Abstract:
This invention provides a method of welding a flange nut to a workpiece surface using an improved design of a welding electrode. The electrode comprises a parts-receiving recess adapted to receive a stem portion of the nut, while maintaining a clearance between the recess and the stem. This clearance allows electrical current to be directed toward the flange portion of the nut. As a result, electrical contact between the workpiece and projections underlying the flange is greatly improved, thereby produce higher quality spot welds and increasing the life of the electrode tip.

Description:
TECHNICAL FIELD 
     This invention generally relates to welding flange nuts to other objects and, more specifically, to a method of welding a flange nut to a workpiece by directing current flow through weld projections on the flanged portion of the nut. An improved electrode design facilitates use of the method. 
     BACKGROUND OF THE INVENTION 
     Flanged nuts have integral washers that simplify handling. They are useful in assembling components using a bolt fastener and the flange can bridge an oversized hole in an attached object. Flange nuts are commonly pre-attached to an object so that the object can be bolted to something else. This method of attachment is useful in joining automotive body parts using welded flange nuts, which facilitates the assembly of the vehicle. 
     Projection welding is a common practice for joining a flange nut to a metal sheet or other workpiece. Integral projections on the metal sheet contacting side of the flange provide fusible metal for the weld. Two counter-acting welding electrodes with flat contact surfaces hold the flange nut and workpiece together at a joining region with the projections on the flange pressing against the surface of the workpiece. A controlled electrical power source delivers a pulse of high amperage AC (or rectified AC) current through the facing electrodes, through the interposed nut, and the workpiece. Typically a 60 Hertz welding current is applied for several cycles of electrical current application (i.e., a fraction of a second). Although the entire nut and underlying workpiece area are heated, the higher density and momentary current flow through the projections tend to selectively melt the projections. When the current flow is stopped, the melt solidifies and the projections are fused to the workpiece. 
     Currently, flange nut welding processes use a copper welding electrode with a flat, full faced electrode tip to engage the top of the flange nut. The flat welding electrode tip is large enough to fit flange nuts of different sizes and it is effective in applying clamping pressure on the flat surface of the top of the nut&#39;s stem. However, the current flow through the stem may be sufficient to soften it during the brief welding period and the hot electrode can stick to the workpiece. The counteracting electrode, which engages the opposite side of the workpiece, usually experiences no such sticking. 
     Thus, it is an object of the present invention to provide an improved method for welding a flange nut to a workpiece, such as a sheet metal part. The purpose of the method is to avoid sticking of nut metal to the electrode and thus to increase the reliability of the process in production and to increase the useful life of the electrode tip. It is a further object of the present invention to provide an improved electrode tip design for contacting the flange nut during welding and to facilitate the practice of the process. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of welding a flange nut to a surface of a metal sheet, or other workpiece, using an electrode design for the nut that engages only the flange portion of the nut. 
     A flange nut, comprising a stem, a flange at one end of the stem, and a hole extending through the stem, is welded to the workpiece at a predetermined attachment location. Typically, the workpiece will have a bolt hole, or the like, at the welding location so that after attachment of the flange nut the workpiece can be bolted to another object. 
     The flange portion of the nut, having pre-formed underlying projections for welding, is placed against the surface of the workpiece so that the projections lie against it and separate the nut from the workpiece by the height of the projections. Opposing electrodes engage the nut and the opposite side of the workpiece, pressing them together for effective welding force and current flow. A suitable welding current is then directed to the electrodes and through the flange portion of the nut, through the weld material projections from the flange, and through the abutting workpiece. The duration of current flow is brief, typically less than a second. The current density in the relatively small projections is sufficient to fuse them without fusing the larger flange material that carried them. The molten metal from the melted projections flows between the flange and workpiece surface or is expelled under the pressure of the electrodes. When current flow is stopped the molten metal is cooled and solidified by heat flow to the cooler surrounding metal of the nut and workpiece. 
     The weld material projections are integral with the flange and formed of the same material. They provide sufficient metal for the weld. Often the flange is round and formed by upsetting one end of the stem of the nut. The projections are formed at the same time and may, for example, be shaped like truncated cones spaced in a circle around the bottom surface of the flange. However, in a preferred embodiment of the invention, the weld projections are tapered and spaced circularly on the bottom of the flange. 
     The method of the present invention preferably employs a welding electrode having an improved design for better weld quality. The electrode for engaging the flange nut comprises a recess formed in the face of the electrode tip that enables the rim of the electrode tip to fit over the stem portion of the nut without touching it. As a result, the hollow electrode tip establishes clamping pressure and electrical contact with only the flange portion of the nut. The current then passes mainly through the flange and its underlying projections to the workpiece metal. The contact of the annular electrode tip with the flange of the nut does not result in flange metal adhering (sticking) to the electrode during the projecting welding operation. 
     Industrial processes rely on the durability of their welding electrodes to produce hundreds of individual welds having superior weld quality without constant replacement of the electrodes. Electrode sticking usually degrades an electrode tip and impedes current flow. The method and electrode design of the present invention reduces electrode failure and welding inefficiencies by redirecting the electrical current away from the stem portion of the nut. The result is improved weld quality and improved life of the electrode tip. 
     These and other objects and advantages of this invention will become apparent from a detailed description of the preferred embodiment that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view of a flange nut-to-workpiece and welding electrode assembly for use in practicing the present invention. 
     FIG. 2 is a side view, in cross-section, of the welding assembly showing how the hollow, flat rimmed electrode tip fits over the stem portion of the flange nut to press against the upper surface of the flange without contacting the stem. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The welding method and electrode design of the present invention provide improved welding current contact between the workpiece and welding projections from the flange nut. Weld quality is increased and electrode sticking is reduced. This process is generally referred to as projection welding. Projection welding is a form of resistance welding in which current flow and heat are localized in weld metal projections that are integral with an object to be welded. 
     A preferred embodiment of the present invention is illustrated in FIGS. 1 and 2. FIG. 1 shows the opposing electrodes, as well as the flange nut and sheet metal to be joined, in a separated but juxtaposed position so that their shapes and interaction can be more clearly illustrated. FIG. 2 shows the objects and electrodes in a sectional view assembled for welding in accordance with this invention. 
     Flange nut  10  is seen as an oblique, generally side view in FIG. 1. A typical flange nut  10  comprises a stem  12  that is often hexagonal in cross-section and a larger diameter round flange  14 . Flange  14  can act like an integral washer. A central hole  16  extends through stem  12  and is adapted to receive a fastener (not shown), such as a guide pin, a bolt, a screw, or the like. Hole  16  is illustrated as threaded but it may be plain. Depending on the type of fastener used and the attachment part to be joined to the workpiece after the nut has been welded thereon, hole  16  can be provided in any shape, size or form. 
     Flange  14  of nut  10  comprises an upper circular surface  20  surrounding stem  12 . Upper surface  20  of flange  14  will be contacted by a welding electrode as described below. Flange  14  also has a lower circular surface  22  that carries three integral underlying arcuate projections  24 . Projections  24  will ultimately be welded to sheet metal part  30  as will be described. 
     A projection is a small mass of welding material, comprising any shape or size, and is typically the same material as that of flange nut with which it is formed. In this example, projections  24  are formed integrally with lower or joining surface  22  of flange  14  and located for welding of the flange nut to a sheet metal article or other part. For illustrative purposes, three quadrilaterally-shaped (in cross-section) projections  24  that taper toward the workpiece surface are shown in FIGS. 1 and 2. However, more or less projections of any suitable shape can be used in practicing the method of the present invention. 
     Generally, a flange nut  10 , comprising underlying welding projections  24 , is placed on a facing surface  31  of a selected workpiece  30  at a joining region. In this example, workpiece  30  is a sheet metal article with a bolt hole  32  at the intended flange nut  10  attachment location. Flange nut  10  with its welding projections  24  is positioned over a bolt hole  32  in contact with surface  31  of sheet metal workpiece  30 . Only the flat bottom surfaces  34  of projections  24  actually engage surface  31  prior to welding. The assembled weld nut  10  and workpiece  30  are pressed together by welding electrode  40  and counter-electrode  50 . Electrode  40  engages the flange nut  10  and counter-electrode  50  engages the opposite side of workpiece  30 . Electrodes  40  and  50  are used to press flange nut  10  and workpiece  30  into tight engagement for welding and to deliver a suitable pulse of welding current for fusion of projections  24  to accomplish the weld. Resistance to electrical current by the workpiece metal generates heat in the flow path of the current, thereby causing the metal located at the joining region to melt. Nut  10  is then pushed, by the electrode force, into the molten metal of workpiece  30 . Upon cooling, the molten material solidifies and forms a weld. 
     Weld quality often depends on suitable electrical contact area of projections  24  with workpiece  30 . The three projections  24  are arcuate in shape (suitably about 60 degrees of arc) with square, or quadrilateral or triangular cross-sections that taper toward a flat workpiece contacting surface  34 . Projections  24  are sized in proportion to varying flange sizes in order to accomplish a consistently high quality weld. Furthermore, projections  24  are arranged on weld surface  22  to uniformly engage workpiece  30 . 
     When flange nut  10  is placed on workpiece  30  prior to welding, flange nut  10  is positioned such that projections  24  formed on weld surface  22  of flange  14  directly contacts workpiece  30 . If a guide pin is to be inserted through the workpiece and weld nut as a locating device, flange nut  10  will be placed on workpiece  30  over an opening  32 , directly aligning central hole  16  of nut  10 . Backup  50 , also comprising a hole  58 , is arranged in a similar fashion for insertion of a guide pin. 
     Electrode  40  is specially designed for pressure application and weld current delivery to flange nut  10 . Only the tip portion of electrode  40  is shown in FIGS. 1 and 2, the remainder of the electrode being of suitable standard configuration for the welding of workpieces. Specifically, the welding tip  42  of electrode  40  is a hollow circular cylinder sized to receive stem  12  of nut  10  without contacting stem  12 . The end of welding tip  42  has a flat contact surface  44  for pressure and current supplying engagement with upper surface  20  of flange  14 . Electrode tip  42  is generally cylindrically shaped, however it can be shaped to suitably fit over flange nuts of varying conformations. Furthermore, tip  42  may be adapted to receive adjustable electrical insulator or conductive inserts  46  where each insert is sized and shaped to accommodate a specific flange nut stem design. The insert  46  shown in FIG. 1 is simply a cylinder shaped conductor for increased electrode contact with flange  20  without contacting hexagonal stem portion  12  of flange nut  10 . Depending upon the fit between the electrode tip  42  and stem  12  it may be desired to use an insert  46  to prevent contact between the tip  42  and stem  12 . These adjustable inserts can be inserted in hollow electrode tips  42  by any suitable means, such as snapping or screwing. Alternatively, the adjustable inserts can simply be a series of removable electrode tips where each tip comprises a recess sized and shaped to accommodate a specific flange nut design. Again, the electrode tips can be attached to electrode  40  by any suitable means. 
     In accordance with this invention, electrode tip  42  is intended to contact only the upper surface  20  of flange  14 . The purpose of hollow tip  42  is to deliver the welding current only through flange  14  and projections  24  to workpiece  30  and counter-electrode  50 . Thus, when electrode  40  is placed over nut  10  (i.e., where stem  12  fits inside electrode tip  42 ), it is preferred that no surface of stem  12  touches or contacts any inner surfaces of tip  42 . Otherwise, any direct contact between electrode  40  and stem  12  will direct a substantial amount of electrical current toward stem  12  rather than toward projections  24  located on flange  14 . As a result, current density will be lower at the projection-to-workpiece interface, thereby creating spot welds having inadequate weld quality. 
     Generally, when a conventional electrode directs electrical current through the stem  12  of a flange nut and a workpiece, the electric current traveling through weld surface  20  of flange  14  is a relatively low density current. This requires longer welding current flow cycles, which unnecessarily heats the stem portion of the nut leading to electrode wear and poor welding results. The subject process and electrode design concentrates the welding current where it is needed to reduce electrode degradation and improve welding results. 
     Electrode contact with and current flow through stem  12  will cause significant temperature increases at the contacted surface. This, in effect, causes a conventional electrode  40  to stick to the top surface of the nut which can eventually destroy the electrode tip. As illustrated in FIG. 2, electrode tip  42  is designed to receive nut  10  while maintaining a clearance  56  between the outer surfaces of stem  12  and the inner surfaces of tip  42 . Electrical current flow from tip  42  is localized at flange  14 . Welding current is specifically directed through projections  24  to workpiece  30 . The relatively high current density caused by the practice of this invention and the electrical resistance of projections  24  promotes rapid and effective melting of the intended weld material in the projections. The effective pressure delivered by tip  42  to flange  20 , the shape of projections  24  and the concentrated heating at their contact with workpiece surface  31  leads to the reliable formation of effective welds between nut  10  and workpiece  30 . After termination of the welding current pulse the molten metal is cooled by the surrounding metal masses and flange nut  10  is welded to workpiece  30  as a result of the coalescence. Uniform division of welding current and relatively even heating of all projections is provided, thereby creating a high quality weld and minimal damage of the electrode tip. 
     While the invention has been described in terms of a preferred embodiment, it is not intended to be limited to that description, but rather only to the extent of the following claims.