0001 The invention relates to a method and to an installation for making a workpiece comprising at least one tubular part obtained by bending a metal sheet. This method and this installation are particularly adapted to the manufacture of a shadow mask support frame for a cathode-ray display tube. 0002 Such a frame is used for example in the domain of the manufacture of colour televisions. The colour display cathode-ray tubes comprise a metal foil pierced with a plurality of holes or slots, called shadow mask and disposed between the electron gun and the display screen. Such a shadow mask is supported by a frame, generally rectangular in shape, which holds it in position in the vicinity of the display screen and, if necessary, ensures that the mask is taut in order to limit the deformations resulting from the local overheatings created by the electron beams. 0003 It is known from EP-A-0 809 272 to produce a shadow mask frame from two thin metal strips joined to each other and comprising ribs for rigidification. Such a frame is light and rigid, which allows it to be used for a taut shadow mask and for a crimped shadow mask. Application FR-A-2 790 140 discloses a shadow mask frame comprising tubular parts formed by bending a metal sheet. The sheet may be bent by different methods, in particilar by hand, but a manual bending is not compatible with high-rate industrial production. Furthermore, the known methods of bending with the aid of machines comprising an apron, sometimes called support strap, on which a metal sheet to be bent is held with the aid of a holding clamp, and an articulated bending flap, do not allow tubular parts to be easily manufactured. 0004 It is an object of the present invention to propose a method of bending with the aid of a bending machine making it possible to manufacture workpieces comprising at least one tubular part, particularly a workpiece constituting one or more uprights of a frame supporting a shadow mask. 0005 In this spirit, the invention relates to a method for making a workpiece comprising at least one tubular section obtained by bending a metal plate along at least one longitudinal edge, characterized in that it comprises steps consisting in pre-bending the plate along at least two bending edges then in exerting, during bending, a clamping force of at least a part of the plate with respect to a bending member. 0006 Thanks to the invention, the metal plate is efficiently displaced by the bending member, without risk of slide that may result in an imprecise bending at the level of the edge. Thanks to the pre-bending, the plate is maintained in position on the apron or supporting beam on which it rests, without risk of buckling during the subsequent bending. The effort of clamping or of holding of the plate with respect to the bending edge makes it possible to hold the plate in position with respect to its environment without resorting to fixing clips which could be used inside tubular parts only with difficulty, as they would hinder the bending operations. 0007 According to advantageous but non-obligatory aspects of the invention, the method incorporates one or more of the following characteristics: 0008 the pre-bending is effected by immobilizing the plate on an apron by means of a retractable holding clamp. In addition, a step of preparation of the pre-bending may be provided, by weakening the metal sheet along the bending edges. Such weakening may be effected by marking or punching through a part of the thickness of the metal sheet, or by any other suitable method. 0009 the plate is bent along different edges over the length of the workpiece to be manufactured, with the result that tubular sections of different basic profiles are formed. This makes it possible to produce workpieces of elaborate geometry, such as shadow mask support frames. 0010 the plate is bent about at least one virtual geometrical axis defined by the cooperation of the bending member and of guiding means associated therewith. 0011 the clamping force is essentially exerted by the outside of a tubular section of the workpiece, in the absence of a tightening effort exerted by a holding clamp or like equipment. This takes into account the fact that the use of a holding clamp is delicate with a tubular workpiece, and even impossible if it presents sections of different basic profiles. 0012 the clamping force is directed substantially in the direction of the bending edge. 0013 the clamping force is substantially perpendicular to the bending edge. 0014 the clamping force stresses a part of the metal plate parallel to itself and perpendicularly to the bending effort exerted on this part by the bending member. 0015 the clamping force is adapted as a function of the position of the bending member. This aspect of the invention makes it possible to take into account the dimensional variations of the plate in the course of the bending operation, such variations resulting in particular from the localized stresses at the level of the edge and of its radius of curvature. It also makes it possible to avoid an interference between the elements exerting a clamping force at the level of two opposite edges of the plate. 0016 the method further comprises a step consisting in causing a first part of the plate to overlap a second part thereof and in reducing the clamping force exerted on the second part before these parts are welded together. This aspect of the invention ensures that the overlapping parts are in elastic contact during welding, which allows this welding to be efficient and lasting. 0017 the method also comprises a step consisting in bending the plate about at least one virtual axis defined as the geometrical axis of a cylindrical cradle for guiding the bending member. The use of a virtual axis makes it possible to manoeuvre the bending member without resorting to a hinge capable of interfering with other parts of the installation dedicated to a bending along another edge. In particular, it may be provided to bend the plate about different bending axes depending on the length of the workpiece, using bending members guided by cradles of different geometrical axes corresponding to these bending axes. 0018 The method comprises a step consisting, after formation of a plurality of tubular sections, in shaping them into a closed frame by bending joining sections, between two adjacent tubular sections, perpendicularly to the principal directions of these tubular sections. 0019 The method comprises a step consisting in applying, by suction or magnetic attraction, a part of the sheet metal plate against a part of the bending member. 0020 The invention also relates to an installation for carrying out the method described hereinabove and, more specifically, to an installation comprising at least one member for bending a metal plate along a longitudinal edge of a tubular section of the frame, characterized in that this bending member is equipped with means for exerting on a part of the plate a clamping force countering a sliding of a part of the plate along the bending member, while this bending member is articulated about a virtual geometrical axis located inside the corresponding tubular section. 0021 According to advantageous but non-obligatory aspects of the invention, the installation incorporates one or more of the following characteristics: 0022 a plurality of bending members are adapted to bend the plate along distinct edges, with the result that tubular sections of different basic profiles are formed. 0023 the clamping means are constituted by at least one heel forming a stop for abutment of the plate. This heel may extend over substantially the whole length of the edge. It is also possible to provide a plurality of heels distributed over the length of the bending edge and separated by spaces for receiving holding heels used for bending the plate along another edge. 0024 The clamping means have a variable position with respect to the bending edge. This may come from the fact that the installation comprises means for adjusting or adapting the effort exerted by the clamping means on the plate. 0025 The bending member comprises at least one surface for bearing against the plate and at least one circular-base cylindrical surface adapted to cooperate with a circular-base cylindrical surface of a cradle, these cylindrical surfaces being centred on a virtual axis of bend of the plate. The use of such a cradle makes it possible to guide the bending member without resorting to a hinge capable of interfering with adjacent devices. In that case, the installation may be provided to comprise a plurality of bending members distributed over the length of the plate and cooperating with cradles of which the cylindrical surfaces have parallel geometrical axes offset with respect to each other. This makes it possible to bend the plate along distinct longitudinal edges, over its length, which enables tubular sections of different geometries to be shaped, corresponding for example to distinct sides of a frame to be made. The bending member or members are advantageously equipped with an outer rib forming a cylindrical surface adapted to cooperate with the cylindrical surface of the corresponding cradle, these cylindrical surfaces being substantially of the same radii. According to an advantageous form of embodiment of the invention allowing tippings of great amplitude, the bending member or members are guided with respect to the cylindrical surface of the cradle by telescopic segments which slide in one another. The surfaces of the ribs, the telescopic segments and/or the surfaces of the cradles may be equipped with sliding balls or rollers. 0026 The installation comprises a tool for shaping the workpiece, this tool including elements for supporting tubular sections of the workpiece provided with means for receiving and immobilizing these tubular parts, these elements being articulated with respect to one another. This tool allows the final shaping of the frame after the tubular parts have been produced. At least one of the articulated elements may be provided to be adjustable in length, which makes it possible to adapt the tool to the manufacture of frames of different dimensions. The articulated elements may also be provided to be adapted to form with one another a closed figure of predetermined geometry, corresponding to the geometry of a frame formed by the manufactured workpiece. The closed nature of the figure obtained makes it possible to obtain an increased geometrical precision thanks to a univocal positioning of the different elements constituting this tool. According to an advantageous form of embodiment of the invention, at least one of the articulated elements is equipped with a member, of cross-section substantially in the form of a U and adapted to receive at least one shim for blocking a tubular section in place in this element, means being provided to immobilize the shim on this member. 0027 At least one of the bending members is equipped with a blade forming shears for cutting out the plate during bending thereof. 0028 Finally, the invention relates to the use of the method or of the installation mentioned hereinabove for manufacturing a shadow mask support frame for cathode-ray tube.

0029 The invention will be more readily understood and other advantages thereof will appear more clearly in the light of the following description of two forms of embodiment of an installation for manufacturing a shadow mask support frame and of its method of implementation, given solely by way of example and made with reference to the accompanying drawings, in which: 0030 FIG. 1 schematically shows a shadow mask frame on which is disposed a partially shown shadow mask. 0031 FIG. 2 schematically shows a perspective view of a part of an installation for manufacturing the frame of FIG. 1, the frame not having been shown. 0032 FIG. 3 schematically shows a part of the frame of FIG. 1 when it is in place in the installation of FIG. 2. 0033 FIG. 4 is a section along line IV-IV in FIG. 2, the frame being shown in place in the installation. 0034 FIG. 5 is a section similar to FIG. 4 during a first step of the method of the invention, prior to the configuration of FIG. 4. 0035 FIG. 6 is a section similar to FIG. 4 during a subsequent step of the method of the invention, later than the configuration of FIG. 4. 0036 FIG. 7 is a section similar to FIG. 4 during a second subsequent step of the method of the invention. 0037 FIG. 8 is a section along line VIII-VIII in FIG. 2, the frame being shown in place in the installation. 0038 FIG. 9 is a section similar to FIG. 8 during a subsequent step of the method of the invention. 0039 FIG. 10 is a section similar to FIG. 8 during a second subsequent step of the method of the invention. 0040 FIG. 11 is a view in perspective of another part of the installation of the invention, a frame in the course of manufacture being shown above this part of the installation. 0041 FIG. 12 is a view on a larger scale of detail XII in FIG. 11, the frame being in place on the tool. 0042 FIG. 13 is a view similar to FIG. 12 during a subsequent step of manufacture of the frame. 0043 FIG. 14 is a plan view of the part of the installation shown in FIGS. 11 to 13, in the course of use. 0044 FIG. 15 is a partial schematic representation in section of a part of a plate in the course of bending. 0045 FIG. 16 schematically shows a perspective view of a part of the installation. 0046 FIG. 17 is a schematic section of a part of an installation in accordance with a second form of embodiment of the invention. 0047 FIG. 18 is a view similar to FIG. 13 for the installation of FIG. 17, and 0048 FIG. 19 is a section along line XIX-XIX in FIG. 18.

0049 FIG. 1 schematically shows a shadow mask 1 mounted on a shadow mask supporting frame 2. The frame is generally rectangular in shape and comprises two lateral uprights 3, 3 and two end uprights 4, 4. The shadow mask is constituted by a thin metal foil, for example of iron-nickel alloy with a low coefficient of expansion, pierced with holes 5 and fixed by welding on upper edges 6, 6 of the end uprights 4, 4. Different forms of shadow mask are possible as a function of the cathode-ray tube with which it is to be associated. In the example shown, the shadow mask is substantially planar, but it may equally well be curved into a portion of cylinder. The mask I may in addition be taut, i.e. subjected to a tension parallel to the lateral uprights 3, 3 of the frame 2. 0050 The uprights 3, 3, 4 and 4 are formed by bending a sheet metal so as each to constitute a tubular element. These four uprights may be obtained by bending a single metal plate. It is also possible to provide using one bent plate for each upright or one plate for two contiguous uprights. 0051 In the example illustrated in the Figures, a single metal plate 10 is used for making the four tubular uprights 3, 3, 4, 4. This plate is partially visible in FIG. 3 after steps of cutting out assembly tongues 11 and of pre-marking longitudinal edges 12 to 17. 0052 At the level of the edges 12 to 14, the plate 10 forms the end upright 4. At the level of edges 12 and 15 to 17, the plate 10 forms the lateral upright 3. In practice, the part of the plate 10 shown in FIG. 3 corresponds to the corner 7 of the frame 2 at the level of which the join is made between the uprights 3 and 4. 0053 The plate 10 is shaped in the configuration of FIG. 3 by operations of cut out, stamping and bending using, in particular, punches intended to create in the plate 10 an edge 12 extending over substantially the whole of its length while the edges 13 to 17 extend over only a part of the plate 10, the edges 13 and 15 being parallel to each other but offset, i.e. distant from the edge 12 by different distances d1 and d2. 0054 As is more particularly visible in FIG. 5, the pre-bending of the plate 10 is effected by immobilizing this plate on an apron or support strap 101 thanks to a holding clamp 107 and by bending the plate 10 as represented by arrows F, which has the effect of creating the lines of bend 12 and 13. Another holding clamp is used at the level of the upright 3, which makes it possible to continue the creation of the line of bend 12 and to create the line of bend 15. This second holding clamp is also configured in order to allow the creation of the lines of bend 16 and 17. 0055 These operations of pre-bending make it possible to attain the configuration of FIG. 4 in which the holding clamp 107 is retracted or withdrawn from the interior space of the plate 10 as represented by arrow F. It will be noted that the shape of the holding clamp 107 is compatible with its withdrawal from the interior of the plate 10 in the configuration of FIG. 4. 0056 Thanks to this operation of pre-bending, the plate 10 is immobilized with respect to the installation of the invention, as it is in abutment along the two bending edges 12 and 13 or 12 and 15 which are substantially parallel. 0057 Pre-bending may be prepared by a marking of the lines of bend, for example by a stamping through a part of the thickness of the plate 10, in particular of 25%. 0058 In general, the pre-bending of the plate 10 may be effected on the machine shown in the Figures or on another machine, without departing from the scope of the present invention. 0059 In the event of an upright being made whose section has more than three sides, such as upright 3, preliminary pre-bendings are necessary in order to make the edges of the type of edges 16 and 17. Such a preliminary pre-bending is also used in the case of the upright 4 for making the edge 14. 0060 The installation 100 of the invention comprises the apron 101 on which is placed a central part 21 of the plate 10. Studs, of which only one is visible in the Figures with reference 102, extend from this plate 101 and are intended to traverse openings 22 provided in the part 21, so as to position the plate 10 on the base plate 101. 0061 From the part 21 there extend two flanges 23 and 24 of the plate 10 intended to form the upright 4. From this same same central part 21 there extend two other flanges 25 and 26 intended to form the upright 3. 0062 The flanges 23 and 25 are connected to the part 21 along the edge 12. The flanges 24 and 26 are respectively connected to the part 21 along the edges 13 and 15. The flange 23 is connected to a border 27 by the line of bend or edge 14 while the flanges 25 and 26 are both divided into two panels 25a, 25b and 26a, 26b respectively, by the lines of bend or edges 16 and 17. 0063 The installation 100 comprises a bending jaw 103 intended to bend the flange 23 down in the direction of part 21 thanks to an effort represented by arrow F1. The jaw 103 comprises plate 103a whose length, parallel to a longitudinal axis X-X of the installation 100, is adapted to the length of the flange 23. The face 103a1 of the plate 103a intended to be in contact with the flange 23 is substantially planar. The plate 103a is equipped, on its face 103a2 opposite the face 103a1, with two ribs 103b extending opposite the flange 23 in directions substantially perpendicular to axis X-X. These ribs 103b each form a convex surface 103c in the form of a portion of cylinder with circular base centred on a virtual axis a. Virtual is understood to mean the fact that the axis a is not defined by a hinge, but as being the geometric focus joining the centres of curvature of the surfaces 103c. This axis is located inside the upright 4 and is parallel to axis X-X. 0064 The installation 100 also comprises a cradle 113 defining a cylindrical inner surface 113c and with circular base against which the ribs 103b are in sliding abutment by their surfaces 103c. The axis of symmetry of the surface 113a is also the axis a. The radii of curvature R103 and R113 of the surfaces 103c and 113c are equal. It is thus possible to pivot the jaw 103 in the cradle 113 about the axis a, as represented by arrow F2 in FIG. 4, by relative slide of the surfaces 103c and 113c. 0065 A second jaw 104 comprises a plate 104a intended to come into abutment, by a face 104a1, against the flange 24 and to exert thereon an effort F3 making it possible to bend it down in the direction of the base plate 101 and of the central part 21. The plate 104a is provided, on its face 104a2 opposite the face 104a1, with ribs 104b of which an outer or convex surface 104c is cylindrical with circular base and adapted to come into abutment against an inner or concave cylindrical surface with circular base 114c of a cradle 114. c denotes the common central geometrical axis of the surfaces 104c, this axis being parallel to axis X-X and located in the interior volume of the upright 4. 114c and R104 and R114 denote the respective radii of the surfaces 104c and 114c, these radii being of the same length. The jaw 104 may pivot about the axis c being guided by the cradle 114, as represented by arrow F2. 0066 Referring more particularly to FIGS. 4, 6 and 7, it will be understood that it is possible to shape the plate 10 in a tubular section with substantially triangular base thanks to efforts F1 and F3. 0067 However, from the position of FIG. 4, if the efforts F1 and F3 are exerted while the flanges 23 and 24 rest respectively against the plates 103a and 104a without any other effort, the plate 10 risks rising in the direction of arrow F4 in FIG. 4, because the flanges 23 and 24 risk sliding along the surfaces 103a1 and 104a1, as respectively represented by arrows F5 and F6. This might result in an imprecise bending at the level of the edges 12 and 13 of which the centre of curvature would in that case no longer merge with the geometrical axes a and c. 0068 In order to avoid relative movements between the flanges 23 and 24 and the surfaces 103a and 104a, the jaws 103 and 104 are each equipped with a heel 103f, 104f adapted to exert on the flanges 23 and 24 an effort, respectively denoted F7 and F8, directed substantially towards the edges 12 and 13, i.e. countering a slide of the flanges 23 and 24 in the direction of arrows F5 and F6. The efforts F7 and F8 therefore constitute efforts of clamping or of holding of the flanges 23 and 24 with respect to the jaws 103 and 104. In particular, the flanges 23 and 24 do not risk projecting radially outside the jaws 103 and 104. 0069 In other words, the heels 103f and 104f form stops to the displacement of the flanges 23 and 24 in the direction of arrows F5 and F6. 0070 It will be noted that the efforts F7 and F8 are exerted by the outside of the upright 4. In this way, the plate 10 is maintained on the apron 101 without using an added element, such as a holding clamp, this being favourable as a holding clamp would be delicate to place in position and to remove once the plate is bent. In addition, a holding clamp could not be extended up to the level of the upright 3. 0071 The invention therefore allows a bending of the plate 10 into a tubular structure without introduction of a wedging device inside its different sections. 0072 The heel 103f is in one piece with the plate 103a, this heel being intended to cover the border 27 of the flange 23. This heel 103f extends over the whole length of the jaw 103 parallel to axis X-X, which guarantees that the upper edge of the upright 4, on which the mask 5 is intended to be stretched, is bereft of irregularities. To that end, the heel 103 presents a regular surface towards the border 27 as the heel 103f serves for forming this border which must be as regular as possible for the mask 1, which is stretched on the upper edge of the upright 4, not to present a wave or element in relief capable of disturbing the image generated in the cathode-ray tube. 0073 The 104f, which extends over the length of the jaw 104, is capable of moving perpendicularly to the plate 104a and to the flange 24, as represented by arrow F9 in FIG. 4. 0074 Functioning is as follows: 0075 When the plate 10 has been positioned on the apron 101 and pre-bent thanks to the holding clamp 107, the jaw 104 is displaced by a pneumatic, electric or hydraulic jack, with the result that, thanks to the effort F3 exerted by the plate 104a, it bends the plate 24 down in the direction of the part 21. The heel 104f exerts on the flange 24 an effort F8 directed towards the edge 12, which guarantees a correct positioning of the flange 24 with respect to the jaw 104. The position of FIG. 6 is then attained, where the angle between the flange 24 and the part 21 is less than 90. From this position, the jaw 103 is actuated so that it bends the flange 23 down in the direction of the part 21 by exerting the effort F1, as shown in FIG. 7. The jaws 103 and 104 are guided in their displacements F2, and F2 thanks to the cooperation of the convex cylindrical surfaces 103c, 104c and concave ones 113c, 114c. 0076 In order to avoid an interference between the heels 103f and 104f, the heel 104f is moved away from the flange 24, as represented by arrow F9 in FIG. 7. In effect, it is possible to eliminate the effort F8 in the configuration of FIG. 6 as no displacement of the flange 24 is necessary between the configurations of FIGS. 6 and 7. Moreover, the fact that the angle is less than 90 ensures an efficient positioning of the flange 24, including in the event of elimination of the effort F8. 0077 In other words, in the configurations of FIGS. 6 and 7, the effort F8 is not indispensable as the plate 104a efficiently holds the flange 24 in position with respect to axis c. 0078 According to a variant embodiment of the invention, it is possible to provide for a plurality of heels 104f to be distributed over the length of the plate 104a, parallel to axes X-X and c. 0079 Reference will now be made to FIG. 15 which is an enlargement of the detail XV in FIG. 4. The plate 10 has a non-zero thickness e. The passage from the position shown in solid lines to the position shown in dashed and dotted lines, which corresponds substantially to that of FIG. 7, has the effect of reducing the height h of the flange 23 in the configuration in dashed and dotted lines with respect to the length l of this flange in the solid line configuration as the neutral axis f of the plate 10 forms an arc of circle centred on the axis a in the configuration shown in dashed and dotted lines. If it is considered that axis a is located inside the zone intended to form the edge 12 with a distance d with respect to the neutral axis f and taking the example of a plate curved from a planar configuration to arrive at a configuration bent at 90, the difference between the height h and the length 1 is d. In the present case, it is less insofar as the edge 12 is already pre-marked in the configuration of FIG. 4. However, it is not zero. 0080 Taking into account this difference in values between the height h and the length l, the value of the effort F7 should be adapted so that it efficiently immobilizes the plate 10 in the zone of the edge 12 all along the movement of rotation of the jaw 103 about axis a. To that end, the heel 103f is provided to be elastically loaded in the direction of axis a or of the edge 12. In that case, the heel 103f is not in one piece with the plate 103a. According to another approach, a cam system may be provided to allow the intensity of the effort F7 to be varied as a function of the angular orientation of the jaw 103 about the axis a, in the course of the pivoting F2. 0081 According to another approach, it is possible to compensate this difference in value between the height h and the length l by offsetting the pivot axis a in a direction perpendicular to the principal plane of the apron 101. In that case, the heel 103f may be in one piece with the plate 103a of the jaw 103. 0082 Of course, the heel 104f may also be provided with means for varying the intensity of the effort F8 as a function of the position of jaw 104 about axis c in the course of pivoting F2. In a variant, the position of the axis c may also be provided to be variable in order to compensate the variations in length and/or height of the flange 24. 0083 Referring more particularly to FIGS. 8 to 10, it is noted that the lateral upright 3 is of quadrangular section. A jaw 105 is provided to cooperate with the flange 25 and comprises a base plate 105a of which one face 105a1 in contact with the flange 25 makes it possible to exert an effort F11 to bend the flange 25 down in the direction of elements 21 and 101. Like jaw 103, the jaw 105 is equipped with ribs 105b defining cylindrical convex surfaces 105c centred on the axis a and making it possible to cooperate with a cradle 115 defining a concave surface 115c for slide of the surfaces 105c, the surface 115c being, like surfaces 105c, of circular base and centred on axis a. The jaw 105 is equipped with a plurality of heels 105f distributed over its length parallel to axis X-X, these heels being provided to come into contact with the flange 25 at the level of edge 17, constituting a stop for a possible slide of the flange 25. The heels 105f exert on the flange 25 a clamping effort F12 directed towards the edge 12 and parallel to panel 25a. The effort F12 has the same function as the efforts F7 and F8 evoked previously. It makes it possible to efficiently position the flange 25 with respect to the axis a all along the operation of bending effected thanks to the effort F11. 0084 In the same way, a jaw 106 is provided to cooperate with the flange 26 and comprises a plate 106a forming a face 106a1 for abutment against the flange 26 and equipped with ribs 106b whose convex outer radial surfaces 106c cooperate with a concave surface 116c of a cradle 116. The surfaces 106c and 116c are centred on a geometrical axis or virtual axis b constituting the centre of the edge 15 and located, like axis a, in the interior volume of the upright 3. 0085 The jaw 106 is equipped with heels 106f making it possible to retain the flange 26 in position with respect to the plate 106a in the course of bending of the edge 15. The plate 106a is not planar but forms a concave zone for receiving the panels 26a and 26b, the plate 106a itself being formed by two panels oriented one with respect to the other with an angle corresponding to the angle of orientation of the panels 26a and 26b with respect to one another. The heels 106f are disposed in order to come into abutment against the free edge 26c of the panel 26b. 0086 The heels 106f are distributed over the length of the plate 106a. 0087 The heels 105f on the one hand and 106f on the other hand, are positioned in quincunx with respect to each other, being offset along the axis X-X, a heel 105f being opposite a free gap 106g defined between two heels 106f, while a heel 106f is disposed opposite a free gap 105g defined between two heels 105f. 0088 The bending of the upright 3 may be explained with reference to FIGS. 8 to 10. In the configuration of FIG. 8, an effort is exerted on the jaw 105 by any appropriate means, with the result that it is displaced and exerts the bending effort F11 of the flange 25 in the direction of the part 21, the heels 105f overlapping the zone of join 25c, defined between the panels 25a and 25b and which includes the edge 17. The effort F11 makes it possible to bend the flange 25 down as far as the position of FIG. 9 where the panel 25a is substantially perpendicular to the part 21, the panel 25b being substantially parallel to this part. In the course of the bending operation made beween the configurations of FIGS. 8 and 9, the effort F12 efficiently retains the flange 25 in position with respect to the jaw 105 and to the edge 12. 0089 The jaw 106 is in that case activated, in order to pass from the position of FIG. 9 to that of FIG. 10, an effort F13 of bending of the flange 26 in the direction of the part 21 being exerted by the plate 106a while an effort F14 is exerted by the heels 106f on the flange 26 parallel to this flange and substantially in the direction of the edge 15. 0090 As previously, the efforts F12 and F14 may be modulated as a function of the orientation of the jaws 105 and 106 about axes a and b. In a variant, the axes a and b may be offset with respect to their representation in FIGS. 8 to 10 in order to compensate the variations in length of the flanges 25 and 26. 0091 In addition, in the configuration of FIG. 10, the effort F12 may be released or reduced by a movement of the heel 105f in a sense of moving away with respect to the edge 12, with the result that, due to the elasticity of the plate 10, the panel 25b comes into firm elastic abutment against that face of the panel 26b oriented towards the part 21. It is in that case possible to spot weld the panels 25b and 26b thanks to a high energy beam directed between two adjacent heels 105f and 106f. 0092 In order to attain the elastic abutment of the panels 25d and 26d, it is also possible to provide for the angle of bend between the panels 25a and 25b to be slightly more open than that obtained in the configuration of FIG. 10, with the result that an elastic abutment is obtained in this configuration without it being necessary to release the effort F12. 0093 As is more clearly visible in FIGS. 11 to 14, the frame 2 is conformed, after shaping of the different uprights 3, 3, 4 and 4, by bending. When the tubular uprights have been made, the plate 10 extends substantially in the direction X-X in four sections joined by zones of join 8, 8 and 8. A tool 20 is provided for shaping the frame 2 and comprises four flaps 201, 202, 203 and 204 intended each to receive one of the uprights 3, 3, 4 or 4. 0094 These flaps 201 to 204 are each formed by two massive pieces 201a, 201b, 202a, 202b, 203a, 203b, 204a, 204b joined by a screw/nut system 201c, 202c, 203c or 204c for adjustment of the flaps 201 to 204 in length. Guide rods 201c, 202c, 203c and 204c are associated with the systems 201c to 204c. The length of each flap 201 to 204 is adjusted, parallel to its largest dimension X1 to X4, to the length of the upright that it must receive. The flaps 201 to 204 are provided to be adjustable in length in order to allow frames 2 of different sizes to be manufactured. 0095 The pieces 201a and 201b are respectively provided with angles 201d, 201e for receiving the upright 4. Jaws 201f, 201g are provided to immobilize the upright 4 against the angles 201d and 201e. In the same way, the flaps 202, 203 and 204 are respectively equipped with angles 202d to 204e and with jaws 202f to 204g. As is more particularly visible in FIGS. 11 and 12, the jaws are shaped as a function of the geometry of the flanges 24 and 26. 0096 The flaps 201 and 202 are articulated together about an axis Y1 essentially perpendicular to the principal direction X-X of the uprights 3, 4, 3 and 4. The axis Y1 is also perpendicular to the direction X1 along which the flap 201 is adjustable in length. In the same way, the flaps 202 and 203 are articulated together about an axis Y2 while the flaps 203 and 204 are articulated together about an axis Y3, these axes Y2 and Y3 being substantially perpendicular to axis X-X and to the directions X1, X2, X3 and X4 for adjusting the different flaps in length. 0097 Three jacks 205, 206 and 207 respectively join the flaps 201 and 202, the flaps 202 and 203 and the flaps 203 and 204, with the result that they are adapted to exert thereon efforts of closure represented by arrows F16, F17 and F18 in FIG. 14. These efforts make it possible to take the tool 20 into a substantially rectangular configuration in a plane perpendicular to axes Y1 to Y3, which makes it possible to bend the frame 2 at the level of the zones 8, 8 and 8 and to form corners such as corner 7, as represented in FIGS. 12 and 13. In effect, the effort F17 has the effect of bringing together the sides of the flaps 202 and 203 joined by the jack 206, which results in a bending of the plate 10 along a line 18 parallel to axis Y2. The tongues 11 arranged on the upright 4 are thus made to overlap the edge 3a of the upright 3, these tongues and this edge in that case being able to be welded by a high energy beam. 0098 In the configuration of FIG. 14, the tool 200 forms a rectangle whose flaps 201 to 204 constitute the four sides. This rectangle is closed at the level of a zone of join between the flaps 201 and 204 where there are provided locking means (not shown) associated with guiding means, likewise not shown. In this way, the rectangle shown in FIG. 14 is defined precisely by the cooperation of the flaps 201 to 204, with the result that the geometry of the frame 2 thus obtained is also defined precisely. 0099 According to an advantageous aspect of the invention shown solely in FIG. 16, the jaw 106 is equipped with a blade 106h forming shears with the plate 101 and making it possible to cut out, during the bending of the upright 3, the plate 10 in the zone of join of the uprights 3 and 4 so as to create a notch 19 visible in FIG. 11 compatible with the movement of bend about axis Y2. Due to the use of the blade 106h, the notch is created after the operations of shaping the tubular uprights 3, 3, 4 and 4. Before the use of this blade, the plate 10 presents a continuity in the zones 8, 8 and 8 of join between the uprights 3 and 4. In other words, the incorporation of the blade 106h on the jaw 106 enables the notch 19 to be made just before the use of the tool 200, which avoids too great stresses at the level of the line 18 during production of the tubular parts. In practice, the blade 106h may be with double cutting edge, with the result that it makes it possible to cut out the plate 10 along two parallel lines 28 and 29 defining therebetween the notch 19 up to a bore 30 previously made in the plate 10. Of course, the jaws 103, 104 and 105 may also be equipped with blades forming shears. 0100 As a function of the geometry of the frame 2, i.e. as a function, in particular, of the length of the uprights and of their cross-section, different sets of jaws 103 to 106 and of cradles 113 to 116 may be used, the jaws and the cradles being placed in position jointly in the installation 100, which makes it possible to effect a rapid standard exchange of the assembly of the bending members of this installation. 0101 The use of the jaws 103 to 106 that may slide on cradles 113 to 116 of different geometrical axes, such as axes a and c, makes it possible to produce non-aligned edges, such as the edges 13 and 15, which would not be possible with conformation jaws articulated on common hinges. 0102 In the Figures, the jaws 103 and 105 and the cradles 113 and 115 have been shown as two distinct units. However, it might be question of the same unit forming bending member at the same time for uprights 3 and 4, as the axis of bend a is the same for the whole edge 12 over the length of the plate 10. This is why the plates 103a and 105a, on the one hand, and the cradles 103d and 105d, might be constituted in one piece. 0103 When the plate to be bent is thin and in order to avoid a phenomenon of buckling of its flanges under the effect of the clamping efforts, it may be provided to apply the metal sheet against the plates 103a to 106a of the jaws 103 to 106 by suction or by magnetic attraction. To that end, the jaws 103 to 106 may be provided with channels opening out on the faces 103a1, 103a2, 105a1 and 106a1 and connected to a source of vacuum. The jaws may also be equipped with permanent magnets or with electro-magnets allowing an immobilization of the flanges 23 to 26. 0104 As shown in FIG. 17 for an installation according to a second form of embodiment of the invention, the jaws, such as jaws 104 may be guided by cradles, such as cradle 114 thanks to telescopic circular segments 1041 and 1042 which make it possible, for a given geometry of a cradle 114, to obtain an efficient guiding for a movement F20 of pivoting of the jaw 104 about axis c of high amplitude. The segment 1041 is fast with the jaw 104 while it is fitted inside the segment 1042, this segment 1042 itself being mobile in abutment against the inner cylindrical surface 114c of the cradle 114. 0105 The segment 1042 is hollow while segment 1041 is solid and of width less than the width of the inner volume of the segment 1042. 0106 The segment 1042 is provided with rollers 1042g disposed along its inner surface 1042h and adapted to cooperate with the edge 1041c of the segment 1041. These rollers facilitate the relative slide between the segments 1041 and 1042. Moreover, the edge 1042c of the segment 1042 is also provided with rollers 1042i adapted to cooperate with the inner surface 114c of the cradle 114. This facilitates pivoting of the segment 1042. 0107 According to variants of the invention (not shown), the segment 1041 and/or the cradle 114 may be equipped with balls or rollers for slide. 0108 The invention has been described, with reference to the first form of embodiment, with jaws equipped with ribs of which the end surfaces 103c, 104c, 105c and 106c are provided to slide against the cylindrical surfaces 113c to 116c of the cradles 113 to 116. However, the ribs 103b to 106b may be equipped with runners incorporating balls or rollers, such balls or rollers being provided to roll on the surfaces 113c to 116c. 0109 Other modes of guiding the jaws by the cradles may be envisaged. 0110 As is more particularly visible in FIGS. 18 and 19, the angles 201d to 204e and the jaws 201f to 204g of the first form of embodiment may be replaced by profiles of cross-section substantially in U-form, of which two are shown in FIG. 18, with references 202m and 203m. These profiles have a cross-section suitable for receiving the uprights 3 and 4respectively, being fixed on flaps similar to flaps 202 and 203 of the first embodiment. 0111 Shims 202p and 203p are respectively associated with the sections 202m and 203m for wedging the uprights 3 and 4 in position. These shims have a substantially triangular section with a truncated angle, i.e. in fact a trapezoidal section. When the uprights 3 and 4 have been placed in position in the profiles 202m and 203m, the shims are introduced in the profiles as represented by arrows F20, the geometry of the shims 202p and 203p being such that they are each provided with a surface 202g, 203g adapted to rest against certain flanges 24 or 26 of the profiles 3 and 4. 0112 Each shim 202p or 203p is provided with a blind hole 202r, 203r for receiving a pin 202s, 203s provided to pass through an orifice 202t, 203t of the profiles 202m and 203m. The introduction of the pin 202s in the orifice 202t and the blind hole 202r is represented by arrows F21. Once the pins 202s and 203s are in place in the holes and orifices 202r, 202t, 203r and 203t, the shims are immobilized on the profiles 202m and 203m and maintain the uprights 3 and 4 in place. 0113 The invention has been shown with jaws whose inner surfaces 103a1 to 106a1 are planar while the flanges 23 to 26 are also planar. Of course, these surfaces and these flanges may be skew as a function of the geometry desired for the uprights 3, 3, 4 and 4. 0114 The invention has been shown when employed for manufacturing shadow mask support frames. Such frames may be made in one, two or four parts, each part forming one, two or the four uprights of the frame.