Abstract:
An electrode for the projection welding of a weld nut to a workpiece has an electrode welding head with parts including a replaceable welding interface disc fitted therein. The electrode can be disassembled so that the conductive electrode interface within the welding head may be replaced when it becomes worn.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 61/016,527 filed Dec. 24, 2007. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to the art of projection welding. More specifically, it applies to a welding head used during projection welding to attach a fastener such as a weld nut, a stud or similar item to a work part. The present invention specifically addresses a configuration by which worn parts in such a welding head may be readily replaced. 
       BACKGROUND TO THE INVENTION 
       [0003]    Projection welding is a resistance welding process which uses heat obtained from resistance to a high electrical current through the work parts held together under pressure by electrodes to effect welding. In so doing a coalescence or welding of the metals is obtained. In spot welding, the size and position of the welds are determined by the size of the electrode tip and the contact point on the workpieces. In projection welding the size and position of the weld or welds are determined by the design of the component to be welded. The welding current and applied force are concentrated in a small, concentrated contact interface formed between the parts to be welded. 
         [0004]    In one application of projection welding special nuts, called “weld nuts”, are employed that have projections on the portion of the weld nut to be welded to the sheet metal, also known as the substrate or work piece. The use of projections formed on one face of a pilot nut ensures that a much higher current density is achieved within the small projections, so that the projections are raised to a temperature whereat the metal liquefies. In combination with the pressure from the electrodes sandwiching the nut and substrate together, this causes a strong weld by which the nut is permanently fixed to the substrate. 
         [0005]    Weld nuts come in two types. Weld nuts with a protruding cylinder or pilot ring, known as “self-piloted” or “piloted nuts”, are self-aligning to a hole in a work piece into which the piloting ring is fitted. A piloted weld nut has generally a flat, annular side surface on one end and on the other end, a parallel, peripheral, annular outer surface surrounding an inner protruding central cylinder that serves as a pilot ring. The pilot ring is topped by its own flat, annular, pilot ring end face. The annular outer surface surrounding the pilot ring carries the protrusions where welding is to occur. 
         [0006]    Non-piloted weld nuts lack the annular pilot ring but otherwise include the protrusions that are necessary for welding. Studs may also be welded to workpiece by resistance welding. While the balance of this disclosure is directed to weld nuts as an example, the invention is equally applicable to the welding of any form of faster and the like to a workpiece. 
         [0007]    In carrying-out the welding process a weld nut is held against a metallic work piece, typically at a point where a hole has been formed in the work piece, by a welding electrode head. For purposes of discussion this may be referred to as the “active” electrode, although it is characterized electrically only by the fact that it contacts the weld nut directly. A weld nut can be placed either on the topside surface of the workpiece or on the underside surface of the workpiece. On the other side of the work piece opposite to the location of the nut, a “counter-electrode” completes the electrical circuit to effect the weld. The key objective is to cause the welding current to flow through the weld nut and across the points of its protrusions into the workpiece, so that resistive heating will melt the protrusions and fuse their material with that of the workpiece. 
         [0008]    A weld nut must be properly aligned in order to be properly welded to the work piece. To assist in locating the nut with respect to the electrode head, and in locating both the head and nut with respect to a hole formed in a work piece, one of the two electrodes generally contains a centrally mounted alignment pin. This pin passes through the center of an electrode and is generally insulated from the electrode itself by a non-conductive sleeve lining a central bore through the core of the body of the electrode. The electrical isolation of the pin prevents arcing between the tip of the pin in the workpiece and/or a weld nut. Such pins often are arranged to slide within the core of the electrode, permitting them to be advanced for alignment and withdrawn at the moment of welding. 
         [0009]    Most typically, this alignment pin is present on the lower of the two electrodes, even in cases where the nut is applied to the workpiece on the topside surface of the workpiece. In this case, the alignment pin extends through the hole formed in the workpiece to penetrate through the hole in the weld nut and assists in the alignment process. 
         [0010]    In the past, both active and counter-electrodes have generally been formed of a single, unitary piece. Both such electrodes have been shaped to provide a current-delivery interface at one end that is generally annular in shape. The central opening in this annular shape is occupied by the alignment pin in the case of an active electrode that is placed on the underside of the workpiece, and is penetrated by the tip of the alignment pin in the case of a counter-electrode placed on the upper side of the workpiece. When the active electrode is on the upper side of the workpiece, the alignment pin extends upwardly through a central bore in the counter-electrode and through the hole in the workpiece to penetrate through the open annular central region of the active electrode. 
         [0011]    Such prior art active and counter-electrodes have generally been formed of unitary parts made of solid copper. When the annular current delivery interfaces of such electrodes become worn, they must be replaced. Wear at the electrode interface occurs as a result of the vaporization of copper due to arcing. This can form annular grooves in the surface of the annular electrode interface. At a certain stage, such grooves interfere with reliable welding and the electrode heads of the prior art design have to be replaced. As such heads are generally massive, machined to a specific shape and made of copper, the replacement cost is significant. 
         [0012]    It would be desirable to provide an improved welding head that can accommodate wear without incurring the full cost of replacing the traditional welding head. This invention addresses that objective. 
         [0013]    The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims that conclude this Specification. 
       SUMMARY OF THE INVENTION 
       [0014]    According to one feature of the invention, an electrode welding head for a projection welding system is provided in the form of multiple parts wherein the conducting interface for the electrode head is provided by a replaceable part, typically in the form of a replaceable electrode interface disc. This disc carries the conductive interface surface, preferably annular in shape, which serves as the current delivery interface of the electrode head. Its role is to serve as an electrode interface through which the welding current is passed when a fastener, e.g. a weld nut, is successfully engaged by an electrode head when the fastener is in its proper location and with a correct orientation towards the workpiece to which it is to be welded. 
         [0015]    More particularly, a welding head according to the invention has a body supporting the replaceable interface disc on its side opposite the welding interface and a containment member, typically and preferably with a central annular opening that surrounds and positions the disk against the electrode body. The containment member is removably coupled to the body of the welding head for easy replacement of the interface disc. 
         [0016]    Preferably, this containment member overlies at least portions of the outer edge of the replaceable disc. The outer circumferential edge of the replaceable interface disc may be beveled, stepped or otherwise shaped in order to provide a portion of the discs circumference which will underlie the inner circumferential boundary of the central opening in the containment member. A portion of the disc providing the electrode interface may protrude through this central opening. 
         [0000]    Most conveniently this replaceable conductive disc is made of copper or other high conductivity material such as metal or blend of metals. 
         [0017]    Optionally, all of these members of the electrode head may be conductive, as being made of copper. Alternately the containment member may be non-conductive and preferably, in such case, is formed as a steel body insulated with a non-conductive material, as by being covered with a ceramic coating. A containment member made of steel with a durable insulative coating will serve to protect the electrode head from collisions in the course of its manipulation and reduce the risk of inadvertent shorting of the welding circuit. 
         [0018]    The containment member may be threaded, optionally with a female thread so as to be engageable with a male thread carried by the body of the electrode head. Both the containment member and the body of the electrode head may carry flattened surfaces to assist in the engagement of this threaded connection. Thus a pair of opposed flats may be cut into the threads for gripping the threaded part while the electrode head is assembled. The containment member itself may have the outward appearance of a hexagonal nut with the same objective. The replaceable interface disc can thereby be compressed between the body and the overlying portion of the containment member by tightening this threaded coupling. When the body part is made of electrically conductive material as part of the electrical circuit, as is preferred, this form of threaded engagement can serve to draw the replaceable interface disc into tight physical and electrical connection with the body and thereby provide an efficient solid conductive path for delivering current to the replaceable interface disc. 
         [0019]    Preferably, the containment member provides an annular face surface which is intended to be directed towards the workpiece and the interface surface of the replaceable welding disc protrudes slightly above the plane of this containment member face surface. This accommodates wear of the replaceable disc and reduces the prospect that a portion of the containment member&#39;s annular face surface may inadvertently contact a weld nut due to the presence of such wear of the replaceable disc. Optionally and alternately, the interface surface on the replaceable disc may be recessed within and behind the face surface of the containment member. In such case, it may be particularly desirable that the containment member either be made of a non-conductive material, or be provided with a non-conductive coating to insulate the containment member from the welding circuit. 
         [0020]    The central region in the replaceable interface disc is generally preferably open to receive a portion of an alignment pin. In the case of an upper electrode, the tip of such an alignment pin will generally extend into a well or recess in the body part of the electrode head located beneath the central opening in the interface disc. In the case of a lower electrode, the alignment pin will generally extend through the bore formed in the central core of the body part, and be slidingly fitted therein. 
         [0021]    An insulating sleeve in the form of a hollow cylinder may be fitted within this bore to guide the sliding motion of the alignment pin and isolate the alignment pin electrically from the welding circuit. The inner insulating sleeve may be held within the assembly of the electrode head by the combination of a protruding rim formed on the outside of the sleeve and a rim-receiving ledge formed on the inside face of the bore formed through the principal body of the electrode head. Preferably, the work-piece end of the insulating sleeve terminates within the body just short of the interface disc to ensure that this insulated part does not interfere with the solid, conductive, contact between the interface disc and the body. 
         [0022]    However, when an alignment pin is not present in the electrode head, the body of the electrode head need not have a central bore or be provided with a recess. 
         [0023]    A principal advantage of the invention is that when the replaceable electrode interface becomes worn, the electrode head may be disassembled by uncoupling, e.g. unthreading, the outer containment member from the inner body, and simply removing and replacing the copper electrode interface disc, which will generally be the only worn part. Such an arrangement represents a considerable saving over the use of unitary prior art electrode heads. 
         [0024]    The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preferred embodiments, in conjunction with the drawings, which now follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is an assembled cross-sectional side view of a one-piece, prior art, lower, active electrode positioned against a weld nut located on the underside of the workpiece to which it is to be welded with a sliding alignment pin present within the lower active electrode and a one-piece, prior art, upper counter-electrode located on the upper side of the workpiece. 
           [0026]      FIG. 2  is a variant on  FIG. 1  depicting an assembled cross-sectional side view of a one-piece, prior art, upper, active electrode positioned against a weld nut located on the upper side of the workpiece to which it is to be welded and a one-piece, prior art, lower counter-electrode on the lower side of the workpiece with a sliding alignment pin present within the lower counter-electrode. 
           [0027]      FIG. 3  is an exploded cross-sectional assembly view of an electrode welding head according to the invention having a replaceable electrode interface disc penetrated by an alignment pin slideably mounted within the body portion of the electrode head. 
           [0028]      FIG. 4  is an exploded cross-sectional assembly view of an electrode welding head without an alignment pin having, according to the invention, a replaceable electrode interface disc with a central opening which may be penetrated by the advancing tip of an alignment pin there through. 
           [0029]      FIG. 5  is an assembled cross-sectional side view of the electrode head of  FIG. 3 . 
           [0030]      FIG. 6  is an assembled cross-sectional side view of the electrode head of  FIG. 4 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0031]      FIG. 1  is an assembled cross-sectional side view of a one-piece, prior art, lower, active electrode  10  positioned against a weld nut  11  located on the underside of the workpiece  13  to which it is to be welded with a sliding alignment pin  16  present within the lower active electrode and a one-piece, prior art, upper counter-electrode  12  located on the upper side of the workpiece  13 . 
         [0032]    The lower electrode body  15  is made of a unitary part of highly conductive material. In this prior art invention, the alignment pin  16  slides up within the lower electrode and serves to locate the weld nut and keep it in a position for welding. Welding current is ideally delivered through the annular current-delivery interface of the lower electrode body&#39;s upper face  17 A and the weld nut&#39;s face  18 . 
         [0033]      FIG. 2  is an alternate variant of  FIG. 1  wherein the upper electrode  12  is the active electrode and is positioned against a weld nut  11  located on the upper side of the workpiece  13  to which it is to be welded. A one-piece, prior art, lower counter-electrode  10  is positioned on the lower side of the workpiece  13  with a sliding alignment pin  16  present within the lower counter-electrode. The upper electrode body  12  is also made of a unitary part of highly conductive material as with the lower electrode  10  in  FIG. 1 . The depiction of this upper electrode body  12  is schematic in that this electrode may also have a more complex form, similar to the counter-electrode  10 . 
         [0034]    In this prior art invention, the alignment pin  16  slides up and penetrates through the workpiece  13  via a bore in the workpiece where the weld nut  11  is intended to be welded. The positioning of the alignment pin  16  may be effected by the application of air pressure applied through parts into which the electrode  10  is fitted (not shown). This mechanism may include a small hole drilled through the support structure of the electrode holder (not shown) which carries the body  15 . This air hole serves to introduce air pressure from beneath the base of the alignment pin  16 , effecting its advancement and resiliently pushing the pin upwardly to its most upwardly advanced position. In the absence of such air pressure, the pin will generally drop down under gravity. The bore  14  is fitted with a non-conductive lining sleeve  14 A to electrically isolate the alignment pin  16  from the welding current. This alignment pin  16  when advanced penetrates weld nut  11  in order to keep it in position during welding. Welding current is ideally delivered through the annular current-delivery interface of the upper electrode body&#39;s lower face  17 B and the weld nut&#39;s face  18 . 
         [0035]      FIG. 3  is an exploded cross-sectional assembly view of an electrode welding head  20  according to the invention. The electrode head has a replaceable electrode interface disc,  22 , which is made of a conductive material. The electrode interface disc  22  therefore serves as the annular current-delivery interface of  FIGS. 1 and 2  and delivers current to the weld nut  11  via the contact of the interface disc&#39;s face  25  and the weld nut&#39;s face  18 . 
         [0036]    The electrode head  20  is penetrated by an alignment pin  16  as in  FIGS. 1 and 2 . Also as in  FIGS. 1 and 2 , the alignment pin  16  is slideably mounted within the body portion  21  of the electrode head  20  in the insulated sleeve  24 . The body portion  21  contains a set of threads  28  for affixing the electrode head  20  to a welding machine. The body portion  21  contains a further set of threads  29  in order to affix a containment member  23  to the body portion  21 . This containment member  23  holds the replaceable electrode interface disc  22  in contact with the electrode body  21 . In this manner welding current is delivered from the welding machine, through the electrode body  21 , to the interface disc  22  to weld the weld nut  11  to the workpiece. 
         [0037]    The insulated sleeve  24  is inserted within the body portion  21  in order to minimize arcing of the welding current between the alignment pin  16  and the weld nut  11  in cases where the alignment pin  16  is formed of electrically conductive material. Alternately, the alignment pin  16  may be provided with an insulated coating, as, for example, a ceramic coating over steel. The inner insulating sleeve  24  is held within the assembly of the electrode head  20  by the combination of a protruding rim  24 A formed on the outside of the sleeve  24  and a rim-receiving ledge  24 B formed on the inside face of the bore  14  formed through the principal body of the electrode head. 
         [0038]    The rim  24 A on the insulating sleeve  24  has a workpiece facing end  40  and said end terminates within the electrode body  21  and replaceable interface disk  22  just short of the disc face  25  of the interface disc  22  to ensure that the insulated sleeve  24  does not interfere with the solid, conductive, contact between the interface disc  22  and the nut  11 . 
         [0039]      FIG. 4  is an exploded cross-sectional assembly view of an active upper electrode welding head  30  that, unlike  FIG. 3 , does not have an alignment pin. This upper electrode head  30  has, according to the invention, a replaceable electrode interface disc  32  with an optional central opening, such that the advancing tip of an alignment pin  16  can penetrate therethrough. This alignment pin  16  would be part of the counter-electrode (not pictured in  FIG. 4 ) and would advance upwards through the workpiece from the counter-electrode below. The replaceable electrode interface disc,  32 , which is made of a conductive material, serves to provide the annular current-delivery interface as an  FIGS. 1 and 2  as it delivers current to the weld nut via the contact of the interface disc&#39;s face  35  and the weld nut&#39;s face  18 . 
         [0040]    The body portion  31  of the electrode head of  FIG. 4  may contain a taper  38  so that the electrode head  30  may be affixed to a welding machine. The body portion  31  contains a further set of threads  39  in order to affix a conductive sleeve  33  to the body portion  31 . This conductive sleeve  33  holds the interface disc  32  in contact with the electrode body  31 . In this manner welding current is delivered from the welding machine, to the electrode body  31 , thence to the interface disc  32 , and is then administered to the weld nut  11  through protruding end face  35 , so as to effect a weld of the nut to the workpiece. 
         [0041]    An insulated sleeve  34  may be inserted within the body portion  31  in order to minimize arcing of the welding current between the alignment pin  16  advancing upwardly from a lower counter-electrode and the body portion  31  when the alignment pin  16  penetrates the body portion  31 . Insulating sleeve  34  is suitable when the alignment pin  16  is formed of electrically conductive material. Alternately, if the alignment pin  16  is non-conductive, the space occupied by insulated sleeve  34  may be integrally formed with the body portion  31  of this electrode  30   
         [0000]    The respective variants of the electrode welding head are shown assembled in  FIGS. 5 and 6 .  FIG. 5  is an assembled cross-sectional side view of the electrode head of  FIG. 3 . The inside, female threads  27  of the containing member  23  in  FIG. 5  mesh with the outer, male threads  29  of the body portion  21 . The working face  25  of the interface disk  22  may protrude from the body  23 , be flush with the top of the containing member  23  or, if the weld nut  11  is of a smaller diameter, be recessed within the body  23  to receive a weld nut  11  within a well. Not pictured for purposes of clarity is the alignment pin  16 , which penetrates the central bore  14  of the body portion  21 . 
         [0042]    The annular current-delivery interface, face  25 , located on the replaceable disc  22 , juts out slightly from the containment member  23 . The containment member  23  holds the replaceable disc  22  in a contacted position with the body portion  21  so that welding current may be delivered to the face  25  in order to effect a weldment. However, as the face  25  will wear in use, it is convenient for it to protrude typically by 5 to 15 thousandths of an inch. 
         [0043]      FIG. 6  is an assembled cross-sectional side view of the electrode head of  FIG. 4 . The inside, female threads  27  of the containment member  33  mesh with the outer, male threads  39  of the body portion  31 . The tip of an alignment pin  16  of a lower electrode would then penetrate the recess  37  of the body portion  31  and the optional insulated sleeve  34 . 
         [0044]    The annular current-delivery interface, face  35 , located on the replaceable disc  32 , preferably juts out from the containment member  33  by a slight degree in the same manner as referenced previously. The containment member  33  holds the replaceable disc  32  in a contacted position with the body portion  31  so that welding current may be delivered to the face  35  in order to effect a weldment. 
       CONCLUSION 
       [0045]    The foregoing has constituted a description of specific embodiments showing how the invention may be applied and put into use. These embodiments are only exemplary. The invention in its broadest, and more specific aspects is further described and defined in the claims, which now follow. 
         [0046]    These claims, and the language used therein, are to be understood in terms of the variants of the invention which have been described. They are not to be restricted to such variants, but are to be read as covering the full scope of the invention as is implicit within the invention and the disclosure that has been provided herein.