HIGH FREQUENCY CONTACT WELDING APPARATUS WITH INCLINED CONTACTS Field of the Invention This invention relates to high frequency contact welding apparatus and methods, wherein the forming and location of the contacts are chosen to eliminate or substantially eliminate overlap. undesirable heating and lag of the joining part (s). BACKGROUND OF THE INVENTION Appliances and methods for welding forging for joining a pair of metal parts or one-piece edge portions bent to form a tube, wherein the surface portions of the metal part or parts to be welded are they advance towards a welding point with a space between them, they are heated by high frequency electric currents supplied to the surfaces by means of contacts on opposite sides of the space and contacting the metal part or parts in advance of the welding point, they are Well known in the specialty. See, for example, US patents. Nos. 2,821,619; 2,873,353; 2,886,691; 2,898,440; 2,992,319; 3,047,712; 3,056,882; 3,375,344; 3,391,267 and 4,241,284 which are incorporated herein by reference.
In the high-frequency heating apparatus of the prior art, the downstream edges of the contacts extend substantially 90 ° to the advancing paths of the metal to be heated. Furthermore, the contacts are inclined in such a way that only a small portion of the contact faces adjacent to the downstream edges contact the metal to be heated. In this way, there is a very high current density in these portions of the contact faces. Furthermore, as will be explained further below, with high frequency currents and with the edges downstream of the contacts extending 90 ° with respect to the path of the metal to be heated, a large proportion of the current leaves the contacts in the portions of the closest contact face of the heating current path in the metal. Therefore, the current density in the portions of the contact faces closest to the heating current path in the metal is much higher than elsewhere in the current faces. The use of high frequency electric currents, ie currents of a frequency of 10 KHz and above and particularly 400 KHz and above, for heating the surfaces to be welded together, has certain well-known advantages compared to the use of current Direct or current of 500 Hz or in minors. For example, the "surface effect" or "Kelvin effect" causes most of the current to circulate on the surface of a part where it is most useful, and this effect increases with frequency. In addition, the surfaces contacted by the contacts do not need to be clean, that is, for example they may have incrustations or oxides. When direct currents or low frequency currents are used, ie 500 Hz, there is little surface effect. In addition, the current path, when the proximity effect is not involved, is determined primarily by the resistance of the path because the inductive reactance of the current paths is zero or small. In this way, the current density per unit area of the contact face contacting the part is substantially uniform. Accordingly, to decrease the current density, one simply has to increase the contact area. The current density when the contact engages the metal part is important in at least two aspects. In this way, if the density is too high, undesirable melting of the metal in or near the contact area may occur. This fusion is not desirable for forging welding because it deteriorates the metal and can cause the metal to harden by self-extinguishing after it is cooled. If there is no fusion, the metal may discolor, have burn marks, cause self-extinguishing hardened areas or melt the metal of the contacts on the metal part whose metal contact must be eliminated. In current practice with high frequency currents, the current can be several thousand amperes with welding speeds, ie advance speed of 7.62 to 152.40 meters / minute (25 to 500 feet / minute). It has been found that with these large currents, and therefore a high contact current density, the previously mentioned problems have been encountered but have been tolerated for some purposes, due to the production speeds available. Due to various pieces of equipment required for mechanical reasons, for example forming rollers, forging welding pressure rollers, etc., the space available for the contacts and their assemblies are limited. However, while increasing the size of the contact face of the contacts causes some improvement with high frequency currents, there is a need for further improvement. Even with an increase in the size of the contact face and the high frequency currents, most of the current is concentrated in the portion of the face closest to the metal surface to be heated and particularly in the nearest downstream corner to the surface to be heated. SUMMARY OF THE INVENTION It has been discovered and confirmed by tests, that by tilting the edges downstream of the contacts supplying the high frequency electrical current to surfaces of metal parts or opposite edge surfaces of a part (hereinafter referred to sometimes as "metal surfaces"), for welding with forging as a whole, the burning, hardening, discoloration, etc. can be substantially eliminated or substantially reduced from the side of the parts where the contacts couple said parts, thus providing a product of a better soldier. In the prior art, the edges downstream of the contacts extend substantially perpendicular to the paths followed by the metal surfaces to be heated as the metal surface is advanced. By inclining the edges downstream of contact it is meant that the downstream edges are oriented such that they extend at an acute angle of from about 35 ° to about 55 °, preferably about 45 ° to the routes followed by the surfaces of metal to heat, the angle is intermediate to the edges downstream and the point of welding. To be more precise, the edges downstream are in planes perpendicular to the metal surfaces that are contacted and these planes intersect the routes of the metal surfaces to be heated to an acute angle, such that the angles are defined between the planes and portions of these routes downstream of the contacts. At an angle less than about 35 °, the current increases at the downstream end of the contact edge and causes the aforementioned undesirable effects at an angle greater than about 55 °, the desired current distribution is not obtained and again the undesirable effects. Thus, for example in a known process for welding a tube or pipe seam, a single piece of metal is bent to bring the edge surfaces adjacent to each other but with a space between them, as the metal is advanced to a point of welding where the surfaces in the forging welding temperature are forced together. The electric heating current is supplied to the edge surfaces from a current source by a pair of contacts, one on one side of the space and the other on the other side of the space and both that couple the metal in advance to the point of weld . Each contact has a linear edge, for example rectilinear closest to the welding point, ie a downstream edge, which engages the metal and extends at an angle of about 35 ° to about 55 ° with respect to the surface route of the edge of the metal as it is advanced. The angle is chosen such that the current density at the edge portion of the contact is more nearly uniform across the width of the edge portion., ie, the dimension of the edge portion in the direction transverse to the edge surface of the metal. Similarly, if the metal surfaces to be heated advance towards a welding point in overlapping relationship with a space between them and the heating current is supplied by contacts on opposite sides of the space and to couple the metal in advance of the welding point, the edges downstream of the contacts extend at an angle from about 35 ° to about 55 ° with respect to the paths of the metal surfaces to be heated as the metal surfaces are advanced. Also, if an edge surface of a metal part is to be welded to a surface of another part, which is not an edge surface, such as in the manufacture of structural members, for example T, H or double T beams, or as in the welding of a fin to a tube, the principles of the invention apply. In this way, as the parts are advanced towards a welding point, forging with a space between them and the electric current is supplied to the parts by contacts on opposite sides of the space and coupling the parts in advance of the welding point, the edges downstream of the contacts are tilted as previously described. Therefore, according to the invention, the inclination of the contact or the contacts is chosen such that each portion of the edge of the contact closest to the welding point, has between this portion and the welding point, a route for the welding current that substantially has the same impedance, the resistance component of the impedance is small with respect to the reactive component. In other words, the orientation of the downstream edge of a contact is chosen such that the impedance of all the paths from the edge downstream to the welding point is more nearly equal so that the heating current from the contact to the surface of adjacent metal to be heated is substantially uniform. In this way, the heating current is not concentrated at a point of contact and therefore the problems described previously are substantially reduced. If desired, each contact may comprise two or more conductively connected parts, which may be individually desired to be pressed to the metal part, such as by springs or by a pneumatic cylinder. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood by reference to the following detailed description of preferred embodiments of the invention, this description will be; to be considered in connection with the accompanying drawings in which: Figure 1 is a fragmentary diagrammatic perspective view illustrating high frequency forging welding of the prior art of edge confining surfaces to a metal strip or strips using contacts to supply high frequency electric current from a source to the metal strip or metal strips; Figure 2 is similar to Figure 1, with the contact modifications of the invention; Figure 3 is a graph relating to the fingerprint of a contact on a metal surface of Figure 1 and illustrating the current distribution in the edge portion downstream of the contact; Figure 4 is similar to Figure 3, but illustrating the current distribution in the edge portion downstream of the contact when the contact is placed according to the invention; Figures 5 to 8 are end views of contacts illustrating various cross-sectional shapes that the contacts have; Figure 9 is a fragmentary and diagrammatic perspective view of a contact support mode; Figure 10 is an elevation view of a fragmentary diagramatic end of an alternate embodiment of the contact supports; Figure 11 is a fragmentary and diagramatic bottom view illustrating contacts of the invention with two parts; Figure 12 is a fragmentary and diagrammatic perspective view of the forging weld joining overlapping portions of a sheet of metal or sheets of metal;
Figure 13 is a fragmentary and diagrammatic perspective view of tubing fabrication when spirally winding a tape and forging welds for edge portions of the tape as a whole; Figure 14 is a fragmentary and diagrammatic perspective view of the forging of a fin in a tube; Figure 15 is a fragmentary and diagrammatic perspective view of the manufacture of a T-shaped element by forging welding an edge portion of a metal strip to an intermediate portion of another metal strip; Figure 16 is a fragmentary and diagrammatic perspective view of the forging weld in conjunction with the lips in a sheet of metal or in sheets of metal; and Figure 17 is a fragmentary and diagrammatic perspective view of the forging of two longitudinal metal strips to a tube. DETAILED DESCRIPTION OF PREFERRED MODALITIES The principles of the invention apply to various types of high-frequency contact welding systems, for example, overlapping and overlapping welding of a spiral or longitudinal tube seam, structural members such as T-members, double T and H and spiral fins in a tube, tubular cable liner, lip tubes, either with a longitudinal or spiral lip and welding from one metal strip to another metal strip as the strips move longitudinally either with a flap or coating welding. Figure 1 illustrates diagrammatically the forging welding of the prior art at a welding point 1 of a pair of edge surfaces 2 and 3, which may be the edge surface of a pair of metal strips or the opposite edge surfaces from a single strip of metal that has been bent to form a tube. The edge surfaces 2 and 3 are advanced in the direction of the arrow 4 and are separated by a space 5 in advance of the welding point 1. To take advantage of the "proximity effect", the space is relatively small and the angle 6 between the edge surfaces may be approximately 4 ° to approximately 7 °. A welding seam 7 is present after the welding point 1. High frequency electric current, for example current of a frequency of at least 10 KHz is supplied to the edge surfaces 2 and 3 by a pair of contacts 8 and 9 in sliding coupling with the upper surfaces 10 and 11 of the metal part or parts with a contact 8 on one side of the space 5 and the other contact 9 on the other side of the space 5. The contact 8 is adjacent to the edge surface 2 and the contact 9 is adjacent the edge surface 3. Normally, there is a small spacing between the edge surfaces and the respective contact as illustrated. Contacts 8 and 9 have contact faces contacting surfaces 120 and 11 and having downstream rectilinear edges 12 and 13 placed substantially on lines 14 and 15, respectively. These lines 14 and 15 respectively are substantially perpendicular to the plane of the edge surfaces 2 and 3 and therefore the paths of the heating current in the metal. From contacts 8 and 9, the high frequency current circulates in the metal part or parts on a plurality of routes adjacent to the edge surfaces 2 and 3, only three of the routes per contact, routes 16 to 18 and 19 to 21, are indicated in dotted lines in Figure 11. It will be noted that the current flow paths have different lengths. With direct current or low frequency current, the current quantities in each path are determined only by the resistance of each path and therefore the current in each path does not vary significantly. However, with high frequency current, the quantities of current in each route are determined not only by the resistance of each route, which due to the surface effect is higher than the direct current resistance, but also by the reactance of each route. further, due to the proximity of the contact 8 and the contact 9, the current density is greater in the corners downstream of the contacts 8 and 9 that are closer together due to the proximity effect. It has been found that currents on routes 16, 17, 19 and 20 are much larger than currents 18 and 21 and larger than currents on intermediate routes to routes 17 and 18 and routes 20 and 21. both, the current density at the edges 12 and 13 of the contacts 8 and 9 varies from a large value at the end of the edge 12 closest to the edge surface 2 to a significantly smaller value at the end of the edge 12 more away from the edge surface, particularly at current frequencies of 400 KHz and above. The current density on the edge 13 varies similarly. It has been found that this variation in current distribution: is the cause of the contact welding problems described previously. After discovering the cause of the contact welding problems of the prior art, I have conducted experiments and found that the problems can be eliminated or substantially reduced by tilting the downstream edge of one or both, preferably both contacts, in such a way that the running edge. below it extends at an angle from about 35 ° to about 55 °, preferably about 45 ° to the route of the metal to be heated, as the metal is advanced. This angle is intermediate to the contacts 8 and 9 and the welding point 1. Figure 2 is a diagrammatic illustration of the inclined contact according to the invention. Reference numbers in Figure 2 that are the same as the reference numbers in Figure 1 designate the same elements. Although the contacts 8 and 9 may have the same size, respectively as the contacts 8 and 9 in Figure 1, the downstream edges 12 and 13 may be longer as illustrated in Figure 2. In Figure 2, the edges Downstream of the contracings 8 and 9 extend at acute angles 22 and 23 with respect to the paths of the edge surfaces 2 and 3 as they are advanced. Preferably, both angles are about 45 ° but the angles may be in the range of about 35 ° to about 45 ° and do not need to be the same. With angles less than about 35 ° and more than about 55 °, the current distribution is altered in such a way that the previously described problems arise again. It will be noted from a comparison in Figures 1 and 2 that the lengths of the current paths that are spaced from the ends of the contacts 8 and 9 closest to the surfaces 2 and 3, are shorter in Figure 2 than in the Figure 1. According to this, the impedances of these routes are smaller and the magnitude of the current flow is larger. Therefore, for the same amount of heating current in the surfaces 2 and 3, the current is better distributed through: the contact faces of the edges 12 and 13 and the currents in the routes 16, 17, 19 and 20 may be less and therefore the overheating, burning, wear, etc., of the metal surfaces 10 and 11 may be substantially eliminated by the currents in the routes 16, 17, 19 and 20. FIGS. 3 and 4 also illustrate diagrammatically the differences between the contact current density with contacts of the prior art and high frequency current and the contact current density with the contacts of the invention. Although for simplicity the effects with contact 8 are described, in connection with Figures 3 and 4, similar effects result with contact 9. As previously mentioned, the faces of the contacts are inclined with respect to the metal surface to be heated , such that only a small portion of the contact face adjacent the downstream edge currently couples the metal surface at any given time. As well, for the most part, the electric heating current leaves the contact face near the edge downstream of the contact. Figure 3 illustrates the footprint 8a of the contact 8 shown in Figure 1. Although all of the current will leave the contact 8 in the downstream portion 24 of the area defined by the footprint 8a. The dimension d in Figure 3 may be in the order of .076 cm (.030") at 300-400 KHz. The current distribution in the portion 24 is illustrated by the graph in the upper part of Figure 3. this way, the current in the portion 26 is relatively large at the end of the footprint 8a closest to the surface 2 to be heated and is relatively small at the opposite end.
In contrast, when the downstream end of the contact 8 is tilted as illustrated in Figure 4, the current density in the portion 24 can be almost constant as illustrated by the graph at the top of Figure 4 or at least , the difference between the current magnitudes at opposite ends of the portion 24, can be substantially smaller, whereby the problems discussed above can be substantially reduced or eliminated. From the foregoing, it will be noted that only a relatively small portion of the contact face is necessary to supply heating current on the metal surfaces 10 and 11. Therefore, it is necessary that the cross section of a contact be only one size and shape that provide the necessary physical strength and wear life. As illustrated in Figures 5 to 8, which show end views of the contacts 8a-8e, the contacts 8a-8e facing the contact end engaging the metal surface, the contacts can have various shapes in section transversal The contact 9 can also have these shapes inverted. Because the contacts are connected to the high frequency energy source by transmission lines, it is not convenient to tilt the contact support. Accordingly, it is preferred to use contacts with downstream edges for example the edge 12 inclined at the desired angle as illustrated in Figures 5 to 8, without changing the position of the contact support. Preferably, the downstream edge is rectilinear, but if it is desired to modify the current density at the contact face, the downstream edge, for example the edge 12 can be curvilinear as illustrated in Figure 8. The contacts used for supplying heating current from the source of the metal to be heated, can be mounted on supports in any convenient way. For example, contacts 15 and 16 illustrated in U.S. Pat. No. 3,056,882 can be replaced by contacts 8 and 9 of Figure 2 present, such that the edges downstream of; the contacts extend at an angle of approximately 45 ° to 55 ° with respect to the advance paths of the metal surfaces to be heated. In this manner, the contacts 8 and 9 are pressed against the metal on opposite sides of the space between the metal surfaces to be heated by spring 60, as described in US Pat. No. 3,056,882.
Alternatively, contacts 15 and 16 of the U.S. Patent. No. 3,056,882 can be replaced by copper alloy bars 25 and 26 which extend through supports 27 and 28 and hold in place by fastening screws, only screw 29 is visible in Figure 9, but a similar screw is present in the support 27. In this way, as the bars 25 and 26 wear out, the fastening screws can be loosened, the bars 25 and 26 move downwards and the screws are tightened. The supports move towards the metal surfaces 10 and 11 by springs as described in US Pat. No. 3,056,882. As a further alternative, the supports can be held in fixed positions and slidably receive the contact rods moving with air towards the metal by the piston rods of air-operated piston and cylinder structures, as illustrated diagrammatically in Figure 10. In this way, after the supports 30 and 31 are placed in the positions shown in Figure 10, the supports 30 and 31q are held in fixed positions. The contact rods 25 and 26 extend through openings in the supports 30 and 31 and are slidably received in these openings. The contact rods 25 and 26 move towards the metal to be heated by the piston rods 32 and 33 operable by pistons in the air cylinders 34 and 35. Although the contact rods 25 and 26 of square cross-section have been shown in FIG. Figures 9 and 10, it will be apparent to those skilled in the art that bars of other cross sections such as those shown in Figures 5 to 8, may be used in place of the contact rods 25 and 26. As previously mentioned, each of the contacts it can be made in a plurality of connected parts in conductive form, each part is movable separately towards and away from the metal to be heated and each part is separately derived or displaced towards the metal. It has been found that with this construction of the contacts, less arc occurs between the contacts and the contacted metal. Figure 11 illustrates diagrammatically two-piece bar contacts that can be employed, for example in the embodiment shown in Figure 10. In Figure 11, the support 30 slidably receives two bars 25a and 25b, which are longitudinally movable with each other and they move separately towards the metal to be heated, such as by two pneumatic cylinder and piston structures 34. Similarly, the support 31 slidably receives two bars 26a and 26b movable longitudinally to each other and displaced separately towards the metal to be heated, such as by two structures of cylinder and air piston 35. The bars 25a and 25b and the bars 26a and 26b are preferably spaced apart from each other by insulated high temperature spacers 36 and 37. The parts or bars 25a and 25b have downstream edges 12a and 12b respectively which extend at an acute angle of about 35 ° to about 55 ° to the route of the metal to be heated and which are aligned with each other. The parts or bars 26a and 26b similarly have downstream edges 13a and 13b that are similarly oriented and aligned. The supports 30 and 31 are made of conductive metal, for example copper, interconnected to drive the contact parts held in this manner. Although the bar parts 25a and 25b and the bar parts 26a and 26b have been shown to be square cross-sectional, it will be apparent to those skilled in the art that the parts may have other cross-sectional shapes. As previously mentioned, the principles of the invention apply to contact welding methods and apparatuses that are different from the methods and apparatuses shown in Figures 1 to 11, and described in connection therewith. Figures 12 to 17 schematically illustrate other applications of the invention. For example, if it is desired to form pipe with a weld joint or weld two metal parts together with the metal superimposed on the weld as described in US Pat. No. 2,886,691, the contacts 8 and 9 are positioned as illustrated in Figure 12, ie with their downstream edges 12 and 13 extending at an angle of about 35 ° to about 55 °, preferably about 45 ° respect to the routes of the superimposed portions of the metal surfaces to be heated as they are advanced to the welding point. Figure 13 illustrates the location of the contacts 8 and 9 in the welding of the helically formed pipe as described in US Pat. No. 2,873,353, Figure 13 specifically illustrates the lap weld but the location of the contacts 8 and 9 is the same for superposition welding 5. In this way, as the tube 38 with a helical weld seam is rotated in the direction from the arrow 39, a metal strip 40 is advanced in the direction of the arrow 41 towards the welding point Ib. The downstream edges 12 and 13 of the contacts 8 and 9 extend at an angle of about 35 ° to about 55 °, preferably about 45 ° with respect to the paths of the edge surfaces 42 and 43 as they are advanced to the point of Welding Ib. Figure 14 illustrates the location of contacts 8b and 9 in the welding of a fin 44 in a tube as described in U.S. Pat. No. 3,047,712, except that the contact 8b engages the side of the fin 44 instead of the upper edge of the fin 44. Depending on the width of the fin 44, the contact 8 or 8a or any of the contacts previously described, can be used to contact the fin 44. In the relatively narrow fin 44, it may be preferable to use the contact 8a as illustrated in Figure 14. The edges downstream 12 and 13 of the contacts 8a and 9 are oriented as previously described with respect to the routes of the metal to be heated as this metal of the tube 44 and the fin 45 are advanced to the point of welding it. Figure 15 illustrates the location of contacts 8 and 9 in the fabrication of welded structural elements as described in U.S. Pat. Nos. 2,821,619 and 3,391,267, the welding of a T-shaped element is specifically shown in Figure 15, but the location of the contacts with respect to the H-shaped elements is apparent from Figure 15 and U.S. Pat. No. 3,391,267. In this way, two pairs of contacts of the invention replace the two pairs of contacts used in the welding of H-shaped structural elements. In Figure 15, a pair of metal strips 46 and 47 are advanced towards a welding point Id in the direction of the arrow 4 with an intermediate space in advance of the welding point Id. The downstream edges of the contacts 8 and 9, such as the edge 12 of the contact 8, extend at an angle from about 35 ° to about 45 ° and preferably about 45 ° with respect to the metal routes of the strips 46 and 47 to be heated. Since the route adjacent to the contact 9 where the heating current flows is not at the edges of the strip 47, it may be convenient to increase the length of the heating current path in the strip 47 with respect to the path of the current in the strip 46. Accordingly, the contact 9 may be more upstream of the point of welding Id than the contact 8 as illustrated in Figure 15. Figure 16 illustrates the location of the contacts 8 and 9 for the welding of the lips 48 and 49 together, which can be formed on the opposite edges of a metal sheet formed in a tube as illustrated in the US Patent No. 2,992,319, or on the edges of a pair of sheets to be held together. In this way, there are a pair of metal surfaces 50 and 51 terminating at lips 48 and 49, for welding by forging together at a soldering point. The lips 48 and 49 are advanced towards the welding point with an intermediate space and the heating current circulates in the facing surfaces of the lips 48 and 49 and is supplied by contacts 9 and 8 respectively that couple the surfaces 50 and 52. The downstream edge of the contact 9 extends at an angle from about 35 ° to about 55 °, preferably about 45 °, with respect to the path of the surface of the hot lips 48 and the downstream edge of the contact 8. it extends at a similar angle with respect to the path of the surface of the lip 49 which is heated. Figure 17 illustrates the location of contacts 8 and 9 when a pair of metal strips are welded to a metal tube as described in U.S. Pat. No. 3,375,344. In this way, as the tube 52 and the strips 53 and 54 move in the direction of the arrow 4, the strips 53 and 54 are heated on their faces 53a and 54a and the longitudinal surface portions of the adjacent tube 52 are heated by electrical current supplied through the contacts 8 and 9 connected to a source of high frequency electrical energy by terminals 55 and 56 that are superimposed on the tube 52. In advance of the welding points lf and lg, there are spaces between the strips 53 and 54 and the tube 52. At the welding points lf and lg, the strips 53 and 54 are pressed adjacent the tube 52 to form forging welds therebetween. The edges downstream of contacts 8 and
9 extend with respect to the routes of the metal to be heated, at an angle between about 35 ° and about 55 °, preferably about 45 °. In the previously described embodiments, the edges downstream of the contacts may extend at the same angle with respect to the heated metal paths and advance. However, the edges downstream of the contacts can extend different acute angles with respect to these routes. Also, in some cases, such as when marking one of the metal pieces is not objectionable, the edge downstream of the contact that engages this piece of metal can be extended to 90 ° with respect to the route of the metal to be heated, as in FIG. previous technique
Furthermore, in the previously described embodiments, both contacts may have the same cross-sectional shape, or the cross-sectional shape of one contact may be different from the cross section of the other contact. It will be understood that various methods and devices for cooling the contacts and their supports that are known in the art can and will normally be used in the apparatus of the invention. Although preferred embodiments of the present invention have been described and illustrated, it will be apparent to those skilled in the art that various modifications may be made without departing from the principles of the invention.