Patent Publication Number: US-6710300-B2

Title: Contact tip assembly for mig gun

Description:
This application is a divisional of U.S. patent application Ser. No. 09/648,201 filed Aug. 25, 2000 now U.S. Pat. No. 6,559,416. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains to wire welding, and more particularly to apparatus that conducts welding power to a weld wire. 
     2. Description of the Prior Art 
     To properly perform wire welding, it is imperative that the conduction of welding power from a welding gun to the weld wire be controlled. Historically, welding power conduction occurred inside a copper contact tip in the gun. The contact tip had a hole through which the weld wire passed. The relatively long length of the contact tip and a small clearance between the contact tip hole and the weld wire increased the probability of smooth and consistent power conduction. 
     However, it has long been recognized that a problem existed in conducting the welding power from the contact tip to the weld wire. The problem had several aspects. The first was the lack of constant contact between the contact tip and the weld wire. Under the great majority of operating conditions, the weld wire was indeed in contact with the contact tip. Nevertheless, in some instances there were momentary conditions in which no contact occurred. In those situations, the welding current could arc across the clearance between the contact tip and the weld wire. The arcing was detrimental to the welding process. 
     Some prior welding machines relied on the inherent cast of the weld wire to produce a constant contact between the contact tip and the weld wire. However, non-uniformities of the weld wire cast, combined with movements in space of the welding gun during operation, inevitably resulted in momentary breakages of contact between the contact tip and the weld wire. 
     A second aspect of the welding power conduction problem concerned the size of the area of the interface between the contact tip and the weld wire. As mentioned, for the great majority of time during a welding process, the weld wire was in contact with the contact tip. However, even during those times of contact the areas of contact were usually very small. The typically large welding currents resulted in very high current densities, which tended to produce tiny microwelds between the contact tip and the weld wire. The small welded areas between the contact tip and weld wire were usually quickly remelted or mechanically broken. However, a stick-slip type of weld wire feeding resulted. The stick-slip feeding of the weld wire caused by repeated microweld creation and breakage contributed to poor welding performance. 
     A third aspect of the problem involving welding power conduction in prior welding guns was that is was possible, and even likely, that at some times during a welding operation the weld wire contacted the contact tip at more than one interface. AL any particular moment, the locations of the interfaces were randomly variable along the length of the contact tip. Because of the greater electrical resistance of the weld wire than the contact tip, the amount of welding power conduction was greatest at the interface that was closest to the downstream end of the contact tip. As a result, almost all of the preheating of the weld wire occurred between the downstream interface and the workpiece. Since the interface furthest downstream invariably changed location within the contact tip during a welding operation, the amount of preheating of the weld wire also varied. Changes in the weld wire preheating changed the characteristics of the welding arc and therefore were potentially detrimental to satisfactory welding. 
     Further aspects of the problems involving prior contact tips included wear of the tip clearance hole. The wear contributed to random and inconsistent interfaces between the contact tip and the weld wire. The weld wire frequently had shavings and other debris clinging to it. When the foreign matter entered the contact tip it could become trapped and prevent the weld wire from feeding properly. 
     Related problems included spatter from the welding arc that could fly up and stick to the end of the contact tip, where it was likely to build up and stick to the weld wire. Burnback caused by a failure anywhere in the welding machine could destroy any contact tip. Similarly, excessive heat from the welding arc could cause a contact tip to reach a plastic state and wear out prematurely. 
     It is therefore well recognized that a contact tip can fail in a number of ways. For example, U.S. Pat. No. 3,112,393 mentions the tendency of a weld wire to arc and fuse to the contact tip, which results in a short life for the contact tip. 
     Prior contact tips were often treated as low cost consumable items. Yet, a contact tip failure could disrupt an entire production line. In addition, a malfunctioning contact tip caused inconsistent or improper operation of other components of a welding machine such as the weld wire feeder or the gas flow. Frequently, other welding machine components than the contact tip were searched and mis-diagnosed as the source of the problem. 
     Accordingly, numerous attempts have been made to solve one or more aspects of the contact tip and the weld wire interface problem. For example, to assure a constant interface, power carrying wire feed rollers were used in some welding machine guns to ensure a constant location of the interface. The feed rollers were not the complete answer, however, because additional interfaces were present at the contact tip. As mentioned, the electrical resistance of the contact tip was less than the resistance of the weld wire. Hence, even with conductive feed rollers the most downstream interface between the contact tip and the weld wire remained random and uncontrollable. 
     Other proposed solutions included a non-circular clearance hole in the contact tip, such as is shown in U.S. Pat. No. 5,635,091. U.S. Pat. No. 5,278,392 shows a contact tip that is covered with a graphite cover. U.S. Pat. Nos. 4,945,208 and 4,978,831 teach a chrome plated bushing covering the downstream end of a contact tip to eliminate carbon residue on the tip. 
     A solution for arcing and microwelding proposed by U.S. Pat. No. 3,112,393 is a contact tip having a helical clearance hole for the weld wire. A somewhat similar proposal is described in Great Britain patent 2,074,069A. That patent shows a contact tip with one or two projections inside the contact tip clearance hole. The projections force the weld wire to change direction inside the contact tip and thereby assure good contact with the contact tip. Another helix-related contact tip is described in U.S. Pat. No. 4,733,052, in which a contact tip is in the form of a rectangular strip wound into a helix. A central longitudinal opening through the helix is smaller than the diameter of the weld wire, such that the weld wire is resiliently squeezed by the contact tip. 
     U.S. Pat. 4,563,569 describes a welding gun in which a weld wire guide tube is transversely pivotable within the gun body. Springs within the gun urge the guide tube transversely such that the weld wire is urged into transverse contact with the clearance hole in the contact tube, which is fixed relative to the gun body. 
     In U.S. Pat. 3,102,947, arcing between the contact tip and the weld wire is prevented by lining the contact tip with an insulative sleeve. Welding power is transferred to the weld wire upstream of the contact tip. 
     U.S. Pat. No. 4,988,846 describes a contact tip with an arcuate groove cut in the wall between two coaxial clearance holes for the weld wire. The weld wire contacts and is deflected by the groove surface, thus assuring an intimate interface between the contact tip and the weld wire. The same principal is used in U.S. Pat. No. 3,142,746, which discloses a ball that partially closes the clearance hole in the contact tip. The ball is biased radially against the weld wire to assure a constant interface between the contact tip and the weld wire. 
     U.S. Pat. No. 4,731,518 teaches a welding gun with a reverse radius in the head tube between the gun handle and the contact tip. The reverse radius causes the weld wire to be biased against the clearance hole in the contact tip. 
     Japan patent JP-11170052-A describes three parallel cylindrical columns arranged in a triangle to form a central hole through which a weld wire passes. The three columns are held in place against each other by an elastic band surrounding the columns. The elastic band, in turn, is captured inside a tube. The elastic band urges the columns radially inwardly against the weld wire. 
     Despite the long felt need for a controlled and reliable interface between a contact tip and a weld wire, and despite the numerous attempts to solve the interface problem, none of the prior solutions is entirely satisfactory. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, an alternate current path for a metal inert gas (mig) gun reliably assures a stable interface between a contact and a weld wire. This is accomplished by apparatus that includes fingers that maintain positive contact with the weld wire under all operating conditions 
     According to one aspect of the invention, the fingers are an integral part of a conductive insert that fits in a welding gun diffuser. The fingers are on an end of a tubular body that is split longitudinally. The body has a nominal outer diameter that is slightly greater than an associated inner diameter of the diffuser. Installing the insert in the diffuser causes the body and fingers to collapse radially slightly. The fingers are then spaced apart a distance less than the diameter of the weld wire. Feeding the weld wire resiliently springs the fingers radially apart. The natural inwardly restoring force of the insert material produces a reliable interface for conducting welding power from the diffuser to the weld wire. The insert thus provides additional stable paths for the welding power from the welding gun to the weld wire. The current densities along the additional paths are reduced. In turn, the reduced current densities reduce the likelihood of microwelds occurring at the weld wire that could cause stick-slip feeding during welding operations. 
     According to another aspect of the invention, the fingers are incorporated into a contact tip that is part of a contact tip assembly. A body on a first end of the contact tip is threaded to engage a diffuser. The fingers are at the second end of the contact tip and are integral with the body. The outer diameter of the fingers at the contact tip second end define concentric partial frusto-conical surfaces that diverge-toward the second end. An annular member such as a sleeve fits over the contact tip. A frusto-conical surface on the inner diameter of the sleeve matches the frusto-conical surfaces of the fingers. A spring is interposed between the sleeve and the diffuser. The spring urges the sleeve frusto-conical surface against the finger frusto-conical surfaces and thereby biases the fingers radially inwardly against the weld wire, which is fed through the contact tip and the fingers. Not only do the fingers provide a reliable interface that eliminates arcing, but they also assure that the interface is only at the downstream end of the contact tip. Consequently, there is only one consistent location for the interface. Weld wire preheating is thus constant, which greatly aides in controlling the welding process. To be certain that the weld wire does not protrude through any of the spaces between the fingers, a liner may be assembled inside the contact tip surrounding the weld wire. 
     It is a feature of the invention that the contact tip assembly further comprises a non-metallic shield surrounding the contact tip and the sleeve. The weld wire passes through a clearance hole in an end wall of the shield. The shield protects the contact tip from welding spatter and burnback. To prevent burnback at the contact tip, the shield clearance hole is substantially larger in diameter than the weld wire diameter. Any molten material created during a burnback situation is not sufficient to plug the shield hole and thereby cause harm to the contact tip. 
     In a modified embodiment of the invention, the contact tip assembly has a contact tip with fingers that define concentric partial frusto-conical surfaces that converge toward the second end. An annular member in the form of a casing has a first end with a trusto-conical inner surface that matqhes the contact tip frusto-conical surfaces. A spring acts between the casing second end and contact tip to urge the contact tip frusto-conical surfaces against the casing frusto-conical surface. The contact tip fingers are thus biased radially inwardly against a weld wire. The radially inward force of the fingers on the weld wire eliminates arcing and also provides a reliable interface between the contact tip and the weld wire only at the downstream end of the contact tip. 
     An alternate embodiment of the alternate current path according to the invention comprises a contact tip assembly with a pivotable finger incorporated into a contact tip. One end of the contact tip engages a diffuser. The outer diameter of the contact tip is recessed at its second end such that a short length of the weld wire is exposed. The finger is connected to the contact tip for pivoting about an axis perpendicular to the weld wire. The finger has a tab that fits in the contact tip recess and contacts the exposed length of the weld wire. A spring acts between the contact tip and the finger second end in a manner that presses the finger tab against the weld wire. The result is a reliable interface between the weld wire and the contact tip only at the contact tip downstream end. 
     In a further alternate embodiment, the alternate current path comprises a contact tip assembly having a finger that is wedged in a contact tip. The outer diameter of the contact tip is slotted such that a length of the weld wire is exposed The contact tip is further shaped with an angled notch adjacent the recess. The finger fits in the contact tip slot and is held there by an end that fits in the angled notch. The finger has a longitudinal surface that contacts the exposed length of the weld wire to assure a single reliable interface between the weld wire and the contact tip. 
     Other advantages, benefits, and features of the present invention will become apparent to those skilled in the art upon reading the detailed description of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a exploded perspective view of a typical prior mig welding gun that may be modified to advantageously include the present invention. 
     FIG. 2 is an front view of an insert having resilient fingers that provide an alternate current path for a mig welding gun according to the present invention. 
     FIG. 3 is a longitudinal cross sectional view of FIG.  2 . 
     FIG. 4 is a first end view of FIG.  3 . 
     FIG. 5 is a second end view of FIG.  3 . 
     FIG. 6 is an enlarged partial view of the insert of FIGS. 2-5 assembled in a welding gun. 
     FIG. 7 is a longitudinal cross sectional view of a modified contact tip assembly according to the present invention. 
     FIG. 8 is a cross sectional view taken along line  8 — 8  of FIG.  7 . 
     FIG. 9 is a cross sectional view taken along line  9 — 9  of FIG.  7 . 
     FIG. 10 is a longitudinal cross sectional view of a further modified contact tip assembly according to the present invention. 
     FIG. 11 is a first end view of FIG.  10 . 
     FIG. 12 is a second end view of FIG.  10 . 
     FIG. 13 is a cross sectional view taken along line  13 — 13  of FIG.  10 . 
     FIG. 14 is a exploded front view of an alternate embodiment of the present invention. 
     FIG. 15 is an exploded top view of FIG.  14 . 
     FIG. 16 is an exploded end view of FIG.  14 . 
     FIG. 17 is a side view of a further alternate embodiment of the invention. 
     FIG. 18 is a top view of FIG.  17 . 
     FIG. 19 is an end view of FIG.  18 . 
     FIG. 20 is a front view of the finger that is used in the alternate embodiment of the invention of FIGS. 17-19. 
     FIG. 21 is a top view of FIG.  20 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the disclosure hereof is detailed and exact to enable those skilled in the art to practice the invention, the physical embodiments herein disclosed merely exemplify the invention, which may be embodied in other specific structure. The scope of the invention is defined in the claims appended hereto. 
     Briefly by way of background, FIG. 1 shows the major components of a typical prior mig welding gun  1 . The gun  1  includes a handle  3  that is connected by a cable  5  to a welding machine, not shown. A head tube  7  is joined to the handle  3 . A sleeve  9  is threaded onto the downstream end  11  of the head tube  7 . In turn, a hollow diffuser  13  is joined to the sleeve  9 . Threads of a contact tip  15  engage threads in the diffuser  13 . A nozzle  17  surrounds the diffuser  13  and the contact tip  15 . A long flexible weld wire  19  is fed from the welding machine through the cable  5 , handle, head tube, and out the contact tip. Welding power from the welding machine is conducted through the cable, handle, and head tube to the diffuser and contact tip. The welding power is conducted to the weld wire  19  as it slides through the contact tip. 
     In accordance with the present invention, an alternate path is provided for conducting the welding power from the welding gun diffuser  13  to the weld wire  19  in addition to the path through the contact tip  15 . Looking at FIGS. 2-5, the alternate current path is provided by an insert  21 . In the illustrated construction, the insert  21  is fabricated from a thin piece of resilient metallic material. The insert has a tubular body  23  with a longitudinal axis  22  and an end  24 . The nominal diameter of the tubular body  23  is slightly greater than the inner diameter of the diffuser (FIG.  1 ). 
     The insert  21  at its second end  26  is trimmed on opposite sides of the longitudinal axis  22  at reference numerals  25  to make a pair of fingers  27 . The fingers  27  are bent in reverse bends  29  and  31  toward the axis  22  such that the fingers are close to each other. The body  23  is slit longitudinally at reference numeral  32  between the end  24  and one of the trim lines  25 . 
     When the insert  21  is installed in the inner diameter  33  of the diffuser  13 , FIG. 6, it collapses slightly such that the spacing between the fingers  27  is less than the diameter of weld wire  19 . Consequently, the weld wire spreads the fingers apart when the weld wire is initially fed through the insert. The restoring force of the resilient insert material produces a firm and reliable path for conducting welding power from the diffuser to the weld wire. The current densities between the weld wire and the insert fingers are less than if there were only a single interface between the weld wire and the contact tip  15 . As a result, the probability of arcing or microwelding of the weld wire to the contact tip or to the insert is greatly diminished compared with using only the prior contact tip  15  for power conduction. 
     Turning to FIGS. 7-9, a path for conducting welding power from a diffuser  13 ′ to a weld wire  19 ′ utilizes a contact tip assembly  34 . In the preferred embodiment, the contact tip assembly  34  comprises a contact tip  36  having a first end  37  and a second end  39 . A body  38  at the first end  37  has external threads that engage internal threads of the diffuser  13 ′. Flexible fingers  35  at the contact tip second end  39  are electrically conductive with the body  38 . As illustrated the flexible fingers  35  are integral with the body  38 . Alternately, the flexible fingers are not necessarily integral with the body, but the fingers must at least be electrically conductive with the body. The fingers  35  define respective concentric partial frusto-conical external surfaces  41  that diverge toward the second end  39 . 
     As illustrated, the contact tip  36  has three fingers  35 , although more or fewer fingers are within the scope of the invention. The fingers are separated by longitudinal slits  42  in a tubular portion  46 . The slits  42  terminate at respective edges  43  of the body  38 . The slit tubular portion  46  acts as a number of flexible necks between the body and a head  68  of each finger. The fingers have respective first arcuate inner surfaces  44  that may be continuations of an inner diameter  47  of the body and that cooperate to make a first inner diameter of the finger necks. Near the contact tip second end  39 , the finger heads  68  have respective second inner surfaces  45  that cooperate to make a second inner diameter. The second inner diameter may be slightly less than the diameter of the weld wire  19 ′. Alternately, the second inner diameter may be equal to or slightly larger than the diameter of the weld wire  19 ′, in which case a spring  52  urges the inner surfaces  45  into contact with the weld wire, as will be explained shortly, at a slightly different location. The second inner surfaces  45  may be relatively narrow in the transverse direction such that they approach being knife edges. Alternately, and as shown, the inner surfaces may be relatively wide in the transverse direction. In that case, the inner surfaces  45  are arcuate in shape to form an inner diameter that has generally the same diameter as the weld wire. The fingers also have respective arcuate outer surfaces  48  that cooperate with the first inner surfaces  44  to define a wall thickness of the finger necks at the tubular portion  46 . The wall thickness of the necks is sufficiently thin such that the fingers can bend in cantilever fashion about the body. 
     An annular member in the form of a sleeve  49  surrounds most of the contact tip fingers  35 . The sleeve  49  has an inner diameter  50  with a frusto-conical surface  51  at one end. The sleeve frusto-conical surface  51  mates with the frusto-conical surfaces  41  of the fingers. A compression spring  52  is interposed between an end  53  of the diffuser  13 ′ and the second end  54  of the sleeve. The spring  52  urges the sleeve frusto-conical surface  51  against the finger heads frusto-conical surfaces  41 . As a result, the fingers are biased radially inwardly such that the surfaces  45  make intimate contact with the weld wire  19 ′. The sleeve outer diameter  62  is preferably relieved for part of its length from the end  54  to a step  63 . 
     To prevent the weld wire  19 ′ from protruding between the contact tip slits  42 , a liner  55  is also part of the contact tip assembly  34 . The liner  55  has an outer diameter  58  that fits loosely in the inner diameter  44  of the finger necks. An inner diameter  59  of the liner loosely receives the weld wire. Preferably, the liner extends for a majority of the length of the contact tip body  38  and tubular portion  46 . The liner is made from an insulative material. 
     The final component of the contact tip assembly  34  is an annular shield  56 . The shield  56  has a wall  61  with internal threads that mate with external threads on the diffuser  13 ′. The shield surrounds a part of the diffuser and the fingers  35 , and the sleeve  49 . The shield has an end wall  57  close to the contact tip second end  39 . The shield end wall  57  has a hole  74  through it that is substantially larger in diameter than the diameter of the weld wire  19 ′. Several radial holes  60  pass through the wall  61  of the shield near the sleeve step  63 . The shield is made of an insulative material, such as a ceramic material having a high melting temperature. The inner end wall  70  of the shield is spaced from the end  39  of the contact tip  36  to allow debris and spatter to accumulate therein without hindering the operation of the contact tip. 
     In use, the contact tip assembly  34  is assembled to the diffuser  13 ′ in a manner generally similar to the installation of a conventional contact tip. A conventional nozzle  17 ′ surrounds the diffuser and the contact tip assembly. The contact tip assembly assures a reliable interface between the finger surfaces  45  and the weld wire  19 ′. The relatively long length of the finger surfaces  45  reduce welding power density between the contact tip  36  and the weld wire. Further, the interface between the contact tip and the weld wire occurs only at the surfaces  45 , which is at a constant distance from a workpiece. Consequently, preheating of the weld wire between the workpiece and the contact tip is uniform for all welding conditions. The spring biased fingers  35  scrape off any grit or other foreign material that is on the weld wire before it passes between the surfaces  45 . The welding quality is thus not compromised by foreign matter at the welding arc. 
     Arrows  64  and  65  represent the paths of the flows of the inert gas used with mig welding. Most of the gas flows outwardly through radial holes  66  in the diffuser  13 ′, arrows  64 . Some of the gas, arrows  65 , flows downstream around the weld wire  19 ′ inside the liner inner diameter  59 , through the contact tip slits  42 , past the spring  52 , and out the shield holes  60 . The paths  64  and  65  recombine as the gas leaves the nozzle, arrow  67 , to surround the welding arc. The hole  74  in the shield wall  57  allows the weld wire  19 ′ to pass through but prevents burnback of the weld wire from reaching the contact tip  36 . The hole  74  is sized tq be relatively large such that only an extremely large burnback ball could plug the hole and fuse the weld wire to the shield. 
     FIGS. 10-13 depict a further modified construction for a contact tip assembly  69  according to the invention. A contact tip  71  has a body section  75  with a first end  72 . Threads  73  at the first end  72  on the body section  75  engage a conventional diffuser, not shown. The contact tip body section has an inner diameter  77  that is slightly larger than the diameter of a weld wire  79 . On the outer diameter of the body section is a flange  81 . 
     The contact tip  71  further has a number of fingers  83  that are integral with the body section  75 . As illustrated, there are four fingers  83 , but more or fewer fingers are also acceptable. The fingers are separated by radial slits  85  from the contact tip second end  87  to the body section  75 . The contact tip fingers have respective head portions  90  that connect to the body section by respective flexible necks  93 . The head portions  90  have respective partial frusto-conical outer surfaces  91  that converge toward the second end  87 . The head portions also have respective inner surfaces  89  that together define a circular passage concentric with the body section inner diameter  77 . 
     Surrounding the contact tip  71  except for the threads  73  is an annular member in the form of an insulated casing  95 . The particular casing  95  illustrated has an interior surface  97  consisting of a cylindrical surface  103  and a frusto-conical interior surface  105 . The casing further has a cylindrical exterior surface  106  parallel to the interior surface  103 , and a frusto-conical surface  108  that is generally parallel to the frusto-conical interior surface  105 . Alternately, other slopes could be used, including faceted sides and pyramidal sides. The casing frusto-conical interior surface  105  converges at the same angle as the frusto-conical surfaces  91  of the contact tip fingers  83 . 
     The contact tip assembly  69  further comprises a snap ring  107  in the cylindrical interior surface  103  of the casing  95 . A spring  109  is interposed between the snap ring  107  and the flange  81  of the contact tip  71 . The spring  109  urges the frusto-conical interior surface  105  of the casing against the contact tip fingers  83 . That action biases the fingers radially inwardly against the weld wire  79 . In that manner, there is always a stable interface between the finger surfaces  89  and the weld wire. Consequently, the transfer of welding power from the contact tip to the weld wire is achieved without arcing, microwelding, or variable preheating of the weld wire. 
     FIGS. 14-16 show an alternate contact tip assembly  111  that reliably conducts welding power to a weld wire  115 . The contact tip assembly  111  has a contact tip  116  with threads  117  that engage a diffuser, not illustrated in FIGS. 14-16. A hole  118  extends through the contact tip  116  between its ends  120  and  131 . The diameter of the hole  118  is slightly larger than the diameter of the weld wire  115 . 
     The contact tip  116  is formed with three pockets. A first pocket  119  near the threads  117  receives a spring  121 . The first pocket  119  does not break into the hole  118 . A second pocket  123  may be obround in shape. A first hole  125  extends transversely from the second pocket  123  to the outside of one side of the contact tip  116 . A second hole  127  extends from the second pocket oppositely from and concentric with the first hole  125 . Preferably, the second hole  127  is threaded. The second pocket does not break into the hole  118 . The third pocket  129  is elongated and is at the contact tip second end  131 . The third pocket  129  breaks into the hole  118 . 
     The contact tip pockets  123  and  129  are designed to accommodate a finger  133 . The finger  133  has a lug  134  that fits in the pocket  123 . A screw  135  passes through the contact tip hole  125  and a hole  137  in the finger lug  134 , and threads into the contact tip hole  127 . When assembled, a finger tab  139  fits in the body pocket  129 . The spring  121  biases a surface  141  of the finger tab  139  against the weld wire  115 . In that manner, the weld wire is consistently in contact with both the hole  118  in the contact tip and the surface  141  of the finger tab. The result is uniform welding power transfer, without arcing or microwelding of the weld wire, during a welding process. 
     In a further alternate embodiment as depicted in FIGS. 17-21, a contact tip assembly  143  comprises a contact tip  145  and a finger  147 . The contact tip  145  is generally cylindrical in shape, having two ends  149  and  151 , a longitudinal axis  152 , and a longitudinal hole  153 . The hole  153  receives a weld wire  154 . The contact tip has threads  155  near the end  149 . A transverse slot  157  is cut through the contact tip. The slot  157  has a transverse wall  159  generally perpendicular to the longitudinal axis  152 , and a flat bottom wall  161  parallel to the longitudinal axis. The bottom wall  161  opens into the hole  153 . There is an angled wall  163  connecting the bottom wall opposite the transverse wall. The angled wall  163  terminates in a short end wall  165  that is generally parallel to the transverse wall  159 . The contact tip also includes a longitudinal slot  167 . The longitudinal slot  167  intersects the slot  157  at the transverse wall  159 . 
     The finger  147  has a side edge  169  and a bottom edge  171  perpendicular to the side edge. An angled edge  173  meets the bottom edge  171  at a tip  174 . The angled edge  173  connects to the side edge  169  by means of a chamfer  175 . There is also a chamfer  177  between the side and bottom edges. The width of the finger is slightly less than the width of the contact tip longitudinal slot  167 . 
     In use, the finger tip  174  is placed under the angled wall  163  of the contact tip  145  with the finger  147  also being partially within the contact tip longitudinal slot  167 . The finger is simultaneously tilted and inserted into the slots  157  and  167  such that the finger bottom surface  171  is in flat intimate but non-binding contact with the weld wire  154 . In that manner, conduction of welding power from the contact tip to the weld wire occurs in a reliable manner at a controlled interface. 
     Thus, it is apparent that there has been provided, in accordance with the invention, a current path for mig welding guns that fully satisfies the aims and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims.