamping means 26 is movable on the slide guide 23 in the first direction above the work surface 12, whereby the range of motion of the clamping means 26 makes it possible to ensure the correct positioning of the bar B for every curving and bending operation.

[0073] The clamping means 26 may also be of the “vice” type as described above for the holding means 22 . According to one aspect of the invention, the machine 10 comprises a tensioning unit 34 mounted on the at least one support slider 11 and movable over the working surface 12 in a direction perpendicular to the movement axis Z. The tensioning unit 34 is configured to pull the bar B in the direction of the movement axis Z or to pull the at least one support slider 11 in the same direction.

[0074] The tensioning unit 34 comprises a plurality of tensioning rollers 38 aligned in a direction perpendicular to the axis of movement Z to define a variable passage gap therebetween for the passage and tensioning of the bar B.

[0075] If the support slider 11 is not equipped with an autonomous drive means, the support slider 11 can be moved by the pulling action exerted by the tensioning unit 34 on the bar B. For example, in order to move a support slider 11 without a tensioning unit 34, the clamping means 26 of that support slider 11 must necessarily clamp the bar B and the clamping means 26 of the other support slider 11 must not clamp the bar B. In order to move a support slider 11 with a tensioning unit 34, the associated clamping means 26 must not clamp the bar B and the clamping means 26 of the other support slider 11 must clamp the bar B.

[0076] According to the embodiment shown in FIGS. 1 to 5, the tensioning unit 34 is associated with the clamping means 26 and is movable therewith. Even if not shown, the machine 10 may be associated with a programmable control, management and command unit configured to control, manage and command the possible movement elements of the support slider 11 .

[0077] According to a possible solution, the moving member of the support slider 11 may be selected in the group comprising a worm screw mechanism, a rack mechanism, a chain mechanism or similar or equivalent moving members.

[0078] An embodiment of the present invention also relates to a method for processing at least one bar B having a mainly horizontal development in a bar processing machine 10, at least one bending operation on the bar B by a bending unit 13 suitable for bending at least a longitudinal portion of the bar B; performing at least a first bending operation on the bar B by a first bending unit 14 comprising a first contrast element 17 having a first bending radius R1 and a first bending element 18 selectively rotatable around the first contrast element 17; and at least one further bending operation on the bar B by a second bending unit 15, 16 comprising a second contrast element 19, 27 and a second bending element 20, 28 selectively rotatable about the second contrast element 19, 27 to bend the bar B about the second contrast element 19, 27. The second contrast element 19, 27 has a bending radius R2, R3 different from the bending radius R1 of the first contrast element 17 so as to bend the bar B with a different bending radius.

[0079] It will be apparent that modifications and / or additions of components may be made to the bar processing machine described hereinabove without departing from the field and scope of the present invention. Although the invention has been described with reference to some particular examples, it is certainly clear to a person skilled in the art that it is possible to achieve many other equivalent forms of machines for processing bars having the characteristics set out in the claims and therefore all falling within the field of protection defined thereby.

Claims

1. A machine for processing at least one bar (B) having a mainly horizontal development, comprising at least two support sliders (11), at least one of which is movable reciprocally towards / away from the other along a movement axis (Z) and each of which is provided with a processing surface (12), on which at least one bending unit (13) is provided suitable for bending at least a longitudinal part of said bar (B) and at least a first bending unit (14), said first bending unit (14) being provided with a first contrast element (17) having a first bending radius (R1) and a second bending unit (15) for bending said bar (B) around said first contrast element (17). and a first bending element (18) selectively rotatable about said first contrast element (17) to bend said bar (B), at least one of said support sliders (11) comprises at least a second bending unit (15, 16) on said working surface (12) with a second contrast element (19, 27) and a second bending element (20, 28), said second bending element (20, 28) selectively rotatable about said second contrast element (19, 27) to bend said bar (B) about said second contrast element (19, 27), said second contrast element (19, 27) having a bending radius (R2, R3) different from the bending radius (R1) of said first contrast element (17), The bar processing machine further comprises a tensioning unit (34) attached to at least one of the support sliders (11), the tensioning unit (34) is movable on the working surface (12) in a direction perpendicular to the movement axis (Z) and configured to pull the bar (B) in the direction of the movement axis (Z); the tensioning unit (34) comprises a plurality of tensioning rollers (38) aligned in a direction perpendicular to the movement axis (Z), the tensioning rollers (38) defining a variable passage gap therebetween for the passage and tensioning of the bar (B); The working surface (12) of each support slider (11) is provided with a clamping means (26) capable of holding at least a part of the bar (B); At least one of the support sliders (11) is movable by being pulled in the same direction as the movement axis (Z) by a pulling action of the pulling unit (34) when the clamping means (26) of the support slider (11) holds the bar (B).

2. 2. The machine according to claim 1, characterized in that at least some of the units (13, 14, 15, 16) consisting of the bending unit (13), the first bending unit (14) and the second bending unit (15, 16) can be arranged in a retractable manner with respect to the working surface (12) of the respective support slider (11) and are movable between a state protruding from the working surface (12) and a position retracted from the working surface (12).

3. 2. Machine according to claim 1, characterized in that at least one of said support sliders (11) provides a single drive means arranged to drive at least said first bending unit (14) and said second bending unit (15, 16) simultaneously or sequentially.

4. 4. A machine according to claim 3, characterized in that the drive means comprises a motor member with a pinion (32) engaging a toothed crown (33) associated with the first bending unit (14) and enabling rotation of at least the first bending element (18) relative to the first contrast element (17).

5. 2. A machine according to claim 1, characterized in that the tensioning unit (34) is associated with and movable with the clamping means (26) of the support slider (11) on which it is mounted.

6. A machine according to any one of claims 1 to 5, comprising a plurality of holding means (22) for holding said bar (B), said plurality of holding means (22) including at least one central holding means (22) capable of holding a central portion of said bar (B) and capable of being moved and positioned in a first direction perpendicular to said axis of movement (Z) and in a second direction coinciding with or parallel to said axis of movement (Z).

7. 7. A machine according to claim 6, characterized in that it comprises, beside said central holding means (22), holding means (22) which are movable only in a direction perpendicular to said axis of movement (Z).

8. 1. A method for processing at least one bar (B) having a predominantly horizontal development in a processing machine, comprising: The bar (B) is processed by the bar processing machine according to any one of claims 1 to 7, and the processing of the bar (B) is at least one operation of bending said bar (B) by means of a bending unit (13) suitable for bending at least a longitudinal portion of said bar (B); At least one operation of bending the bar (B) by the first bending unit (14) comprising a first contrast element (17) having a first bending radius (R1) and a first bending element (18) selectively rotatable around the first contrast element (17); and at least another operation of bending the bar (B) by another bending unit (15, 16) comprising a second contrast element (19, 27) and a second bending element (20, 28) selectively rotatable about said second contrast element (19, 27) to bend said bar (B) about said second contrast element (19, 27), said second contrast element (19, 27) having a bending radius (R2, R3) different from the bending radius (R1) of said first contrast element (17) so as to bend said bar (B) with a different bending radius.

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