Method for assembling three-dimensional metal structures, machine for the manufacturing thereof, and structures obtained with such a method
This is a divisional of copending application Ser. No. 725,655, filed on Apr. 22, 1985, now U.S. Pat. No. 4,667,707. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method for assembling three-dimensional metal structures, for example for pre-manufactured elements such as panels or ceilings to be used for building. The invention further pertains to apparatus for carrying out such a method, and finally to the structure made with said method. 
     A three-dimensional wire structure is known which comprises a series of plane nettings. Each netting is provided with at least one pair of lengthwise wires and brace wires. The nettings are spaced apart a pre-determined distance by a series of cross wires welded to the nettings and to brace wires. Such a wire structure forms bearing planes for elongated bodies with a corresponding size, made from a light insulating material which is arranged inside the wire structure. The manufacturing of such structures requires a very small tolerance for the various components and a careful alignment between said parts during the welding steps. To fulfill said requirements, many hand operations are necessary to bring the welding units to the appropriate areas where the wires to be welded cross, and to retain the alignment of said parts. Such a method is thus unavoidably costly. Moreover, it is very difficult to obtain simultaneously the alignment of the various structural portions. Severe problems have also been encountered when manufacturing said structures, particularly with the reliability of the welds and the resistance of the structure to static and dynamic stresses. 
     SUMMARY OF THE INVENTION 
     The present invention pertains to an assembly method which is simple and relatively inexpensive for accurately assembling a three-dimensional wire netting having a high stress resistance. The method according to the invention comprises the following steps: 
     (a) preparing a series of plane nettings comprising lengthwise and brace wires welded together; 
     (b) arranging the nettings according to a given crosswise pitch; 
     (c) aligning the nettings to locate corresponding brace wires of the different nettings in the same plane; 
     (d) aligning the lengthwise wires to define corresponding planes of the structure; 
     (e) arranging at least one cross wire in a position where said wire crosses the lengthwise wires or the brace wires of the nettings in different areas of crossing with said lengthwise and brace wires, respectively; 
     (f) positioning at least one welding unit in that area where the nettings cross the cross wire, in such a way that the electrodes of said welding unit face the crossing wires in the crossing area; 
     (g) engaging the crossing wires and said electrodes to weld the cross wire to the netting; 
     (h) moving the welding unit relating to the nettings over a distance which is equal to the cross-wire pitch, while retaining the brace wires in the same plane; 
     (i) repeating steps (f) to (h) for a new cross wire and a new crossing area until the cross wires are welded to the lengthwise or brace wires of the wire structure. 
     According to another feature of the invention, the assembly of the nettings which will form the three-dimensional structure comprises the following steps: 
     (a) preparing straight wires from drums, by straightening and simultaneously twisting said wires; 
     (b) arranging a lengthwise wire group in holders to bring said wires in parallel relationship with one another; 
     (c) welding the lengthwise wires to a first brace wire, in such a way that said brace wires substantially lie in the same plane as said lengthwise wire; 
     (d) feeding the lengthwise wires relative to said holders, over a distance equal to a brace wire pitch, to define marking section; 
     (e) arranging a second brace wire in such a way that said wire crosses the marking section of said lengthwise wires, and in such a way that said second brace wire lies at an accurately-defined spacing from the first brace wire; 
     (f) welding said second brace wire to the lengthwise wires in the respective crossing areas; 
     (g) repeating steps (d) to (f) until all the netting welds have been made. 
     The three-dimensional structure which is obtained with the method according to the invention has a marked resistance to stresses, due to the twisting of the wires and the accuracy of the welding. Said structure has for further feature the accuracy in the planes of the various brace wires, to form very small spaces between the elongated bodies which are located inside the structure proper. Moreover, the assembled structure has a very high therefor &#34;degree of accuracy, ensuring an accurate arrangement of the panels inside the structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other details and features of the invention will be apparent from the following description, given by way of non limitative example and with reference to the accompanying drawings, in which: 
     FIG. 1 is a general diagrammatic view of assembly machines being used according to method of the invention. 
     FIG. 2 is a perspective view of an assembly machine as shown in FIG. 1. 
     FIG. 3 is a partial plan view of the machine as shown in FIG. 2. 
     FIG. 4 is a side view of the machine as shown in FIG. 3, in working condition. 
     FIG. 5 is a front view of certain details of the machine as shown in FIG. 2. 
     FIG. 6 shows details from FIG. 5, during an operating step. 
     FIG. 7 is a side view of elements of the machine as shown in FIG. 2. 
     FIG. 8 shows details from FIG. 7, during an operating step. 
     FIG. 9 is a diagrammatic view along section line IX--IX in FIG. 4. 
     FIG. 10 is a diagrmmatic view showing additional details of the machine as shown in FIG. 2. 
     FIG. 11 is a diagrammatic view of another detail from the machine as shown in FIG. 2. 
     FIG. 12 is a plan view of a variation in a detail of the machine as shown in FIG. 2. 
     FIG. 13 is a diagrammatic side view of the variation as shown in FIG. 12. 
     FIG. 14 shows the element of FIG. 12 during operation of the machine. 
     FIG. 15 shows an operating flow-chart of the machine as shown in FIG. 2. 
     FIG. 16 shows a schematic diagram of the complete machine as shown in FIG. 2. 
     FIG. 17 is a diagrammatic plan view of a second machine of the assembly shown in FIG. 1. 
     FIG. 18 is a diagrammatic side view of the machine as shown in FIG. 17. 
     FIG. 19 is a schematic diagram of the machine as shown in FIG. 17. 
     FIG. 20 is a general schematic diagram of the operations provided with the method according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The assembling method according to the invention is intended for the manufacture for example, but not exclusively, of three-dimensional metal structures 30 (FIG. 2) of that kind as described in European patent application No. 82102021 published on Sept. 29, 1982. Said method is particularly useful to assemble plane nettings 36 made from steel wires with cross wires 37, and provides preferably for the use of a machine 40 allowing assembling the three-dimensional structure. 
     The nettings 36 comprise lengthwise wires 34 and brace wires 35 that are connected together by means of a flat welder 38 (FIG. 1) prior to assembling the three dimensional structure 30. 
     The assembly machine 40 (FIG. 2) comprises a support structure 41 which bears the nettings 36, a supply group 42 from which the cross wires 37 are taken to be welded to the nettings 36, a series of welding units 43, and a feeding group 45 for said structure 30, during the assembly, and a receiving unit 50 (FIG. 1). 
     The machine is further provided with an electronic group 57 (FIG. 1) for sequentially controlling the various assembly and welding steps, a control panel 58, an air unit 59 for performing the commands sent by unit 58, and a cooling unit 60 for cooling welding electrodes in the assembly machines 38 and 40. 
     The following description pertains to the supporting structure. 
     The supporting structure 41 comprises an upright array 79 on which a series of pairs of cross beams 80 are secured at regular spacings. Said cross beams 80 form a series of horizontal superimposed bearing planes 81. The spacings between the planes 81 are equal to one another and define the cross-wire pitch of the plane nettings 36 relative to the already assembled structure 30. 
     The planes 81 (FIG. 3) have an elongated rectangular shape and are provided with two transverse sides 82 and 83, respectively on the left and right as viewed in FIG. 3, and they bear a corresponding plane netting 36. 
     A control element 84 comprised of an extruded part with a rectangular cross-section, is secured to each side 82 and is provided with a control surface 85 which can cooperate with the ends, on the left-hand side of the figure, of the brace wires 35 from the nettings 36. The surfaces 85 of the control elements 84 from the various planes 81 are located in the same plane, and define a vertical marking surface, which represents a side surface plane for the three-dimensional structure 30. 
     Another control element 86, also commprised of an extruded part with a rectangular cross-section, is adjustably secured to side 83 of each plane 81, and is provided with a control surface 87 which can cooperate with the ends, on the right-hand side of the figure, of the brace wires 35 from the nettings 36. The surfaces 87 of the control elements 86 lie in the same plane and define another vertical marking surface for the three-dimensional structure 30, with this marking plane being parallel to the side reference plane defined by surfaces 85. Both of these marking planes are designed to bring the lengthwise wires 34 in the same plane, on the same side of the various nettings 36, and the marking planes lie at right angle to the planes of nettings 36. As shown in FIG. 3, alignment means such as guide elements 92 and 93 are secured at one end, such as the front end, of control elements 85 and 86. Elements 92 and 93 are formed by extruded parts with a U-shaped cross-section, projecting outside the planes 81. Said elements cooperate with the ends of the brace wires 35 as well as with the lengthwise wires 34 from the plane nettings 36, and they serve to define accurately the vertical marking planes of structure 30 and to prevent warping of the nettings 36 in that area next to the outlet end of the guide elements 92 and 93, adjacent to the welding units 43. 
     The control elements 87 and guide elements 93 are movable relative to the sides 83, for example by means of screws 94. In such a way, the spacing between the marking planes of the surfaces 87 and guide elements 92 may be changed either relative to the marking plane of surfaces 85, or relative to the guide elements 93, to define accurately the vertical marking planes of different structures 30 using different nettings 36 of different widths. 
     A supply group follows the supporting structure. Said supply group 42 is divided into two units, each one comprising a hopper 101 (FIG. 11), holding the wires that become brace wires 37. The wires are pulled by gravity into outlet channel 102 (FIG. 11), and eccentric 103 is provided to facilitate this movement of the wires. A device 104 is provided so that the wires 37 fall one by one, and the wires are guided by slanting rods 105 and stop at the base of said rods 105 by engaging shoulders 106. The presence of wire 37 retianed against said shoulders 106, is sensed by a magnetic sensor 107, which conducts an electric signal to control unit 57. 
     Between support 41 and hoppers 101 (FIG. 3), two gripping arms 111 are provided, each one being formed by a parallelpiped-shaped bar with a bearing point at the one end, and an output shaft 112 of an air driving member 113. Said driving member 113 is provided to rotate the gripping arm from a horizontal position to a vertical position. Each gripping arm 111 in the horizontal position thereof, is substantially aligned with and superimposed on wire 37, which is stopped for the time being against shoulders 106 and adjacent the wire proper. A number of magnets 114 are so arranged as to move the wire 37 away from the shoulders 106 and to retain the wire on arm 111 aligned with the axis of the arm. 
     Said position of wire 37 relative to arm 111 remains unchanged even when arm 111 lies in a vertical position. Two sensors 115 and 116 are provided to sense, respectively, the horizontal position and the vertical position of said arm 111, and to convey the information to control unit 57. 
     Facing hopper 101 (FIGS. 2 and 3), two air actuators 121 are arranged, provided with pistons 122 movable in a horizontal plane along a direction at right angle to the marking planes of surfaces 85 and 86. To said pistons 122 are fastened two corresponding support blocks 123 to which are secured in turn two other air actuators 124. Said actuators 124 are provided with pistons 25 which are horizontally movable along a direction slanting at an angle of 45° relative to the marking planes of surfaces 85 and 86. Said pistons 125 include two uprights 126 to which two corresponding series of air-actuated clamps 127 are fastened, which can move the wires 37 away from the arms 111 and retain said wires in parallel relationship with said uprights 126. Sensors 128 are provided to sense the presence of one or a plurality of wires 37 retained by said clamps 127. The supports 123 may be moved away from the pistons 122 from side positions adjacent to the hoppers, to center positions adjacent to the welding units 43 and guide elements 92 and 93. Uprights 126 may in turn be moved away from said pistons 125 from positions removed from hoppers 101 and guide elements 92 and 93 towards areas next to said arms 111 and welding units 43. 
     End sensors 131 and 132 for the actuators 121, sense respectively the side and center positions of blocks 123, and end sensors 133 for the actuators 124 sense the positions of uprights 126 adjacent to the arms 111 and welding units 43. Moreover, magnet sensors 134 sense the presence of wires 37 on said uprights 126, as they are retained by clamps 127. The informations from the sensors 134 are also conveyed to control unit 57. Said hoppers 101 are so arranged with suitable partitioning elements, to each receive two series of cross wires 37, with a length which is slightly shorter than the maximum width of the hopper proper. The slanting rods 105, the gripping arms 111, the uprights 126, the clamps 127, and the various sensors may handle simultaneously two wires 37 aligned and next to one another. This allows the simultaneous assembly of two three-dimensional structures each of which have a height slightly less than half the height of a structure with the maximum height. 
     The welding units 43 (FIG. 3) are divided into two groups which are respectively mounted on two plates 145 and 146. Said plates are slidably mounted on vertical uprights 147 which are arranged to the left and right of guide elements 92 and 93, in such a way as to have each pair of elements 92 and 93 be associated with one pair of welding units 43. 
     Each unit 43 comprises a body 151 in the shape of a hollow parallelepiped, on which a transformer 152, an air actuator 153, a movable electrode 154 and an opposing electrode 155 are mounted. 
     The movable electrode 154 is secured to a piston 157 from actuator 153 which is in turn guided by sleeves that insulate the piston from body 151. The opposing electrode 153 has an L-shaped body and is electrically connected to body 151. Transformer 152 is fitted partly inside the parallelepiped-shaped body 151 and it is provided with a primary winding the terminals of which may be connected to a source of electrical energy. The secondary winding of said transformer is provided with two terminals 159 and 160 connected to electrodes 154 and 155. Terminal 159 is directly connected to electrode 155, while the connection between electrode 154 and terminal 160 is obtained by means of a series of thin copper leaves, bent into a U-shape, which allow the movements of piston 157 relative to said transformer 152. The active portion of each movable electrode 154 (FIG. 5) as shown in 163, is of cylinder shape and lies higher relative to piston 157, said portion being connected thereto by a block 164. The active portion of each electrode 155, shown in 165, is in the shape of a parallelepiped and projects upwards of electrode 155. Inside blocks 164 of electrodes 154 and 155 pass cooling ducts, which are provided with small inlet and outlet openings 166 and 167, connected to the cooling unit 60. 
     In rest condition, portions 163 and 165 of units 43 lie underneath the planes of nettings 36, and each plate 145, 146 is vertically movable relative to the uprights 147. An actuator 170 moves the welding units 43 upward to bring portions 163 and 165 in alignment with the planes of nettings 36. Moreover, two end sensors 168 and 169 (FIGS. 5 and 6) may sense respectively the high and low positions of said units 43. In the variation as shown in FIGS. 12 and 13, the welding units 43 are mounted on two fixed plates 180. Each opposing electrode shown in 171, is provided with a lever arm 173 and swings about a sleeve 172 in parallel relationship with piston 157. The active portions 163 of said movable electrodes 154 are lined up with the planes of the various nettings 36, while the active portions of electrodes 171 lie underneath said planes. The arms 173 swing about a single vertical link 174, which is in turn connected to an air actuator 175. 
     The actuator 175 is so designed as to swing the active portions 164 into alignment with the planes of nettings 36. The sensors 168 and 169 sense in such a case, the respective high and low positions of said electrodes 171. 
     Assembly 45, the so-called feeding assembly (FIGS. 3 and 7), comprises an air actuator 182 provided with two pistons 183 which are movable in parallel relationship with said guide elements 92 and 93. On said pistons 183, a vertical rod 184 is mounted, to which L-shaped horizontal arms 185 are fastened and arranged in the space lying between the planes of nettings 36. Said arms 185 have a lengthwise portion 186 lying in the mean plane between the vertical marking planes defined by said guide elements 92 and 93. 
     On each portion 186, by means of a pivot 187, a toothed lever 190 swings, which is provided at the bottom with a front tooth 191 and a back tooth 192. In a rest position, said lever is retained in horizontal position by gravity and against the action of a stop element 188 depending from said portion 186. Each tooth 191, 192 comprises in the front portion thereof, a corresponding substantially vertical engagement edge 193, and in the back portion thereof, a slanting edge 194. The engagement edges 193 of teeth 191 and 192 are aligned among them on two vertical planes the spacing of which is slightly wider (by about 1 mm for wires from 0.6-0.7 mm) than half the pitch of brace wires 35 and nettings 36. The actuator 182 is so designed as to move the rod 184 and thus levers 190 over a distance equal to half the pitch of said brace wires 35. A pair of end sensors 197 and 196 sense the end limits of the stroke and conveys the informations to said control unit 57. The control edges 193 are designed to cooperate with said brace wires 35 to move the nettings 36 together, while retaining the same plane for the brace wires in the various nettings 36, either before the assembly of structure 30, or during the movement of nettings 36, during the welding operations. 
     More particularly in the rest position, the control edges 193 of teeth 191 or 192 lie adjacent and behind the brace wires 35 of nettings 36. Consequently, during a forward movement of said pistons 183, the edges 193 of teeth 191 or teeth 192, from levers 190 move the nettings 36 towards the front part of the machine, and this is done over a piston stroke equal to half the pitch of the brace wires 35. 
     During the return stroke, the brace wires 35 engage the slanting edges 194, causing the toothed levers 190 to raise, so that those levers may return to a position with the edges 193 behind the brace wires 35 for a new feeding of the nettings 36. During both forwards and backwards movement of levers 190, the teeth 191 and 192 act only once, and on the same brace wire 35. Consequently the cross wires 37 are spaced apart a pitch which is substantially equal to half the pitch of the brace wires 35 in the already assembled structure 30. 
     With reference to FIG. 1, it will be noted that the receiving unit 50 comprises a reversing plane 201 provided with a base 202 for collecting the already assembled structure, and at least one area 203 which can receive a bracket for a second structure, when the assembly machine 40 assembles simultaneously two structures with a reduced height. 
     Unit 57, the so-called control unit 57, comprises a microprocessor 210 (FIG. 16) provided with a series of input-output interface units. The input interfaces receive the data from the sensors sensing the presence of wires and the sensors sensing the stroke end of the actuators; the output interface units are connected to relays, possibly to solid-state switches which control the opening or closing of valves 212-219, arranged between the pressurized air circuit 225 from air unit 59, and actuators 104, 113, 122, 124, 127, 182, 169 or 175, respectively, as well as all the actuators 153 of welding units 43. 
     The microprocessor is provided to operate a power unit 226 which is connected to the source of electrical energy, the primary windings of transformers 152 in units 43. The microprocessor 210 is provided with a programme which controls the performance of the various electro-valves in a pre-determined sequence and depending on the condition of the various sensors. Said microprocessor is moreover connected to a series of adjustment members to allow for a variation in the time intervals of the weldings. 
     The assembling method is shown diagrammatically in FIG. 20, and at steps 211 and 212, the wires 37 are collected from hoppers 101 and the nettings 36 (FIG. 3) are positioned on the bearing planes 81. The lengthwise wires 34 are thus engaged in the various guide elements 92 and 93, until brace wires 35 from the first series lie in front of the control edges 193 of teeth 191. 
     The machine 40 is so designed as to work with two wires 37, or four wires in the case where two structures are being assembled, which wires are already positioned on the arms 111 and this in a vertical position. The blocks 123 lie in the respective sidewise positions and the uprights are removed from the arms 111. At this stage, control unit 57 is ready to start assembling the structure 30. By pressing the start key, the actuators 183 are started, which move the respective levers 190 in the direction of the machine front part, thus causing the not yet assembled neetings 36 to be moved simultaneously. Consequently, as the brace wires 35 from the first series are suitably located in one and the same plane, the lengthwise wires 34 are brought to the respective welding areas. 
     After moving forwards the nettings 36 are sensed by sensors 198, said control unit 57 operates the actuators 124 to bring the uprights 126 adjacent arms 111. Said unit 57 controls the closing of clamps 127 on wires 37, and the transfer of the wires to uprights 126. This condition having been sensed, said unit 57 provides for the operation on the opposite side, of actuators 124, by moving away the uprights 126, and thus the wires, away from said arms 111. Unit 57 starts the actuators 121 working to move the supporting blocks 123 towards the welding area. 
     Said control unit 57 after sensing by means of sensors 131, the new position of the elements, again operates the actuators 124 (FIG. 14), to bring the wires 37 directly adjacent the lengthwise wires 34. As the pistons 125 move at an angle of 45° relative to the electrode axes and wires 34, the wires 37 and uprights 126 may move freely without hampering said parts. The new position of wires 37 is sensed by sensors 133. Said unit 57 causes at this stage, raising of plates 145 and 146, as well as of all the welding units 43, or the vertical links 174 (FIG. 13) with the single electrodes 171, until the active electrode portions 163 and 165 (FiG. 5) are lined up with the lengthwise wires 34 and lie co-axial with the crossing areas of the cross wires 37 and lengthwise wires 34. Said unit 57 further provides for the working of the actuators 113 to bring the arms 111 to a horizontal position, to collect another pair of wires 37 (or four wires in the case of two structures) from hoppers 101. 
     During the following step, said unit 57 provides for the control of all the actuators 153 in all the welding units 43. Consequently, the movable electrodes 154 bring the crossed wires 34 and 37 into engagement with respective opposing electrodes. Said unit 57 thus provides for the powering of the primary windings of transformers 152 and the welding of said wires 34 and 37 in the respective crossing areas. While the electrodes are still retaining the wires 34 and 37, unit 57 starts the actuators 182 for a return stroke towards the back machine part. The levers 190 are thus brought backwards and the back teeth 192 thereof, project slightly rearward of the brace wires 35 of the second series. The new position of said levers 190 having been sensed with the sensors 197, said unit 57 causes the opening of the electrodes and clamps, and causes the actuators 180 or 175 to move in the opposite direction, to remove the electrodes from the path of the brace wires 35. Said unit 57 also returns the arms 111 to the vertical position. Finally, unit 57 removes the uprights 126 and blocks 123 from the welding area, thus returning the machine to the original position thereof. 
     FIG. 3 shows in dot-and-dash lines, the position of blocks 123 adjacent the welding area, the clamps 127 being shown open and away from the welding area. The extension positions of toothed levers 190 and nettings 36 are shown in the same way. FIG. 4 shows the position of actuators 121 and 124 for the welding step, the wires 35 being aligned by toothed levers 190. FIG. 8 shows the positions of the units 43 during the welding step, and in dot-and-dash lines, that step where teeth 192 are pulled rearward over wires 35. 
     FIG. 15 shows diagrammatically, relative to time, the flow of welding current 221, the movement of the movable electrodes 154 relative to the opposing electrodes 157 or 171, and the movement of units 43 or electrodes 171. Said diagram also shows the movements of clamps 127, actuators 122 and 123, wire 37 on arms 105, as well as the movements of arms 111. 
     The following description relates to the plane assembling machine. 
     The lengthwise wires 34 before welding thereof to brace wires 35, are unwound from drums 240 (FIG. 1) with high capacity, and they are first straightened by means of a wire-straightening machine 241. Said known machine will not be further detailed here. It is generally provided with a series of straightening rollers and counter-rollers 242 which straighten the wires. 
     During the straightening, the wires are stiffened under the action of the unwinding and distortions due to the twisting imparted during said step. 
     The assembly machine 38 for the nettings 36 comprises an elongated frame 243 with an inlet area and an outlet area 244. The frame 243 (FIG. 17) is provided with a series of cross-beams 246 which can retain the lengthwise wires 34. Said lengthwise wires 34 are in turn retained summarily at predetermined mutual spacings by suitable guides 247 which are movable along said cross-beams. In the middle area of said frame 243, carefully positioned alignment holders 248 are moreover provided, which have such a length that the lengthwise wires 34 will come to lie in substantially parallel relationship with one another in a marking area next to the holder area proper. The wires 34 lie substantially in the same plane; the mutual spacing between the wires proper is thus very accurate and has very high tolerances relative to the project objectives. Next to the outlet area from the holders 248, there have been provided a device 250 for supplying brace wires 35, and a welding unit 251 (FIG. 18). Said device 250 is so designed as to arrange each brace wire 35 in such a way as to cross the lengthwise wires 34 in parallel relationship with one another, so that each lengthwise wire be as near as possible to or in engagement with the wires 34 in the marking section thereof. Said device 250 (FIG. 18) particularly comprises a hopper 252 provided with an outlet channel 253 which is controlled by an actuator 254, a plurality of slanting bars 256, and a series of small stop flanges 258. 
     The welding unit 251 comprises a fixed electrode 260, lying slightly below wires 34 and cross-wise to the wires in the marking area thereof, and an array of movable electrodes 261. The active portion of said electrodes 261 lies in substantially parallel relationship with the active portion of said electrode 260, and said electrodes 261 are so designed as to move vertically relative to electrode 260 under the action of the corresponding actuators 262. Said electrodes 261 with the downward movement thereof, are so designed as to cooperate with a brace wire 35 and push same against the lengthwise wires 34 which are in turn stopped by the fixed electrode 260 in the respective crossing areas for the sequence welding of wires 34 and 35. The machine 38 further comprises a feeding member 270 which can feed the lengthwise wires relative to electrodes 260 and 261 over a distance which is accurately equal to the brace wire pitch. Said device comprises cross-wise teeth 272 moved by a chain 273 and which can engage the brace wires 35 after welding thereof. The stroke of said teeth 272 is carefully controlled by an extension coder 275 which measures accurately the angle displacements of a shaft 276 on which is secured a gear wheel cooperating with chain 271. Said chain is moved in turn by a servo-mechanism comprising a motor 277 which is controlled by said extension coder according to a programme which is synchronized with the movement of said electrodes 261 and the wire welding. Downstream of device 250, a cutting device 278 is provided, which is operated by an air-control member 279 and comprises shears which can accurately cut said brace wire 35 adjacent the welding areas with the end lengthwise wires 34 lying outside the netting. The nettings proper will thus have very accurate sizes, which then allows the accurate assembling operations for structure 30 on machine 40. 
     The machine 38 may be employed to assemble simultaneously a plurality of nettings. Said operation is performed by using a single brace wire for a plurality of nettings which have their end lengthwise wires located side to side and joined by a single brace wire. Said meetings are then separated during the same cutting step which follows the welding by cutting the brace wire between the end lengthwise wires adjacent to their welding areas. The various steps for feeding the nettings, welding and cutting are controlled from a control unit 281 which comprises a microprocessor 282 (FIG. 19) and a control panel 283. The various working steps, the feeding step and the welding times may be programmed and synchronized on said microprocessor. FIG. 20 summarizes the various assembling steps for structure 30. 
     During step 290, welding and twisting of wires 34, 35 and 37 is performed, and these wires will be cut respectively in steps 291, 292 and 293. During steps 294 and 295, respectively, the wires 34 are located on holders 246 of machine 38, and the wires 35 are located on the suitable hopper 252. The wires 34 and 35 are welded in step 296 and then cut in step 297. Thereafter, while cross-wise teeth 272 return to the welding area during step 298, the nettings may be collected in step 299 and arranged on the bearing planes 81 of assembling machine 40. The cross wires 37 will be collected in turn in hoppers 101 during step 211. There then occurs the assembling step of structure 30 which provides for the stepwise feeding of the cross wires during step 300, while the actuator 104 and arms 111 have located wire 37 in middle position during step 303. 
     There then follows the raising of the electrodes in step 305, the welding in step 306, and the return of levers 190 in step 307. The command to lower the electrodes is then given in step 309, and the collecting on body 50 of the assembled structure, occurs in step 310.