US 4,160,892 AGrant
Method and Apparatus for Seam Welding Overlapped Edges
Issue Date:1979-07-10
•18 Claims
•4 Drawing Sheets
Abstract
Method and apparatus for longitudinally seam welding overlapped edges of metal sheets with continuously renewable flat wire electrodes in which the edges are overlapped a distance of from three to no more than six times the thickness of the edges and the flat wire electrodes have a width from about two to three times the width of the overlap.
Metadata
Assignee
- Opprecht; Paul
Inventors
- Paul Opprecht
- Martin Kaul
Application Information
Application Number:US 6683126
Filing Date:1976-03-18
Priority Date:1975-03-19
Art Unit:213
Classifications
IPC:
B23K 1106
Field of Search:
21964;81;82;83;84;119;117 R
Patent Drawings (4 sheets)
Description
The present invention relates to a method and apparatus for seam welding overlapped edges of metal sheets and more particularly to a method and apparatus, capable of being operated continuously, for seam-welding the overlapped edges of tinned, galvanized or otherwise surface-treated metal sheets or cold-rolled steel plates with continuous and self-renewing flat, wire electrodes.
Processes for seam welding the overlapped edges of sheets are known by passing the edges between two electrodes, wherein the overlapped range of the sheets' edges to be welded is less than the width of the surface of the welding electrodes. These processes are especially suited for pressure-welding of iron sheets lacking foreign-metal coatings. If, however, coated sheets, for instance tinned or galvanized metal sheets, are welded, such processes are hindered by the coating metal being transferred to the welding electrodes, contaminating the contact areas and thereby requiring continuous maintenance of the electrode surfaces. Further, such processes are ill suited to welding thin sheets of thicknesses of from 0.15 to 0.5 mm, because the sheet or plate edges quickly mar the electrode welding surfaces with gouges or the like, which hinders the passage of the welding currents and gives rise to uneven welds and unsuitable bonds.
Processes are also known in which the edges are overlapped so much that the overlapped range exceeds the width of the electrode welding surfaces. Such processes are suitable for welding coated sheets, but because the overlap range is wider than the welded surface, the outer parts of the sheet edges will not be welded and there is the tendency that these overlapping, non-welded edges will bend away from the welded portion. When such a process is used for example in making cans, they may only be thereafter coated with great difficulty with plastics or lacquer because of the loose edges. Dirt may also collect between the non-welded overlapped edges and if the contents are perishable, they may spoil. It is also known to compress the welded seam region to avoid this problem but this requires an additional step beyond the welding step if the desired results are to be achieved.
Attempts have also been made in the past to obtain overlapped seams by first tacking the sheet along its edges before welding the seam, but the process is costly and furthermore, leads to uneven overlap and inferior welding seams. If tacking is dispensed with, overlaps up to 2 mm (wide) are feasible, but always with the risk of gaping of the welded edges. This prohibits its use in certain industries, and in particular, the food and beverage industry.
In accordance with the present invention, there is provided a method for longitudinally seam welding two overlapping metal sheet edges that substantially avoid these problems, which process comprises feeding two sheet edges of similar thickness and in overlapping relationship to a welding station, said overlap being from about 3 to no more than 6 times the thickness of the edges, continuously feeding a pair of flat wire electrodes to the welding station at the same rate as the overlapped edges and passing the overlapped edges between said pair of electrodes so that they are located on opposite sides of the overlapped seam, said electrodes having a width of from 2 to 3 times that of the overlap, compressing the overlapped edges together between said electrodes at the welding station and simultaneously energizing the electrodes to thereby longitudinally seam weld the overlapped edges whereby the seam extends beyond the overlapped region into adjacent portions of the non-overlapped regions of the sheet edges. Preferably, the sheet edges are passed through a guide means to position them in the proper overlapped position prior to feeding to the welding station and the sheets are held externally during welding to overcome the spreading effect of the compressive electrodes.
The present invention further comprises apparatus for accomplishing the above prescribed method for seam welding two overlapping sheet edges which comprises a welding station having two opposed flat wire electrodes, means for feeding said edges in overlapped relationship to the welding station between said electrodes, means for continuously feeding flat wire electrodes to the station at the same rate as the overlapped edges, said edges being overlapped a distance of from about 3 to no more than 6 times the thickness of the sheet edges and said wire electrodes having a width of from about 2 to 3 times the width of the overlapped region of the sheet edges and means for holding the overlapped edges in position at the welding station so that the electrodes can thereby seam weld the overlapped edges.
Preferably the apparatus includes a guide rail located ahead of the welding station for guiding the edges into proper overlapped position and means to hold the edges in said guide rail during passage therethrough. Further, the flate wire electrode feeding means include a pair of electrode welding rollers each having a notch on their outer surface over which the flat wire electrode is guided, said rollers being mounted on axes parallel to the longitudinal direction of the welding seam, with each axis being offset in opposite directions approximately 0.5.degree. to 1.5.degree. from the longitudinal axis of the welding seam.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory but are not restrictive of the invention.
The accompanying drawings which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the description, serves to explain the principles of the invention.
OF THE DRAWINGS:
FIG. 1 is a partial side elevational view of a resistance seam welding machine constructed according to the present invention and showing a welding station having upper and lower flat wire, welding electrode rollers and a Z rail for guiding the overlapped sheet edges into the welding rollers;
FIG. 2 is an enlarged view looking in the direction of movement of the overlapped sheet edges and flat wire welding electrode rollers;
FIG. 3 is a top view of the welding electrode rollers of FIG. 2;
FIG. 4 is a sectional view through the Z rail of FIG. 1 for guiding the overlapped edges of a rolled sheet and schematically illustrating external supporting means;
FIG. 5 is a top sectional view taken through the Z rail along the line V--V of FIG. 4 with the external supporting means removed;
FIG. 6 is a sectional view taken along the line VI--VI of FIG. 4 but showing the overlapped edges of a rolled sheet without the Z rail and additional structure to the external supporting means as well as a centering ring mechanism for holding the overlapped edges in position by the welding station;
FIG. 7 is an enlarged cross-sectional view through the two flat wire welding electrode rollers of FIG. 2 and showing their relationship to the overlapped edges; and
FIG. 8 is an elevational view of the centering ring mechanism of FIG. 6 acting normally to the direction of motion of the sheet at the welding station.
Reference will now be made in detail to a preferred embodiment of the invention, the example of which is illustrated in the drawings.
For the purpose of clarity and simplification, only those parts of the various mechanisms that are essential to an understanding of the present method and apparatus have been illustrated in the drawings.
The present invention relates to lontitudinal seam welding machines that can be operated continuously and in a semi or fully automatic manner for the longitudinal welding of sheet metal borders or edges as basically described for example in Swiss Pat. Nos. 370,175 and 429,982.
With reference to FIG. 1 there is shown a part of a longitudinal, resistance welding machine 1 having a welding station 2 that comprises a lower, fixed welding arm 3 and an upper, pivotable welding arm 5. As best shown in FIG. 2, arms 3 and 5 supply flat wire electrodes 11 to welding station 2. In accordance with the invention, means are provided for feeding these electrodes to the station and as embodied, this means comprises a lower welding electrode roller 7 and an upper welding electrode roller 9, each rotatably mounted on axles 15 and 16 which are journaled in corresponding bifurcated ends of the lower and upper welding machine arms 3 and 5, respectively. Welding electrode rollers 7 and 9 each are provided with a groove 18 (see FIG. 2) to seat wire electrode 11 which is guided over these electrode rollers. The wire electrode is approximately of rectangular cross-section and its width to thickness ratio is approximately from about 1 to 2.5 to 1 to 5 and more preferably from about 3.3 to 4.5. The wire electrode can be initially a round wire that is rolled flat by a means, not shown, on the welding machine. As further shown in FIG. 2, wire electrode 11 projects above the contours of rollers 7 and 9, but otherwise is shaped to fit into grooves 18.
The sheet metal 20 having overlapped edges 21 and 22 to be longitudinally seam welded is shown in FIG. 2 in welding position between the two rollers. The relationship between upper and lower arms 5 and 3 as well as rollers 9 and 7 is also shown in FIG. 3 as will be more fully described below.
In accordance with the invention, means are provided for feeding the edges 21, 22 of the metal sheet 20 into welding station 2 of the machine in overlapped relationship. As embodied and as best shown in FIG. 4, this means comprises a Z rail 13 having upper and lower rail grooves 26 and 27 into which are inserted the edges 21, 22 of metal sheet 20. Means are further provided for holding the said edges in the Z rail grooves and for keeping the edges in overlapped position at the welding station.
In the embodiment shown, the edges 21, 22 are opposite edges of a single rolled sheet or tube 20, such as in the making of cans or the like, and consequently the holding means are arcuate shaped, laterally mounted guidance jaws 32, 33, 34 (see FIGS. 4 and 6) shaped to fit the size of the rolled sheet 20. The jaws are spring loaded, as schematically depicted in FIGS. 4 and 6 by springs 36, 37 and 38 mounted between the jaws 32, 33, 34, respectively, and a part of the frame of welding machine 1. Jaws 32-34 press against the external sides of metal tube 20 pushing edge 21 against the upper groove bottom 29 of rail groove 26 and edge 22 against a lower groove bottom 30 of rail groove 27. Springs 36, 37 and 38 are adjustable and are mounted pairwise as shown in FIG. 6 to compress jaws 32-34 with a force against sheet metal 20 sufficient to achieve accurate guidance of sheet metal edges 21 and 22 and their precise overlap for successful welding. FIG. 6 shows the adjustable displacement "h" for springs 36-38. Lacking those guidance jaws, satisfactory resting of sheet edges 21 and 22 in Z rail 13 cannot be guaranteed. Obviously the pressure from jaws 32-34 may not be excessive or otherwise edges 21 and 22 would be damaged at the Z rail and/or might offer excessive resistance to moving the sheet.
As shown in FIGS. 5 and 6, rail grooves 26 and 27 and groove bottoms 29 and 30, respectively, are not parallel to one another but rather are oriented towards each other in the lontidutinal direction of welding. Therefore, the distance between groove bottoms 29 and 30 will be such that the overlap of the sheet metal edges 21 and 22 of sheet metal 20 to be welded will be, for example, at the exit end (.DELTA.min) with respect to the entrance end (.DELTA.max) in the ratio of approximately 1 to 4 to 1 to 8 depending on the length of the sheet. An initial value of 3.5 mm and a final value of 0.8 mm for overlap for a Z rail length of 335 mm is found to be optimum.
The resulting angle subtended by Z rail's 13 supporting surfaces for the sheet metal edges will be approximately from 20' to 60', and more preferably from 25' to 30'. This overlap, that decreases in the direction of welding, allows a more accurate and consistent welded joint along the overlapped seam.
As best shown in FIGS. 2 and 3, welding electrodes 7 and 9 rotate in a plane parallel to and in the longitudinal direction of welding and at an angle to each other. Preferably the angle of rotation is on the order of approximately one degree (0.5.degree. to 1.5.degree.) on each side of the welding direction; the angle being denoted by .rho., and the total angle by 2.rho. in FIG. 3.
This angular arrangement ensures that the two sheet edges 21 and 22 will be pressed against each other during the welding process, so that more evenly overlapped sheet edges 21 and 22 will be obtained, the amount of overlap being for instance 0.8 mm.
In accordance with the invention and during the welding process, flat wire electrode 11 is continuously fed to roller electrodes 7 and 9 at the same rate as the overlapped edges are fed to the welding station, thus providing a continuously self renewing electrode to the sheet 20 as it passes through the welding station.
The rollers also press down on the overlapped portion in a direction normal to the longitudinal direction of welding so that the thickness of the body in the area of the welding seam will not appreciably exceed the sheet thickness (i.e. not more than about 30%). Of course, the sheet thickness will be double for conventional overlapping welding.
In accordance with the invention and to achieve optimum and consistent seam welding, it was found advantageous to overlap the sheet edges 21 and 22 at the welding station from 3 to 6 times the sheet thickness, and to use wire electrodes 11 having a width of from 2 to 3 times the overlap width. For conventional sheet thicknesses in the range of from 0.18 to 0.24 mm, the electrode width will ordinarily be about 2 mm. Minimum width of the electrode wire is desirable on economic grounds, and that of about 2 mm applies to such conditions. Such an electrode wire can be made from a circular wire with a cross-section of 1.5 sq. mm for instance. Obviously, the width of the wire electrode should be larger for greater sheet thicknesses corresponding to larger overlap widths, and accordingly, a thicker round wire would be required as the starting material.
In accordance with the invention, the means for keeping the edges 21, 22 in overlapped position at welding station 2, after they have been accurately aligned by Z rail 13, preferably comprises a centering ring of rotatable, but adjustable, calibrating rollers 40 (see FIG. 8) adjacent the three guidance jaws 32-34 at the welding station 2 as schematically indicated in FIG. 6. If desired, guidance jaws 32-34 may be replaced by a number of closely mounted, spring-biased centering rings such as rollers 40 which would offer less friction to the passage of the sheet.
The welding compression at welding station 2 rises from zero at the first contact between welding rollers 7 and 9 and sheet edges 21, 22 and rises to a maximum of about 60 to 120 kilograms/sq millimeter (KSM) and preferably around 75 KSM. This compares to about 50 KSM for the corresponding case of lap-seam welding. Measured values for instance were found to be in the ratio of 55:75 KSM. Those values were obtained for sheet thicknesses between 0.15 and 0.25 mm. They are correspondingly higher for larger sheet thicknesses where, for example, the overlap of a sheet thickness of 0.4 mm will be approximately 1.2 mm in the area of welding.
During welding the electrodes 11 are energized and the current from the electrode welding surfaces essentially concentrates in the overlapped region which is narrower than the width of the electrode surfaces so that all of the material within the overlapped region will be covered with welding nuggets and their corresponding structure will extend over the entire overlapped region. The edges of the electrode wires will also cover non-overlapping regions of the sheet edges. The current at the edges of the electrode wires will encounter longer paths from one electrode to the other and correspondingly higher resistances so that the current density and hence the heating at said edges is relatively slight, whereby the electrode edges cause a continuous transition of the crystalline transformation away from the overlapped region and will therefore not induce any weakening notches.
In this manner, tearing of the seams is prevented when flanging the welded sheets, and the possibility is now provided--in view of the absence of protruding edges--to coat such sheets without difficulty, for instance with plastics or lacquers. By making use of continuous and self-renewing electrodes, soiling of their surfaces and their wear will not be a problem, the electrode wire itself forming the electrode welding surface and hence renewing same continuously.
By making use of flat wire electrodes in combination with the defined overlap there is achieved in accordance with the present invention and in one operational sequence, a method and apparatus that allows metal sheets, especially thin ones to be seam welded whether they be coated or not and without further preparation, by means of a compressed seam, which is ideally suited to the welding of tin cans flawlessly and in a continuous manner.
In operation, and with reference to FIG. 7, the two edges of two sheets 43 and 44 that are to be welded are fed between the two tape-like wire electrodes 41 which are guided by and continuously supplied by two corresponding electrode roller carriers 42. Said edges to be welded are made to overlap in the manner and degree as discussed above before entry between the two wire electrodes so that their range of overlap is narrower than the width of electrodes. The rollers rotate with the passage of the overlapped edges, thus continuously supplying fresh electrodes to the welding surface while simultaneously compressing the weld as current is supplied to the electrodes to achieve the desired welded seam.
Preferably, the range of overlap will be so positioned as to be in the center of the wire electrodes, the width of the overlap being about one-half that of the wire electrodes. Thus, if a flat wire approximately 2 mm wide is used as an electrode, then the two sheet edges should overlap about 1 mm and be placed between the electrodes so that the latter will project beyond the overlap region on both sides by about 1/2 mm. This method achieves welding through the entire region of the overlap, preventing gaping of the edges with sharp edges arising at the welding seam. This data applies to sheet thicknesses of from about 0.15 to 0.3 mm.
The present invention represents an extremely significant technical advance because only with the present described process and the corresponding equipment can the manufacturing industry make cans having the seams that are practically the same or very close to butt-weld seams or even to seamless welds. There are no raised free edges which otherwise make the can unsuitable for the food industry because of the danger of contamination.
In practice, it has been found that a machine implementing the method of the invention can make welded sheet metal cans at a sheet rate from 10 to 40 meters per minute depending on the frequency of the welding power.
As shown by microsections taken of the weld seams, such welding process provides a practically homogeneous weld which is close to that of the original sheet in the region of the weld so that even for intensive flanging there will be no crack formation, and this is of particular importance with regard to further processing of such sheets into cans. In this manner are explained the excellent flanging qualities and the possibility of using lesser grades of sheet metal.
Because of the high productivity and the quality of the seam, which has a strength at least equal to that of the sheet metal, achieved by the present techniques, the can blank have subsequent machining properties and the feasibility of covering the seam with lacquers and power coatings is improved thus lending the process for use in the food, beverage and aerosol industries.
Use of the wire process also allows flawless bonding of both black plates and tin plates, without prior preparation and in continuous operation. Tin-free steel sheets may be welded by this method only with a prior brushing operation being required.
The invention in its broader aspects is not limited to the specific details shown and described and departures may be made from such details without departing from the invention and without sacrificing its chief advantages.
Claims
We claim:
1. A method for longitudinally seam welding two overlapping metal sheet edges which comprises feeding two sheet edges of similar thickness to a welding station having opposed rollers on opposite sides of said overlapped sheets, guiding said edges along rigid guide means and biasing said edges toward said rigid guide means by resilient means, other than the metal sheet, which engages said sheet to cause said sheet edges to be in accurate overlapping relationship at said welding station, said overlapped being no more than 6 times the thickness of the edges, continuously feeding a pair of flat wire electrodes to the welding station between the rollers at the same rate as the overlapped edges and passing the overlapped edges in a longitudinal direction between said pair of electrodes so that the electrodes are located on opposite sides of the overlapped seam, said electrodes having a width of from 2 to 3 times that of the overlap, compressing the overlapped edges together between said electrodes at the welding station and simultaneously energizing the electrodes to thereby longitudinally seam weld the overlapped edges whereby the edges are welded with a continuously renewable electrode surface and the seam extends beyond the overlapped region into adjacent portions of the non-overlapped regions of the sheet edges, whereby the outer sheet edges are welded to the adjacent sheet to prevent each outer sheet edge from bending away from the adjacent sheet.
2. The process of claim 1, wherein the wire electrodes and overlapped edges are positioned at the welding station so that their midpoints are aligned.
3. The process of claim 1, wherein the overlapped edges are opposite ends of a rolled sheet and the step of biasing said edges includes supporting the rolled sheet externally prior to entry into and during welding at the welding station to guide the edges into and maintain them in the proper overlapped position.
4. The process of claim 3, wherein the overlapped edges are overlapped initially in excess of the desired distance and are permitted to separate thereafter to the desired distance just prior to arrival at the welding station.
5. The process of claim 1, in which the flat wire electrodes are fed to a pair of grooved roller electrodes located at the welding station which rollers rotate at the same rate as the overlapped edges are fed to the welding station to thereby continuously present fresh electrode surfaces to the overlapped edges, said rollers also compressing the seam during welding at the welding station.
6. The process of claim 1, in which the rollers and electrodes guided thereby are offset from about 0.5 to 1.5 degrees in opposite directions from the longitudinal direction of the welded seam.
7. Apparatus for seam welding two overlapping sheet edges which comprises a welding station having opposed rollers, two opposed flat wire electrodes between said rollers, means for feeding and guiding said edges in accurate overlapped relationship and in a longitudinal direction to the welding station between said electrodes, said feeding and guiding means including rigid means adjacent said welding station for engaging said edges, said feeding and guiding means further including means for resiliently biasing said sheet edges toward said rigid means, means for continuously feeding said flat wire electrodes to the station at the same rate as the overlapped edges, said edges being overlapped a distance no more than 6 times the thickness of the sheet edges and said wire electrodes having a width of from about 2 to 3 times the width of the overlapped region of the sheet edges, and means for holding the overlapped edges in position at the welding station so that the electrodes can thereby seam weld the overlapped edges with continuously renewable electrode surfaces, whereby the outer sheet edges are welded to the adjacent sheet to prevent each outer sheet edge from bending away from the adjacent sheet.
8. Apparatus according to claim 7, wherein the overlapped edges are opposite ends of a rolled sheet and wherein the biasing means includes means for supporting the external surfaces of the rolled sheet to place and maintain the edges in the proper overlapped position prior to and at the welding station.
9. Apparatus according to claim 8, which includes a cluster of rollers acting against opposite external sides of the rolled sheet at the rolling station for holding the edges in proper position during welding.
10. Apparatus according to claim 8, wherein the, feeding and guiding means includes a plurality of spring-loaded jaws acting against and shaped to fit the external surface of the rolled sheet and including in combination a Z shaped guide rail having opposite faced grooves for receiving the opposite edges of the rolled sheet and for guiding them into proper overlapped position, said jaws holding the edges in said guide rail as they are fed longitudinally to the welding station.
11. Apparatus according to claim 10, wherein the Z rail grooves diverge outwardly from entrance to exit end with respect to the longitudinal direction of movement of the overlapped edges to the desired overlapped distance.
12. Apparatus according to claim 7, wherein the means for feeding the electrodes comprises a pair of grooved rollers mounted for rotation on opposite sides of the seam at the welding station over which the wire electrodes are guided to thereby continuously supply fresh electrode surface to the seam to be welded.
13. Apparatus according to claim 12, wherein the axes of the rollers and the electrodes guided thereby are offset from about 0.5 to 1.5 degrees in opposite directions from the longitudinal direction of the welded seam.
14. A method for longitudinally seam welding two overlapping metal sheet edges which comprises the steps of positioning a pair of elongated metal sheet edges in accurate overlapping relationship by guiding said edges along rigid guide means and biasing said edges toward said rigid guide means by resilient means, other than the metal sheet, which engages said sheet, feeding said overlapped sheet edges through a welding station having opposed rollers on opposite sides of said overlapped sheet edges, feeding a pair of flat wire electrodes having a width greater than the width of overlap of said sheet edges through the welding station between the rollers and on opposite sides of said overlapped sheet edges, compressing the overlapped sheet edges between the electrodes at the weld station and forming a substantially continuous weld seam along both sides of said sheet edges at the overlap therebetween and beyond the overlapped region into adjacent portions of the non-overlapped regions of said sheet edges, whereby the outer sheet edges are welded to the adjacent sheet to prevent each outer sheet edge from bending away from the adjacent sheet.
15. The method as defined in claim 14 wherein said electrodes have a width of from about 2 to about 3 times the width of overlap of said sheet edges.
16. In a method for longitudinally seam-welding two overlapping metal sheet edges which comprises the steps of supplying the two metal sheet edges into opposite grooves of a guiding means, bringing said edges into overlapping relationship in such a way that the edges are oriented toward each other in the longitudinal direction of welding, supporting the external surface of said sheet edges and biasing the edges to be welded toward the opposite grooves of said guiding means by resilient means, other than the metal sheet, which engages said sheet and guides the sheet edges in proper overlapped position, feeding the overlapped sheet edges to a welding station and passing the overlapping edges in a longitudinal direction between a continuously fed pair of flat wire electrodes fed over a pair of roller electrodes located on opposite sides of said sheet edges at said welding station, said wire electrodes having a width greater than the width of overlap of said sheet edges in the welding station, the axes of said roller electrodes being offset from about 0.5 to about 1.5.degree. in opposite directions from the longitudinal direction of the welded seam, compressing the overlapped edges together between said electrodes at the welding station, simultaneously energizing the electrodes to thereby form a longitudinal weld seam along both sides of the overlapped sheet edges and beyond the overlapped region into the adjacent portion of the non-overlapped regions of the sheet edges, whereby the metal edges are welded with a continuously renewable electrode surface, and whereby the outer sheet edges are welded to the adjacent sheet to prevent each outer sheet edge from bending away from the adjacent sheet.
17. Apparatus for seam welding two overlapping sheet edges which comprises a welding station having opposed rollers, two opposed flat wire electrodes between said rollers, means for feeding and guiding said sheet edges in accurate overlapped relationship and in a longitudinal direction to the welding station between said electrodes, said feeding and guiding means including rigid means adjacent said welding station for engaging said edges, said feeding and guiding means further including means for resiliently biasing said sheet edges toward said rigid means, means for feeding said flat wire electrodes to the welding station at the same rate as the overlapping sheet edges, said wire electrodes having a width greater than the width of the overlapped region of the sheet edges, and means for holding the overlapped sheet edges in position at the welding station so that the electrodes seam weld the overlapped sheet edges with a continuous seam along both sides of said overlapped sheet and extending beyond the overlapped region into the adjacent portions of the non-overlapped regions of the sheets, whereby the outer sheet edges are welded to the adjacent sheet to prevent each outer sheet edge from bending away from the adjacent sheet.
18. Apparatus according to claim 17 wherein said wire electrodes have a width of from about 2 to about 3 times the width of the overlapped region of the sheet edges.
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