Patent Publication Number: US-2022219258-A1

Title: Spot welding assembly with pivotable electrodes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The subject application is a bypass Continuation-in-Part Patent Application of PCT International Patent Application Serial No. PCT/US2020/053228 filed Sep. 29, 2020 entitled “SPOT WELDING ASSEMBLY WITH PIVOTABLE ELECTRODES” which claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/910,605 filed on Oct. 4, 2019, and titled “Spot Welding Assembly With Pivotable Electrodes.” The subject application also claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 63/167,947 filed on Mar. 30, 2021, and titled “Spot Welding Assembly With Pivotable Electrodes,” U.S. Provisional Patent Application Ser. No. 63/174,267 filed on Apr. 13, 2021, and titled “Spot Welding Assembly With Pivotable Electrodes,” and U.S. Provisional Patent Application Ser. No. 63/209,029 filed on Jun. 10, 2021, and titled “Spot Welding Assembly With Pivotable Electrodes”. The entire disclosures of all the aforementioned applications are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The subject disclosure generally relates to welding assemblies. More particularly, the subject disclosure relates to a spot welding assembly with pivotable electrodes for accommodating variances in work pieces. 
     BACKGROUND OF THE DISCLOSURE 
     Spot welding assemblies are known in the art for welding first and second work pieces to one another. Spot welding assemblies typically include a base for supporting a base electrode and a welding gun for supporting a gun electrode. During a welding operation, the welding gun is moved toward the base in order to draw the first and second work pieces against one another. A current is then applied through the base and gun electrodes and the first and second work pieces in order to melt regions of at least one of the first work piece and/or the second work piece to form a weld therebetween. 
     An issue with such spot welding assemblies, especially when the welding process is automated, is that the electrodes are static, and thus do not compensate for variations in work piece placement and/or dimensional variations among the work pieces, especially when the work pieces are not arranged parallel to a welding surface defined between the first and second electrodes. As such, the first and/or second work piece may not make adequately contact the electrodes, thus leading to an insufficient weld. In these situations, it is known to manually adjust an orientation of the welding gun and/or base to provide improved contact against the work pieces, however, such manual adjustments can be time consuming and labor intensive. 
     In view of the foregoing, there remains a need for improvements to such spot welding assemblies. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the disclosure, a spot welding assembly for welding a first work piece to a second work piece is provided. The spot welding assembly includes a base, and a base electrode coupled with the base for supporting the first work piece. A welding gun is moveable toward and away from the base. A gun electrode is coupled with the welding gun for supporting the second work piece and for locating the second work piece against the first work piece upon movement of the welding gun toward the base to allow the first and second work pieces to be welded to one another. At least one of: the base electrode is pivotable relative to the base; or the gun electrode is pivotable relative to the welding gun. 
     Because one or both of the base and/or the gun electrodes are pivotable, the electrode(s) are able to quickly be oriented flat against the work pieces even when the work pieces are not parallel to an original plane of the base and/or gun electrode(s), such as when the work pieces are improperly located or irregularly dimensioned. Orienting the electrode(s) in this manner ensure sufficient electrical and thermal conductivity during welding in order to provide a sufficient weld. Such adjustments may be provided in both automated and manual welding operations. 
     The subject spot welding assembly may be employed in projection welding applications, wherein at least one of the work pieces has at least one projection that engages the other work piece for focusing a current from the base and gun electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIG. 1  is a perspective view of a spot welding assembly; 
         FIGS. 2A-2B  are front perspective views of work pieces located between base and gun electrodes of the spot welding assembly, illustrating the base and gun electrodes being pivoted in different directions; 
         FIGS. 3A-3D  are front partial views of base and gun electrodes of the spot welding assembly, illustrating the base and gun electrodes being pivoted in different directions; 
         FIGS. 4A-4C  are top, front and side views of a weld pin of the spot welding assembly; 
         FIGS. 5A-5C  are top, front and side views of an upper collar of the spot welding assembly; 
         FIGS. 6A-6C  are top, front and side views of a gun electrode of the spot welding assembly; 
         FIGS. 7A-7C  are top, front and side views of an upper adapter of the spot welding assembly; 
         FIGS. 8A-8C  are top, front and side views of a base electrode of the spot welding assembly; 
         FIGS. 9A-9C  are top, front and side views of a base of the spot welding assembly; 
         FIGS. 10A-10C  are top, front and side views of a retaining collar of the spot welding assembly; 
         FIGS. 11A-11B  are front partial views of the base and gun electrodes of the spot welding assembly, illustrating first and second work pieces being located between the base and gun electrodes in an automatic operation; 
         FIGS. 12A-12B  are front partial views of the base and gun electrodes of the spot welding assembly, illustrating first and second work pieces being located between the base and gun electrodes in a manual operation; 
         FIGS. 13A-13C  are front perspective views of the base and base electrode of the spot welding assembly, illustrating the base electrode being pivoted into different positions; 
         FIG. 14  is a front perspective view of the gun electrode of the spot welding assembly in a centered position; 
         FIG. 15  is a front perspective view of the spot welding assembly, illustrating the base and gun electrodes securing first and second work pieces in pivoted positions; 
         FIGS. 16A-16C  are partial front schematic views of the gun electrode of the spot welding assembly, illustrating the gun electrode being pivoted in various positions; 
         FIGS. 17A-17C  are partial front schematic views of the base electrode of the spot welding assembly, illustrating the base electrode being pivoted in various positions; 
         FIG. 18  is a top perspective view of example first and second work pieces for being located and welded by the spot welding assembly; 
         FIG. 19  is a cross-sectional view of an alternate embodiment of a base electrode and showing a compression spring contained therein in an expanded condition; 
         FIG. 20  is another cross-sectional view of the alternate embodiment of the base electrode and showing the compression spring in a compressed condition; 
         FIG. 21  is a perspective and elevation view of the alternate embodiment of the base electrode; 
         FIG. 22  is a perspective and elevation view of the alternate embodiment of the base electrode taken from a different perspective than  FIG. 21 ; 
         FIG. 23  is a perspective and elevation view of a gun cap for an alternate embodiment of the gun electrode; 
         FIG. 24  is a perspective and elevation view of a gun swivel for the alternate embodiment of the gun electrode; 
         FIG. 25  is a perspective and elevation view of a weld tip of the alternate embodiment of the base electrode; 
         FIG. 26  is a perspective and elevation view of a compression spring for the alternate embodiment of the base electrode; 
         FIG. 27  is a cross-sectional view of the alternate embodiment of the gun electrode; 
         FIG. 28  is a cross-sectional and exploded view of the alternate embodiment of the gun electrode; 
         FIG. 29  is a perspective view of an alternate embodiment of the base electrode and showing the weld pin in an extended position; 
         FIG. 30  is another perspective view of the alternate embodiment of the base electrode and showing the weld pin in a retracted position; 
         FIG. 31  is a cross-sectional view illustrating a spot welding assembly including the base electrode of  FIGS. 29 and 30  in a first operating position and illustrating a sensing device, in this arrangement comprised of fiber optic sensor, disposed below the base electrode for sensing a position of the weld pin; 
         FIG. 32  is a cross-sectional view illustrating the spot welding assembly and fiber optic sensor of  FIG. 31  in a second operating position; 
         FIG. 33  is a cross-sectional view illustrating the spot welding assembly and fiber optic sensor of  FIG. 31  in a third operating position; 
         FIG. 34  is a cross-sectional view of another alternate embodiment of the base electrode and illustrating the weld pin and base swivel in a first orientation and the sensing device comprised of a touch probe disposed in abutting relationship with the weld pin in the first orientation for sensing a position of the weld pin; 
         FIG. 35  is another cross-sectional view of the base electrode of  FIG. 34  and illustrating the touch probe disposed in abutting relationship with the weld pin in a different, second orientation of the base swivel; 
         FIGS. 36A and 36B  are perspective views taken from different perspectives and illustrating a weld pin of the base electrode of  FIG. 34  in an extended position; 
         FIGS. 37A and 37B  are perspective views taken from different perspectives and illustrating the weld pin in retracted positions; 
         FIGS. 38A and 38B  are perspective views illustrating the weld pin of the base electrode of  FIG. 34  from different perspectives; 
         FIGS. 39A, 39B, and 39C  are cross-sectional views of the base electrode of  FIG. 34  and illustrating the weld pin and base swivel in different orientations but at a first height of the weld pin within the passage; 
         FIGS. 40A, 40B, and 40C  are cross-sectional views of the base electrode of  FIG. 34  and illustrating the weld pin and base swivel in different orientations but at a second height that is different than the first height; and 
         FIGS. 41A, 41B, and 41C  are cross-sectional views of the base electrode of  FIG. 34  and illustrating the weld pin and base swivel in different orientations but at a third height that is different than the first and second heights. 
     
    
    
     DETAILED DESCRIPTION OF THE ENABLING EMBODIMENTS 
     Referring to the Figures, wherein like numerals indicate correspond parts throughout the several views, a spot welding assembly  10  is generally shown. With reference to  FIG. 1 , the spot welding assembly  10  generally has a base  12  for supporting a base electrode  14 , and a welding gun  16  for supporting a gun electrode  18 . As shown in  FIGS. 2A-2C, 11A-11B, 12A-12B and 15 , a first work piece  20  and a second work piece  22  are positioned between the base and gun electrodes  14 ,  18 . The first work piece  20  can be supported by tooling  23 . With reference back to  FIGS. 1 and 3A-3C , the welding gun  16  is moveable toward and away from the base  12  in order to make the first and second work pieces  20 ,  22  engage one another, after which a current is applied through the base and gun electrodes  14 ,  18  and the first and second work pieces  20 ,  22  in order to melt at least one of the first and/or second work pieces  20 ,  22  to form a weld therebetween. According to the example embodiment, the first work piece  20  is a flat plate and the second work piece  22  is a nut, but other types of work pieces may be utilized without departing from the scope of the subject disclosure. As presented in the example embodiment, the subject spot welding assembly  10  may specifically be a projection welding assembly, wherein at least one of the work pieces  20 ,  22  has one or more projections for concentrating the current from the base and gun electrodes  14 ,  18  in order to target the weld to specific areas. 
     As shown in  FIGS. 1 and 9A-9C , the base  12  has a bottom portion  24  that is configured to be located on a die. The bottom portion  24  may have various shapes, such as a cylindrical shape (e.g.,  FIGS. 1-2C ) or a cuboid shaped (e.g.,  FIGS. 9A-9C ). As best shown in  FIGS. 3A-3D and 9A-9C , the base  12  further has a generally cylindrical-shaped projection portion  26  that extends upwardly from the bottom portion  24  along an axis A and terminates at a top surface  28 . The top surface  28  is concave shaped (concaving axially downwardly) to define a first hemispherical portion or surface  30  for the base  12 . As illustrated in  FIGS. 3A-3D , the concave shape of the top surface  28  of the base  12  is defined relative to a cross-sectional plane extending along the axis A. The projection portion  26  defines a plurality of first threads  32  adjacent to the bottom portion  24 . A plurality of first wire channels  34  extend through the projection portion  26  to the top surface  28  for receiving one or more wires for transmitting a current to the base electrode  14 . The first wire channels  34  are spaced apart and positioned so that at least one of the first wire channels  34  contacts a convex second hemispherical portion or surface  36  of the base electrode  14  that is disposed in mating relationship with the first hemispherical portion of the base  12  during any position throughout the base electrode&#39;s range of motion. 
     More specifically, as illustrated in  FIGS. 3A-3D and 8A-8C , the base electrode  14  is pivotably and rotatably positioned against the first hemispherical portion or surface  30  of the base  12 . According to the example embodiment, the base electrode is made of a class  2  copper material, but other highly conductive materials could be utilized. The base electrode  14  has a lower end being convex shaped, complementary to the concave shape of the first hemispherical portion or surface  30  of the base  12 , to define a convex second hemispherical portion or surface  36  that mates and nests with the first hemispherical portion or surface  30  such that the second hemispherical portion or surface  36  is pivotable and rotatable 360 degrees relative to and along the first hemispherical portion or surface  30 . The base electrode  14  further has a cylindrical projection  38  that extends upwardly from the second hemispherical portion or surface  36  to a flat upper welding surface  39  for supporting the first work piece  20 . The base electrode  14  further defines a passage  40  that extends axially through the base electrode  12  from the upper, flat surface  39  to the lower, convex second hemispherical portion or surface  36 . In other words the passage  40  extends to and is open at the convex second hemispherical portion of surface  36 . (See e.g.,  FIGS. 3A-3D, 8A-8C, 29, and 36B ). 
     As illustrated in  FIGS. 3A-3D  and  FIGS. 4A-4C , a weld pin  42  is received by and translatable or slideable along the passage  40  of the base electrode  14  and in its neutral position (such as shown in  FIGS. 3B and 41B ) extends from a bottom weld pin end  44  disposed in aligned or flush relationship with the second hemispherical portion or surface  36  and past the upper flat surface  39  to a tip  46 . According to the example embodiment, the weld pin  42  is made of a non-magnetic stainless steel; however, other non-magnetic materials could be utilized. The weld pin  42  extends axially from the bottom weld pin end  44  to a tip  46  that tapers to a point disposed axially outside of the passage  40 . The bottom weld pin end  44  of the weld pin  42  is convex shaped complementary to the convex shape of the second hemispherical portion  36  of the base electrode  12  such that the weld pin  42  also engages and is pivotable along the first hemispherical portion  30  along with the second hemispherical portion  36  of the base electrode  14  during pivoting and rotational movement of the base electrode  14  relative to the base  12 . The weld pin  42  is configured to assist in aligning the second work piece  22  relative to the first work piece  20  by being received by the second work piece  22  and a tubular segment  74  of the gun electrode  18  (discussed in further detail below). As illustrated in  FIGS. 3B and 3D , the pointed tip  46  of the weld pin  42  assists in guiding the weld pin  42  into the second work piece  22  and tubular segment  74 . Alternatively or additionally, the weld pin  42  may be configured to contact the first work piece  20 . For example, the weld pin  42  may be located into a channel of the first work piece  20  to assist in locating the first work piece  20 . 
     As best illustrated in  FIGS. 3A-3D  and  FIGS. 10A-10C , a retaining collar  48  that has a generally tubular shape receives the projection portion  26  of the base  12 . According to the example embodiment, the retaining collar  48  is made of a non-magnetic stainless steel material; however, other materials could be utilized. The retaining collar  48  extends axially between a bottom collar end  50  and a top collar end  52 . A plurality of second threads  54  are defined adjacent to the bottom collar end  50 . The second threads  54  are arranged to be threaded with the first threads  32  of the base  12  for securing the retaining collar  48  to the base  12 . A first flange  56  being generally cylindrical shaped extends radially inwardly from the top collar end  52  to terminate at a first stop  57  being cylindrically shaped and disposed radially adjacent to and thus in slight radially spaced and encircling relationship with the projection  38  of the base electrode  14 . Put another way, the radial spacing of the first flange  56  from the cylindrical projection  38  establishes a cylindrically shaped stop  57  (See e.g.,  FIGS. 3A-3D and 13A-13C ) that limits the pivoting movement of the base electrode  14  relative to the first hemispherical portion  30  of the base  12  in all pivoting directions of the base electrode  14 . More particularly, as further shown in  FIGS. 17A-17C , as a result of the cylindrical portion  38  abutting the stop  57  of the first flange  56  during the pivoting movement, the pivoting is limited to approximately 15 degrees (defined relative to the axis A) in every direction according to the example embodiment, however, the first flange  56  could be customized and sized to limit the pivoting to other preferred angles. In addition to limiting pivoting movement of the base electrode  14 , the retaining collar  48  prevents debris from entering the region of the first and second hemispherical portions  30 ,  36  in order to provide prolonged use of the welding assembly  10 . 
     As best shown in  FIGS. 3A-3D, 7A-7C and 14-15 , an upper adapter  58  is coupled with the welding gun  16 . According to the example embodiment, the upper adapter  58  is made of a class  2  copper material, however, other highly conductive materials could be utilized. The upper adapter  58  of the welding gun  16  extends axially between a proximal end  60  and a distal end  62 , with a plurality of third threads  64  defined adjacent to the proximal end  60  for threadingly fixing the upper adapter  58  to the welding gun  16 . The distal end  62  of the upper adapter  58  is concave shaped to define a third hemispherical portion or surface  66  that concaves into the distal end  62 . As illustrated in  FIGS. 3A-3D , the concave shape of the distal end  62  of the welding gun  16  (as established by the upper adapter  58 ) is once again defined relative to a cross-sectional plane extending along the axis A. The upper adapter  58  further defines fourth threads  68  adjacent to the distal end  62 . A plurality of second wire channels  70  extend generally axially through the upper adapter  58  and terminate at the distal end  62  for receiving wires for transmitting a current to the gun electrode  18 . The second wire channels  70  are spaced apart and positioned so that at least one of the second wire channels  70  contacts a convex-shaped fourth hemispherical portion or surface  72  of the gun electrode  18  during any and all positions of the gun electrode  18  throughout its range of pivoting and rotational movement. 
     As best shown in  FIGS. 3A-3D and 6A-6C , the gun electrode  18  is located against the upper adapter  58 . According to the example embodiment, the gun electrode  18  is made of a class  2  copper material, however, other highly conductive materials may be utilized. The gun electrode  18  has an upper end being convex shaped to define a convex fourth hemispherical portion or surface  72  that mates and nests with the third hemispherical portion or surface  66  such that the convex fourth hemispherical portion or surface  72  is pivotable and rotatable 360 degrees relative to and along the third hemispherical portion or surface  66 . The gun electrode  18  further includes a tubular segment  74  that extends axially from the fourth hemispherical portion  72  and terminates at a flat welding end for receiving and locating the second work piece  22  and the weld pin  42 . 
     As best shown in  FIGS. 3A-3D and 5A-5C , an upper collar  76  that generally has a cylindrical shape is coupled with the upper adapter  58 . According to the example embodiment, the upper collar  76  is made of a non-magnetic stainless steel; however, other non-magnetic materials may be utilized. The upper collar  76  extends axially between a fixed end  78  and a guiding end  80 . Fifth threads  82  are defined adjacent to the fixed end  78  for being threaded with the fourth threads  68  of the upper adapter  58 . The upper collar  76  further includes a second flange  84  that extends radially inwardly from the guiding end  80  and terminates at a second stop  85  being cylindrically shaped and disposed in slight radially spaced and encircling relationship with the tubular element  74  of the gun electrode  18 . Put another way, the radial spacing of the second flange  84  from the tubular element  74  establishes a second cylindrically shaped stop  85  (See e.g.,  FIGS. 3A-3D and 13A-13C ) that limits the pivoting movement of the tubular element  74 , and thus the related gun electrode  18 , relative to the third hemispherical portion  66  of the welding gun  16  (as established by the distal end  62  of the upper adapter  58 ) in all pivoting directions of the gun electrode  18 . More particularly, as further shown in  FIGS. 16A-16C , as a result of the tubular element  74  abutting the second stop  85  of the second flange  84  during the pivoting movement, the pivoting is limited to approximately 15 degrees (defined relative to the axis A) in every direction according to the example embodiment, however, the second flange  84  could be customized and sized to limit the pivoting to other preferred angles. In addition to limiting pivoting movement of the gun electrode  18 , the upper collar  76  prevents debris from entering the region of the third and fourth hemispherical portions  66 ,  72  in order to provide prolonged use of the welding assembly  10 . 
     During use of the spot welding assembly  10 , the pivoting movement and rotation of the base and gun electrodes  14 ,  18  allows the base and gun electrodes  14 ,  18  to be oriented flat against the first and second work pieces  20 ,  22  to provide a large contact area against the first and second work pieces  20 ,  22 , even when the first and second work pieces  20 ,  22  do not lie parallel to an original plane that is defined between the base and gun electrodes  14 ,  18  when they are in an un-pivoted/centered position, such as when they are improperly placed or irregularly shaped. The large contact area provides adequate electrical and thermal conductivity during welding, thus ensuring a successful weld even in non-ideal conditions. For example,  FIG. 2A  shows the base and gun electrodes  14 ,  18  in a centered position with the first work piece  20  positioned generally along the original plane,  FIG. 2B  shows the base and gun electrodes  14 ,  18  in a tilted left position while maintaining flat contact against the first and second work pieces  20 ,  22 , and  FIG. 2C  shows the base and gun electrodes  14 ,  18  in a tilted right position while maintaining flat contact against the first and second work pieces  20 ,  22 . 
     As illustrated in  FIG. 1 , during operation, the welding gun  16  starts in a raised position, and the base and gun electrodes  14 ,  18  are in their centered, un-pivoted state. The first work piece  20  (a plate in the example embodiment) is positioned on the base electrode  14  via either automatic or manual load. At this point, the base electrode  14  may be rotated and pivoted to compensate for any angular deflection caused by the first work piece  20  not being located along the original plane to ensure proper contact of the base electrode  14  against the first work piece  20 . Specifically, the base electrode  14  may be rotated and pivoted to match an angle of the first work piece  20 . Once the first work piece  20  is located, the second work piece  22  (which is a projection fastener in the case of the example embodiment), is placed over the weld pin  42 . The gun electrode  18  is then lowered toward the base electrode  14  with the welding gun  16 . Upon contact of the second work piece  22  against the first work piece  20 , the gun electrode  18  automatically rotates and pivots to match an angle of the base electrode  14  to provide full contact between the first and second work pieces  20 ,  22 . An electrical current is then supplied to the base and gun electrodes  14 ,  18 , melting the second work piece  22  to the first work piece  20 , to create a weld therebetween. It should be appreciated that one of the first and second work pieces  20 ,  22  may present one or more projections that engage the other work piece for concentrating the current and weld. 
     Referring now to  FIGS. 19-22 , an alternate embodiment of the base electrode  114  is generally shown with like numerals, separated by an initial prefix of “1” in the drawings and descriptions below, identifying similar components with the embodiment described above and thus in certain instances not described again herein but incorporated by reference through the use of the “1” prefix here or in the drawings. One feature that distinguishes the base electrode  114  from the embodiment described above is the base electrode  114  is a self-contained assembly which includes multiple pieces, namely a base cap  184  and a base swivel  186  that are separate components and removably connectable to one another to establish the base electrode  114 . The base cap  184  includes the upper welding surface  139  that, in operation, directly contacts the first work piece  20 . As best illustrated in  FIGS. 19-20 , in a preferred arrangement, the base cap  184  is in threaded engagement with the base swivel  186  to establish the removably interconnected relationship, such that the base cap  184  can be easily removed from the base swivel  186 , when worn, and replacement in required. Thus, when the base cap  184  reaches the end of its operating life, the entire base electrode  114  does not have to be discarded and replaced. The base swivel  186  includes the convex second hemispherical portion of surface  136  that is in the frictionless contact with the first hemispherical portion  130  of the base  120  due to the jet of air injected between these components. The base cap  184  and the base swivel  186  are preferably made of the same material, e.g., copper. 
     Further, in contradistinction to the first embodiment of the base electrode  14 , a passage  40  does not extend entirely through the base electrode  14 . Instead, in this second embodiment of the base electrode  140 , the base swivel  186  includes an inner bore  187  which (when the base cap  184  is interconnected to the base swivel  186 ) extends from the upper welding surface  139  to a closed bore bottom  189  disposed in spaced relationship with the lower, convex second hemispherical portion or surface  136  of the base electrode  114 , i.e., the inner bore  187  is not a through hole like the passage  40  in the first embodiment of the base electrode  12 . An upper portion of this inner bore includes female threads to threadedly engage with male threads on the base cap  184 . A lower portion of the inner bore  187  is unthreaded and receives a compression spring  188 .  FIG. 19  shows the compression spring  188  in an uncompressed condition, and  FIG. 20  shows the compression spring  188  in a compressed condition, such as during operation of the welding assembly. The weld pin  142  is disposed in the inner bore and extends from the bottom weld pin end  144  which abuts and is supported by the compression spring  188 , and which biases the weld pin  142  in an upward direction, to the weld tip  146  disposed outside of the inner bore  187  and the base cap  184  when the components are interconnected together. Since the inner bore  187  does not extend all the way through the base swivel  186 , the semi-hemispherical surface  136  of the base swivel  186  extends uninterruptedly within the confines established by its outer perimeter. This configuration also allows the bottom weld pin end  144  of the weld pin  142 , which is where the weld pin  142  contacts the compression spring  188 , to be flat or planar rather than radiused or convex shaped, as is the case in the above-discussed embodiment. 
     In some other embodiments, the base cap  184  and the base swivel  186  may be joined to one another through attachment means other than threads that are detachable to allow the base cap  184  to be replaced when worn. 
     Referring now to  FIGS. 23-24 and 27-28 , an alternate embodiment of the gun electrode  118  is also provided. In this embodiment, the gun electrode  118  is also made as two pieces that are removably coupled with one another to form the gun electrode  118 . More specifically, the first piece of the gun electrode  118  is a gun cap  190  that includes the flat welding surface or end which, in operation, directly contacts the second work piece  22 . The second piece of the gun electrode  118  is a gun swivel  192 , which includes the convex fourth semi-hemispherical portion or surface  172  that allows the gun electrode  118  to articulate and swivel about the third hemispherical portion  66  of the welding gun  16 . Similar to the base electrode  114  described immediately above, the gun cap  190  and gun swivel  192  are preferably threaded with one another to establish the removable interconnection. However, other means of establishing the removable interconnection can be utilized without departing from the scope of the subject disclosure. This two-piece configuration of the gun electrode  18  allows for reduced waste because the gun cap  190  can be replaced after wear without discarding and replacing the entire gun electrode  118 . 
     Still further, in this alternate embodiment, the weld pin  142 , which is shown in  FIG. 25 , is made of a high carbon steel material, such as D2, that is coated to reduce arcing during a welding operation. The coating may be, for example, a black oxide or a diamond-like carbon (DLC) material. 
     Referring now to  FIGS. 29-33 , yet another embodiment of the base electrode  214  is generally shown with like numerals, separated by an initial prefix of “2” throughout the drawings and specification, identifying similar components with the embodiments described above and thus in certain instances not described again herein but incorporated by reference herein through use of the “2” prefix either in the following description or in the drawings. Similar to the embodiment of  FIGS. 19 and 20 , in this embodiment, the base electrode  214  is made as two pieces, namely a base cap  284  and a base swivel  286 , which are threadedly joined together. However, in this embodiment, the passage  240  of the base swivel  286  extends fully through the base swivel  286 , i.e., the passage  240  has the form of a through hole extending through the entire base electrode  214  similar to the first embodiment of the base electrode  14 . Thus, unlike the embodiment of  FIGS. 19 and 20 , the weld pin  242  is biased by air pressure that is delivered through one or more openings in the base  212  rather than a compression spring. 
     As best illustrated in  FIGS. 31-35 , a sensing device  290 ,  390  is disposed below the base electrode  214 ,  314  for sensing a position of the weld pin  242 ,  342  within the passage  240 ,  340  of the base electrode  214 ,  314 . More specifically, the base  212 ,  312  can define an instrument channel  292 ,  392  extending along the axis A and in aligned relationship and communication with the weld pin  242 ,  342  and the passage  240 ,  340  when the base electrode  214 ,  314  is disposed in a central, non-pivoted position (See e.g.,  FIGS. 32, 39B, 40B and 41B ). Even when the base electrode is disposed in a pivoted position, the instrument channel  292 ,  392  remains in communication with the passage  240 ,  340  and thus the weld pin  242 ,  342  disposed therein (See e.g.,  FIGS. 39A, 39C, 40A, 40C, 40A, 40C ). The sensing device  290 ,  390  is disposed in communication with the instrument channel  292 ,  392  and the weld pin  242 ,  342  for sensing a position of the weld pin within the passage  240 ,  340  of the base electrode  314 . Although the sensing device  290 ,  390  is described here in association with the alternative two-piece arrangement of the base electrode  214 ,  314 , the sensing device  290 ,  390  could also be used in association with the first embodiment of the base electrode  14  without departing from the scope of the subject disclosure. Put another way, as will be appreciated in view of the following disclosure, the sensing device  290 ,  390  does not rely on or require implementation of the two-piece base electrode  214 ,  314  described herein and can also be used in association with the first embodiment of the spot welding assembly  10  and related base electrode  14 . 
     In accordance with a first arrangement, and as illustrated in  FIGS. 31-33 , the sensing device  290 ,  390  is comprised of a fiber optic sensor  294  positioned in communication with the instrument channel  292  of the base  212  and configured to emit a light beam upwardly and serially through the instrument channel  292  and the passage  240  and into contact with the bulging or bottom weld pin end  244  of the weld pin  242  to sense a reflection of that beam of light. By sensing the reflection of the beam of light, a controller  295  disposed in communication with the fiber optic sensor  294  is configured to determine the distance between the fiber optic sensor  294  and the weld pin  242  based on the reflected or returned light beam and thus determine the position of the weld pin  242  within the passage  240  of the base swivel  286 . In an exemplary embodiment, the bottom weld pin end  44  of the weld pin  242  is highlighted, such as being painted white, to improve the fiber optic sensor&#39;s  292  ability to measure the distance to the weld pin  242 . 
     The bottom weld pin end  244  (e.g., bulging end) of the weld pin  242  is semi-spherically curved (i.e., convex shaped) with the same radius of curvature as the convex second hemispherical portion  236  of the base swivel  286  such that when the weld pin  242  is in a fully retracted position (See e.g.,  FIG. 30 ), the bulging end  244  of the weld pin  242  is flush with the second hemispherical portion  236  of the base swivel  286 . Thus, as the base electrode  214  articulates during operation (See e.g.,  FIGS. 31 and 33 ), the distance measured by the fiber optic sensor  294  is not affected. 
     Referring now to  FIGS. 36A-38B  yet another exemplary embodiment of the base electrode  314  is generally shown with like numerals, separated by a prefix of “3” throughout the drawings and specification identifying similar components with the embodiments described above and thus in certain instances not described again herein but incorporated herein by reference through use of the “3” prefix either in the following description or in the drawings. This embodiment is similar to the embodiment shown in  FIGS. 29-33  in that the bulging or bottom weld pin end  344  of the weld pin  342  is semi-spherically curved (i.e., convex shaped) such that when the weld pin  342  is in the fully retracted position (see  FIGS. 37B and 41A -C), it is flush with the second hemispherical portion  336  of the base swivel  386 . 
     Referring now to  FIGS. 34-35 , in an alternative arrangement of the sensing device  290 ,  390 , rather than being a fiber optic sensor, the sensing device  392  is comprised of a touch probe  396  or linear variable differential transformer (LVDT) that extends through the instrument channel  392  and into the passage  340  in the base electrode  386  to directly contact the curved, convex bottom surface (i.e., bulging end  344 ) of the weld pin  342 . As shown in  FIGS. 39-41 , in operation, a change in the height of the weld pin  342  relative to a fixed point on the base  312  affects the position of the touch probe  396 ; however, articulation of the weld pin  342  without an accompanying change in height will not affect the movement of the touch probe  396 . For example,  FIGS. 39A-C  show the weld pin  342  at a first height H 1 , accompanied by corresponding linear or axial movement of the touch probe  396 ;  FIGS. 40A-C  show the weld pin  342  at a lower, second height H 2 , accompanied by corresponding linear or axial movement of the touch probe  396 ; and  FIGS. 41A-C  show the weld pin  342  at a still lower, third height H 3 , accompanied by corresponding linear or axial movement of the touch probe  396 . Thus, as shown in  FIGS. 39-41 , the touch probe  396  remains in continuous contact with the bulging, bottom weld pin end  344  of the weld pin  342  during all pivoting and rotational movement of the base electrode  314  and all linear or translational movement (and thus all heights H 1 , H 2 , H 3 ) of the weld pin  342  within the passage  340 . Thus, the touch probe  396  can always sense a position of the weld pin  342  within the passage  340  of the base electrode  314  as determined by the linear or axial position of the touch probe  396  within the instrument channel  392  and passage  340 . In other words, the change in the height or position of the weld pin  342  correspondingly raises or lowers the touch probe  396  which is connected to either an LVDT or similar scaled measuring device to determine the position of the weld pin  342 . A radius of the weld pin  342  also allows the weld pin  342  to swivel without changing the position of the touch probe  396  if there is no linear or axial movement of the weld pin  342 . 
     In some embodiments, a different type of sensing device  290 ,  390  that can sense the position of the weld pin within the base electrode may be employed. Such alternate embodiments may utilize, for example, an ultrasonic sensor. 
     Unless otherwise defined, the term “hemispherical,” as used in the present application, includes any surface extending along a portion of a sphere. The term “hemispherical” is not limited to exactly one-half of a sphere shape. 
     The foregoing description is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.