Single drive and source for adjacently clamping and resistance welding

CROSS-REFERENCES TO RELATED APPLICATIONS

0001This U.S. Non-Provisional patent application is a continuation-in-part and claims the benefit of pending U.S. Non-Provisional Ser. No. 11/178,171 filed on Jul. 8, 2005, entitled SYSTEM AND METHOD FOR CLAMPING AND RESISTANCE WELDING (hereinafter the 171 application), incorporated by reference herein.

BACKGROUND OF THE INVENTION

00021. Technical Field

0003The present invention relates to resistance spot welding systems and methods and more particularly to an improved resistance spot welding system having a single drive and source for clamping a plurality of workpieces at a first location and welding the workpieces substantially adjacent the first location.

00042. Discussion of Prior Art

0005Resistance spot welding systems are used in various industrial applications, including automotive vehicle body construction. These systems function to apply pressure to and transmit an electric current through a plurality of adjacently secured workpieces, such that the resistance of the workpieces generates sufficient heat energy to produce a spot weld therebetween. The assembly is initially secured by first positioning the workpieces in a desired configuration, and then using fixtures to clamp the workpieces together. A typical fixture consists of various types of tooling elements that accurately locate and orient the workpieces with respect to the tool path and restrains workpiece motion in the presence of welding electrode forces.

0006In most systems, part positioning, clamping and welding are performed by a plurality of programmable robots, each having its own drive mechanism, control, and actuation source, within an assembly cell. For example, a first set of robots may be used for handling and clamping the workpieces at predetermined datum locations, while a separate set of robots are used to weld the workpieces to initially produce a plurality of dimension control welds (DCWs). The datum locations are verified during the construction and installation of the assembly cell, and the DCWs are typically produced as close to the datum locations as possible to minimize workpiece deformation. Due to equipment size and configuration (e.g., robotic bulk), however, the electrodes are typically unable to produce these welds near or substantially adjacent the datum location, and must therefore engage the workpieces at locations spaced a minimum distance from the datum locations. Applying compressive welding forces at these non-datum locations results in workpiece deformation that reduces dimensional accuracy.

0007To alleviate this concern extended weld gun arms, which do not interfere with the clamping units, have been increasingly incorporated. However, the longer arms have resulted in an increase in the total size of the gun unit, as the required input force has correspondingly increased. These multi-robotic systems further present various manufacturing concerns, including overcrowding of floor and three-dimensional space within the assembly cell, increased tooling costs, and longer cycle times. While the foremost concern directly impacts operator convenience and efficiency, the later concerns affect overall costs of production. Finally, as processes and product designs become increasingly complex these concerns intensify.

0008In response to these further concerns, the 171 application discloses the general concept of a spot welding system for clamping a plurality of workpieces at and welding the workpieces substantially adjacent predetermined datum locations. The preferred embodiments of the system disclosed therein present concentrically alignable clamping and welding elements during engagement that are configured to produce an annular weld about the datum location. However, while substantially reducing the distance between the weld and datum location, the system disclosed by the 171 application presents a complex structural and mechanical configuration that is difficult to implement.

0009Accordingly, there remains a need in the art for an improved system and method for spot welding a plurality of workpieces substantially adjacent datum locations, so as to reduce workpiece deformation, that can also be efficiently implemented.

BRIEF SUMMARY OF THE INVENTION

0010Responsive to these and other concerns, the present invention presents a resistance spot welding system for reducing workpiece deformation that presents a simplified structural and mechanical configuration. Among other things, the present invention is useful for reducing assembly cell congestion by combining the separate drive mechanisms and actuation sources of conventional weld and clamp units into a single drive mechanism and source. Further, the present invention is useful for allowing the weld unit to compensate for electrode wear by enabling weld gun equalization.

0011A first aspect of the present invention broadly concerns a system for clamping and welding a plurality of workpieces. The system includes a clamp configured to engage the workpieces by applying a clamping force at a first location, so as to retain the workpieces in a relatively fixed condition. The system further includes an electrode positioned and configured to engage by applying a welding force to and passing an electric current through a section of the workpieces, wherein said section is substantially adjacent the first location. A drive mechanism is drivenly coupled to the clamp and electrode, and configured to cause the clamp and electrode to engage the workpieces when actuated. Finally, the inventive system includes a source configured to produce the clamping and welding forces, and actuate the mechanism, so as to transfer the forces through the mechanism to the clamp and electrode.

0012A second aspect of the present invention concerns a method of clamping and welding a plurality of workpieces in a predetermined assembly configuration and reducing assembly cell congestion, wherein at least one datum location for clamping the workpieces is predetermined, so as to minimize deformation during clamping and welding. The method further includes the steps of fixing the workpieces in the assembly configuration, and securing a clamping and welding system having a single drive mechanism in relation to the workpieces. The workpieces at the datum location are then engaged by actuating the drive mechanism of the system, so as to clamp and retain the workpieces in the assembly configuration. Finally, an electric current is applied through and pressure is applied to the workpieces at a second location substantially adjacent the first location also by the actuation of the drive mechanism.

0013It will be understood and appreciated that the present invention provides a number of advantages over the prior art, including, for example, providing localized impression of the workpieces by clamping and subsequently joining the workpieces in a substantially adjacent configuration. Since a single power source and drive mechanism is utilized, the system is more compact, which enables a higher density of weld units, a reduction in the needed number of station fixtures, and increased access for maintenance. The single power source also eliminates the need for separate controls for clamping and welding (i.e., additional control logic, separate valves in cases where a pneumatic drive is utilized, and separate electrical control in cases where an electrical drive is utilized). Thus, the number of robots needed is reduced, further resulting in increased floor space, a reduction in complexity, cycle time, and the need for manpower or hours-per-vehicle (HPV).

0014Other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiment(s) and the accompanying drawing figures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

0015Preferred embodiments of the invention are described in detail below with reference to the attached drawing figures, wherein:

0016<figref idref="DRAWINGS">FIG. 1</figref> is a perspective view of a clamping and resistance spot welding system in accordance with a preferred embodiment of the present invention, and two workpieces being clamped and welded by the system;

0017<figref idref="DRAWINGS">FIG. 1</figref><i>a </i>is a perspective view of a plurality of two workpieces, particularly illustrating exemplary datum locations;

0018<figref idref="DRAWINGS">FIG. 2</figref> is a perspective view of a resistance spot welding system without the clamping element in accordance with a preferred embodiment of the present invention;

0019<figref idref="DRAWINGS">FIG. 3</figref> is a perspective view of a resistance spot welding system with the clamping element and two sets of electrodes, in accordance with a preferred embodiment of the present invention;

0020<figref idref="DRAWINGS">FIG. 4</figref> is an exploded view of a clamping and welding system in accordance with a preferred embodiment of the present invention, particularly illustrating a clamp swing arm, weld swing arm, linearly translating drive mechanism, lower weld arm/connecting plate, and fixed housing structure;

0021<figref idref="DRAWINGS">FIG. 5</figref> is a left elevation view of the simplified mechanical configuration of a resistance spot welding system in accordance with a preferred embodiment of the present invention, particularly illustrating an upper weld swing arm, drive mechanism, connecting plate, fixed housing structure, and two workpieces;

0022<figref idref="DRAWINGS">FIG. 5</figref><i>a </i>is a front elevation view of the system shown in <figref idref="DRAWINGS">FIG. 5</figref>;

0023<figref idref="DRAWINGS">FIG. 5</figref><i>b </i>is a right elevation view of the system shown in <figref idref="DRAWINGS">FIG. 5</figref>;

0024<figref idref="DRAWINGS">FIG. 6</figref> is a left elevation view of the system shown in <figref idref="DRAWINGS">FIG. 5</figref>, particularly illustrating upper and lower weld arms in initial disengaged positions, a plurality of workpieces, and station fixture;

0025<figref idref="DRAWINGS">FIG. 6</figref><i>a </i>is a left elevation view of the system shown in <figref idref="DRAWINGS">FIG. 6</figref>, particularly illustrating the upper weld swing arm in the workpiece engaged position, and the lower weld arm in a disengaged position;

0026<figref idref="DRAWINGS">FIG. 6</figref><i>b </i>is a left elevation view of the system shown in <figref idref="DRAWINGS">FIG. 6</figref>, particularly illustrating the upper weld swing arm in the workpiece engaged position, and the lower weld arm in an engaged position;

0027<figref idref="DRAWINGS">FIG. 7</figref> is an elevation view of the upper clamp and weld swing arms shown in <figref idref="DRAWINGS">FIG. 4</figref>, in the initial disengaged position, particularly illustrating the shared axis of rotation, arm engaging pin (bearing), and the slot openings;

0028<figref idref="DRAWINGS">FIG. 7</figref><i>a </i>is an elevation view of the upper clamp and weld swing arms shown in <figref idref="DRAWINGS">FIG. 7</figref>, in an intermediate disengaged position wherein the clamp arm precedes the weld arm;

0029<figref idref="DRAWINGS">FIG. 7</figref><i>b </i>is an elevation view of the upper clamp and weld swing arms shown in <figref idref="DRAWINGS">FIG. 7</figref>, wherein the clamp arm is in the workpiece engaged position and the weld arm trails in an intermediate position;

0030<figref idref="DRAWINGS">FIG. 7</figref><i>c </i>is an elevation view of the upper clamp and weld swing arms shown in <figref idref="DRAWINGS">FIG. 7</figref>, wherein both arms are in the workpiece engaged position;

0031<figref idref="DRAWINGS">FIG. 7</figref><i>d </i>is an elevation view of the upper clamp and weld swing arms shown in <figref idref="DRAWINGS">FIG. 7</figref>, wherein the weld arm is in an overrated position; and

0032<figref idref="DRAWINGS">FIG. 8</figref> is a table of theoretical welding forces applied to the workpieces by the system for a plurality of given source force, welding shank length, and cylinder diameter combinations.

DETAILED DESCRIPTION OF THE INVENTION

0033As best shown in <figref idref="DRAWINGS">FIG. 1</figref>, the present invention concerns an improved resistance welding system <b>10</b> for clamping and welding a plurality of workpieces, such as the component parts of a body-panel or support roof assembly of an automobile, to produce a spot or seam weld <b>12</b>. The inventive system <b>10</b> includes a single drive mechanism <b>14</b> and a single combined clamping and welding force generating source <b>16</b>. The system <b>10</b> is intended for use within an assembly station, wherein the workpieces are first placed upon station fixture <b>18</b> (<figref idref="DRAWINGS">FIG. 6</figref>) by a human or robotic operator (not shown). The preferred system <b>10</b> is robotically maneuverable into position along multi-axes, configured to receive sensory input, and is programmably controlled. Although described and illustrated herein with respect to spot welding, it is appreciated that the inventive aspects of the system <b>10</b> may be utilized with other compressive joining means, such as weldbonding, riveting, rivetbonding, clinching, clinchbonding, or wherever it is desired to reduce workpiece deformation by joining the workpieces substantially adjacent the clamping location.

0034As shown in <figref idref="DRAWINGS">FIGS. 1 and 1</figref><i>a</i>, a plurality of two workpieces <b>20</b>,<b>22</b> of equal thickness is preferably welded; however, the system <b>10</b> may be utilized to weld a greater plurality or structural components having variable thickness. The workpieces <b>20</b>,<b>22</b> may be formed of a wide range of materials including steel, iron alloys, aluminum alloys, magnesium alloys, titanium and molybdenum, and present thicknesses of variable dimension. However, operable thickness and workpiece dimensioning/welding location are limited by the capabilities of the system <b>10</b> as further described herein. Finally, the workpieces <b>20</b>,<b>22</b> present oppositely engagable upper and lower surfaces <b>20</b><i>a</i>,<b>22</b><i>a</i>, wherein the surfaces <b>20</b><i>a</i>,<b>22</b><i>a </i>are preferably parallel (<figref idref="DRAWINGS">FIG. 1</figref><i>a</i>).

0035Turning to the configuration of the system <b>10</b>, the system <b>10</b> generally includes a clamping element (or “clamp”) <b>24</b> for engaging the workpieces <b>20</b>,<b>22</b> at a first location or imprint, so as to secure the workpieces <b>20</b>,<b>22</b> in a fixed relative condition (<figref idref="DRAWINGS">FIG. 1</figref>). More preferably, the clamping imprint is centered at a datum point <b>26</b> (<figref idref="DRAWINGS">FIG. 1</figref><i>a</i>) determined by suitable methodology, such as a conventional finite analysis method. It is appreciated by those ordinarily skilled in the art that at these points clamping force is optimized and workpiece deformation due to clamping is minimized. A plurality of datum points <b>26</b> is typically determined in combination, so as to counterbalance each other.

0036At least one welding electrode <b>28</b> is configured to produce the weld <b>12</b> substantially adjacent the first location, wherein the term “substantially adjacent” shall mean within 5 cm, and more preferably, within 2 cm of the imprint. As best shown in <figref idref="DRAWINGS">FIGS. 1 through 3</figref>, the system <b>10</b> further includes virtually identical backup (or “second”) electrodes <b>30</b> that engage the workpieces <b>20</b>,<b>22</b> opposite the welding electrode <b>28</b> to complete the electric potential. The preferred electrodes <b>28</b>,<b>30</b> each present a tubular configuration, and are coaxially aligned in a workpiece engaged position (<figref idref="DRAWINGS">FIGS. 1, 5</figref> and <b>6</b><i>b</i>). More preferably, the electrodes <b>28</b>,<b>30</b> each present a tapered welding cap and bent shank configuration as is conventionally utilized.

0037As shown in <figref idref="DRAWINGS">FIG. 2</figref>, the system <b>10</b> may be operated without the clamping element <b>24</b>, and as such the preferred clamp <b>24</b> is removably connected (i.e., easily disconnected and reconnected manually by an operator) to the mechanism <b>14</b>. The welding function and configuration of the system <b>10</b> further present novel and useful structural features and mechanical operation as further described herein. More preferably, the system <b>10</b> is configured so as to interchangeably interconnect and utilize one of a plurality of differing clamps <b>24</b> and electrode sets depending upon application. With respect to the latter, an electrode holder <b>32</b>, which facilitates interconnection, supplies power to and secures the shank of the electrode <b>28</b>, is preferably included. Likewise, a second electrode holder <b>34</b> is provided for the backing electrode <b>30</b>. As shown in <figref idref="DRAWINGS">FIG. 3</figref>, the electrode holders <b>32</b>,<b>34</b> are preferably configured to concurrently secure more than one set of electrodes <b>28</b>,<b>30</b>, where multiple concurrent spot welding is desired.

0038Returning to <figref idref="DRAWINGS">FIG. 1</figref>, the system <b>10</b> also includes appropriate appurtenances such as a welding cable connector <b>36</b> for connecting to and feeding electric potential to the welding electrode <b>28</b>, and coolant ingress/egress nozzles <b>38</b> for receiving fresh, and removing heated, coolant. The mechanism <b>14</b>, source <b>16</b>, clamp <b>24</b>, and welding electrodes <b>28</b>,<b>30</b> are coupled to a fixed housing structure <b>40</b> that maintains the system <b>10</b> in an operable position relative to the workpieces <b>20</b>,<b>22</b>, provides leverage to the drive mechanism <b>14</b>, and prevents motion in one direction. In the illustrated embodiment shown in <figref idref="DRAWINGS">FIGS. 4 through 6</figref><i>b</i>, a connecting plate <b>42</b> interconnects the backing electrode <b>30</b>, a lower weld arm <b>42</b><i>a</i>, the mechanism <b>14</b>, and more preferably the source <b>16</b>, as shown in <figref idref="DRAWINGS">FIG. 4</figref>. The plate <b>42</b> presents a connecting plate prong <b>44</b> for further providing rotational connection to the fixed housing structure <b>40</b>.

0039More particularly, the preferred clamp <b>24</b> is configured to engage by applying a clamping force to a section of the workpieces <b>20</b>,<b>22</b> at a datum point <b>26</b>, and is drivenly coupled to the drive mechanism <b>14</b> and source <b>16</b>. A fixed backing block <b>46</b> (<figref idref="DRAWINGS">FIG. 3</figref>) may be included in the system <b>10</b> and configured to oppositely support the workpieces <b>20</b>,<b>22</b> relative to the clamp <b>24</b>, or the station fixture <b>18</b> may be configured to provide the necessary counter force to the clamping element <b>24</b> (<figref idref="DRAWINGS">FIGS. 6-6</figref><i>b</i>). The preferred electrodes <b>28</b>,<b>30</b> are configured to engage by applying a welding force to and passing an electric current through the workpieces <b>20</b>,<b>22</b> at opposite positions substantially adjacent the clamping location, and are also drivenly coupled to the mechanism <b>14</b> and source <b>16</b>. Due to its close proximity to the conductive electrode <b>28</b> as well as to heat generated during welding, the preferred clamp <b>24</b> includes an insulated outer cover, and more preferably, is entirely formed of a non-conductive heat resistant material.

0040The source <b>16</b> is configured to produce the clamping and welding forces, and actuate the mechanism <b>14</b>, so as to transfer the forces through the mechanism <b>14</b> and to the clamp <b>24</b> and electrodes <b>28</b>,<b>30</b>. Among other technologies, the source <b>16</b> may be pneumatic, hydraulic, or electromechanical in operation. In the illustrated embodiment, the source <b>16</b> is interconnected to the drive mechanism <b>14</b>, so as to cause a linearly translatable member <b>48</b> (<figref idref="DRAWINGS">FIG. 4</figref>) to move along a single longitudinal axis of freedom, wherein it is appreciated that a reversal of pneumatic pressure, for example, causes the member <b>48</b> to translate in the opposite direction to disengage the workpieces <b>20</b>,<b>22</b>. It is understood that a single force is produced and applied both to the clamp <b>24</b> and electrodes <b>28</b>,<b>30</b>, so as to derive the clamping and welding forces as shared.

0041The source <b>16</b> is preferably configured to result in a clamping force of approximately 50 kilogram force/per square centimeter (kgf/cm²) and a welding force of approximately 400 kgf/cm²being produced. Alternatively, the power source <b>16</b> is replaceable depending upon the application. For example, a larger power source may be utilized to drive longer weld arms or weld thicker workpieces. As presented in <figref idref="DRAWINGS">FIG. 8</figref>, it is appreciated that the provision of standard factory 6.12 kgf/cm²(i.e., 6 bar) or 10.2 kgf/cm²(i.e., 10 bar) air pressure as the source <b>16</b> and a pneumatic cylinder diameter size between 80 to 100 mm results in sufficient welding force being applied to the workpieces <b>20</b>,<b>22</b> for most electrode shank lengths, A (<figref idref="DRAWINGS">FIG. 2</figref>), and applications. It is appreciated, however, that the tabulated forces in <figref idref="DRAWINGS">FIG. 8</figref> represent the maximum generated force for the combination, and that for a given application only a fraction of the maximum force may be required. As such, the preferred system <b>10</b> further includes a force reduction element (not shown), such as a pressure regulator where a pneumatic source <b>16</b> is utilized, that reduces the force to the required amount.

0042The mechanism <b>14</b>, clamp <b>24</b> and electrodes <b>28</b>,<b>30</b> are cooperatively configured such that the clamp <b>24</b> engages the workpieces <b>20</b>,<b>22</b> prior to the electrodes <b>28</b>,<b>30</b>. In the illustrated embodiment, the clamp <b>24</b> includes an upper clamp swing arm <b>50</b> (<figref idref="DRAWINGS">FIG. 4</figref>), the first electrode <b>28</b> is connected to an upper weld swing arm <b>52</b>. The swing arms <b>50</b>,<b>52</b> preferably share an axis of rotation <b>54</b> as shown in <figref idref="DRAWINGS">FIGS. 7</figref><i>a</i>-<i>d</i>, so that the arms <b>50</b>,<b>52</b> are rotatable between an initial disengaged position (<figref idref="DRAWINGS">FIG. 7</figref>) and a workpiece engaged position (<figref idref="DRAWINGS">FIG. 7</figref><i>c</i>). To facilitate placement of workpieces <b>20</b>,<b>22</b> upon station fixture <b>18</b>, the vertical space above the fixture <b>18</b> is unobstructed when the system <b>10</b> is in place by providing an initial disengaged position that forms at least a 70 degree angle with horizontal.

0043The mechanism <b>14</b> and swing arms <b>50</b>,<b>52</b> are cooperatively configured such that the arms <b>50</b>,<b>52</b> are caused to rotate from the initial disengaged position to the engaged position non-coextensively (i.e., either at different rates, or at the same rate but including a delay period for the welding swing arm <b>50</b>), so that the clamp arm <b>50</b> reaches the engaged position first. More preferably, the clamp arm <b>50</b> is caused to reach the engaged position when the weld arm forms at least a 15 degree angle from the engaged position (<figref idref="DRAWINGS">FIG. 7</figref><i>b</i>). To effect this motion, the arms <b>50</b>,<b>52</b> and mechanism <b>14</b> are exemplarily configured so as to cooperatively form a cam.

0044More particularly, as best shown in <figref idref="DRAWINGS">FIG. 4</figref>, the clamp arm <b>50</b> defines a clamp arm slot <b>56</b> preferably near the end opposite from the workpiece engaging end, so as to minimize the required linear translation of the member <b>48</b>. The clamp slot <b>56</b> presents a bent longitudinal opening having a constant width. The longitudinal axis of a first section <b>56</b><i>a </i>of the clamp slot opening <b>56</b> presents a first pitch, P₁, as measured relative to and when the arm <b>50</b> is horizontal, while an adjacent upper section <b>56</b><i>b </i>presents a vertical longitudinal axis in the same arm position. The upper weld swing arm <b>52</b> defines a weld arm slot <b>58</b> preferably near the end opposite from the workpiece engaging end. The weld arm slot <b>58</b> preferably presents a straight longitudinal configuration, the same constant width as slot <b>56</b>, and a second pitch, P₂, wherein P₂is not less than (i.e., equal to or steeper than) P₁, but less than vertical in the horizontal arm position.

0045At least one laterally extending swing arm engaging pin (or cam follower) <b>60</b> is fixedly connected to the upper end of the linearly translating member <b>48</b>, so as to be linearly translated therewith and slidingly engagable. More preferably, at least a portion of the pin <b>60</b> is rotatably coupled to the member <b>48</b>, so as to present a bearing that is rollingly engagable. The pin <b>60</b> defines a cross-sectional diameter slightly less than (e.g., 95-99% of ) the widths of the slot openings <b>50</b>,<b>52</b>, so as to be receivable by the slots <b>50</b>,<b>52</b> without intolerable lateral freedom. That is to say, the pin <b>60</b> once received and slots <b>50</b>,<b>52</b> are cooperatively configured such that the pin <b>60</b> is generally able to translate only along the longitudinal axis. Because the arms <b>50</b>,<b>52</b> are translatably fixed at their shared axis of rotation, the linear translation of the pin <b>60</b> when received by the slots <b>50</b>,<b>52</b> causes the arms to rotate at rates according to the current pitch of the section of the slot engaging the pin <b>60</b>, wherein the steeper the slot the less rotational displacement is caused.

0046<figref idref="DRAWINGS">FIGS. 7 through 7</figref><i>d </i>illustrate the translation of the pin <b>60</b> relative to slots <b>50</b>,<b>52</b> and the resulting rotational displacement of the arms <b>50</b>,<b>52</b>. In <figref idref="DRAWINGS">FIG. 7</figref> the arms <b>50</b>,<b>52</b> are at an initial disengaged position and the member <b>48</b> and pin <b>60</b> are preferably at their lowest point of translation. The pin <b>60</b> in this position engages the horizontally vertical section <b>56</b><i>b </i>of the clamp slot <b>56</b>, which is presenting a current pitch less than slot <b>58</b>. As the pin <b>60</b> translates upward due to the application of the source <b>16</b>, the clamp arm <b>50</b> is caused to rotate faster than the weld arm <b>52</b> due to the difference in pitch. <figref idref="DRAWINGS">FIG. 7</figref><i>a </i>shows the arms <b>50</b>,<b>52</b> in an intermediate position, wherein the welding arm slot <b>58</b> is horizontal thereby resulting in the greatest moment about the axis <b>48</b> being experienced. The pin <b>60</b> now engages the first section <b>56</b><i>a </i>of the clamp slot <b>56</b>, so as to maintain an acceptable rate of rotation.

0047<figref idref="DRAWINGS">FIG. 7</figref><i>b </i>shows the clamp arm <b>50</b> in the workpiece engaged position, and the welding arm <b>52</b> trailing in a second intermediary position. In this position the pin <b>60</b> begins to travel up the now vertical section <b>56</b><i>b </i>of the clamp slot <b>56</b> thereby causing no rotational displacement by the clamp arm <b>50</b>. Thus, during operation the mechanism <b>14</b> is drivenly coupled to the clamp arm <b>50</b> only until the clamp <b>24</b> engages the workpieces <b>20</b>,<b>22</b> or shortly thereafter. Concurrently, the pin <b>60</b> continues to engage the weld arm <b>52</b> as the weld arm slot <b>58</b> remains diagonally oriented. Since the pin <b>60</b>, which continues to be driven upward by the source <b>16</b>, prevents the clamp arm <b>50</b> from rotating counter-directionally about the axis <b>48</b>, the clamp arm <b>50</b> is “locked” in the engaged position. When the upward force vector no longer acts upon the clamp arm <b>50</b> (i.e., when the slot is vertical and has not been fully traveled), the preferred clamp <b>24</b> is further configured to generate the clamping force on its own. To that end, in the illustrated embodiment, a compression spring <b>62</b> is included, as best shown in <figref idref="DRAWINGS">FIG. 4</figref>. More particularly, at the workpiece engaging end of the clamp arm <b>50</b>, the clamp <b>24</b> includes a plunger <b>64</b> that is telescopingly coupled to the spring <b>62</b> and clamp arm <b>50</b>. The plunger <b>64</b> is configured to strike the upper surface <b>18</b><i>a </i>of the workpieces <b>20</b>,<b>22</b> as the clamp arm <b>50</b> rotates, thereby compressing the spring <b>62</b> until the clamp arm <b>50</b> reaches its locked position.

0048<figref idref="DRAWINGS">FIG. 7</figref><i>c </i>shows the upper weld arm <b>52</b> in the workpiece engaged position, though the weld arm slot <b>52</b> remains diagonally oriented. As such, the pin <b>60</b>, mechanism <b>14</b> and source <b>16</b> are able to gradually increase the applied force to operable welding amounts, as they attempt to further rotate the weld arm <b>52</b>. The clamp arm <b>50</b> remains locked. As shown in <figref idref="DRAWINGS">FIG. 7</figref><i>d</i>, the slot opening <b>58</b> and mechanism <b>14</b> are cooperatively configured to further allow the weld arm <b>52</b> to rotate past the engaged position where necessary (e.g., when the electrode tip is worn or tip-dressed). More particularly, the member <b>48</b> is able to be further upwardly translated past its point when the weld arm <b>52</b> is in the engaged position and the weld slot <b>58</b> presents a sufficient longitudinal length to allow further pin translation. Likewise, the vertical section <b>56</b><i>b </i>of the clamp slot <b>56</b> is also extended to accommodate. It is appreciated that this capability will allow system usage even where electrode wear and or misalignment has occurred.

0049In another inventive aspect of the illustrated embodiment, it is also appreciated that the electrode <b>28</b> upon engaging the upper surface <b>20</b><i>a </i>of the workpieces <b>20</b>,<b>22</b>, the upper weld arm <b>52</b>, workpieces <b>20</b>,<b>22</b>, mechanism <b>14</b> and source <b>16</b> are cooperatively configured to cause the backing electrode <b>30</b> to tilt upwards and engage the workpieces <b>20</b>,<b>22</b> by providing a degree of rotation about the connecting plate prong <b>44</b> (<figref idref="DRAWINGS">FIG. 6</figref><i>b</i>). The ability to tilt upwards at least 3 degrees (i.e., “equalization”) enables the welding force to be applied to the workpieces <b>20</b>,<b>22</b> by both electrodes <b>28</b>,<b>30</b>, so as to accommodate lower electrode tip-wear, miss-assembly and/or workpiece surface tolerancing; otherwise, where the backing electrode <b>30</b> is spaced from the lower workpiece surface <b>22</b><i>a</i>, the workpieces <b>20</b>,<b>22</b> must be deformed in order for the upper electrode <b>28</b> to reach the backing electrode <b>30</b>. More particularly, to enable the concurrent application of the clamping force and equilization during welding, the axis of rotation <b>54</b> preferably shared by the arms <b>50</b>,<b>52</b> is defined by separate clamp arm and weld unit engaging bearings <b>66</b>,<b>68</b> (<figref idref="DRAWINGS">FIGS. 4 and 5</figref><i>a</i>). That is to say, the separate bearings <b>66</b>,<b>68</b> enable the weld unit to rotate while the clamp arm <b>50</b> remains motionless. Finally, the pin <b>60</b> and prong <b>44</b> are minimally spaced, and more preferably aligned, when the electrode <b>28</b> is in the engaged position (<figref idref="DRAWINGS">FIGS. 5</figref><i>b </i>and <b>7</b><i>c</i>) so as to minimize the force acting upon the clamp arm <b>50</b> during equilization.

0050The preferred forms of the invention described above are to be used as illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments and modes of operation, as set forth herein, could be readily made by those skilled in the art without departing from the spirit of the present invention. For example, it is well within the ambit of the present invention to modify the cam configuration of the system <b>10</b> by utilizing the linear member <b>48</b> of the drive mechanism <b>14</b> to define the slots <b>56</b>,<b>58</b> and providing the pins or cam followers <b>60</b> on the arms <b>50</b>,<b>52</b> themselves.

0051The inventors hereby state their intent to rely on the Doctrine of Equivalents to assess the scope of the present invention as pertains to any apparatus, system or method not materially departing from the literal scope of the invention set forth in the following claims.