Abstract:
A method for sizing a weld tool for forming a resistance spot weld to fasten a joint having at least two sheet metal members is provided. The method includes the steps of providing a plurality of weld force curves, determining a governing metal thickness, determining a total thickness of the sheet metal members, determining a plurality of joint parameters, including a weld gap parameter, a thickness parameter, a strength parameter, an equalization parameter and a sheet stiffness parameter, using the plurality of joint parameters to select one of the plurality of weld force curves, and using the governing metal thickness and the selected weld force curve to calculate a weld force parameter.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to weld tools for resistance spot welding a joint having two or more sheet metal members and more particularly to a method for sizing weld tools. 
     2. Discussion 
     Modern automotive development processes typically employ cross-functional teams having members with diverse backgrounds to review decisions related to the design and manufacture of various components. The diverse backgrounds provides a relatively small team with a broad understanding of almost all of the aspects that are relevant to the design, manufacture and use of the particular component. Accordingly, cross-functional teams have been instrumental in the shortening of the cycle times for developing and producing a vehicle in that the team is frequently able to identify problems and risks in proposed designs and processes prior to physical construction of the vehicle or any related tooling. 
     While cross-functional teams have made significant advancements, several drawbacks have been identified. One drawback concerns the need for a preliminary design having sufficient detail to properly convey the design concept of the component to team members or tooling suppliers. In the case of sheet metal fabrications, such as vehicle bodies, the design effort undertaken to sufficiently detail a given design for presentation to a cross-function team constitutes a majority of the overall effort required to completely design the sheet metal fabrication to completion. As such, the design of the sheet metal fabrication may be relatively mature prior to the disclosure of the design to the team members, eliminating or substantially impairing the option to introduce significant changes in the design should a notable problem be identified by the cross-functional team. 
     In particular, problems are routinely encountered with the sizing of weld tools for resistance spot welding the sheet metal members of a joint together. These problems arise when aspects of the design of the joint or the manufacturing process are not fully understood or compensated for in the design process. These aspects include, for example, the ability of the weld tool to equalize to the joint or the need to deflect one or more of the sheet metal members forming the joint into abutment prior to forming the spot weld. The additional force required in such situations often overstresses the weld tool and significantly reduces its useful life. 
     To avoid reducing the life of a weld tool, it would seem rather obvious to simply procure a larger weld tool for the operation. This alternative, however, is frequently not a viable option due to space constraints that result from the design of the sheet metal fabrication and to a lesser extent, machine tool capabilities. Even where a larger weld tool can be implemented, the machine tool used to orient the weld tool is frequently not sized to handle the additional weight of the larger tool and as such, this change negatively impacts the performance of the machine tool (due to increased inertia and weight) as well as its durability. 
     Accordingly, there remains a need in the art for a quick, efficient and accurate method for sizing weld tools for resistance spot welding a joint having a plurality of sheet metal members. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide a method for calculating a weld force index for sizing a weld tool for resistance spot welding a joint. 
     It is another object of the present invention to provide a method for calculating a weld force index for sizing a resistance spot weld tool which utilizes a series of predetermined parameters for performing the calculation. 
     A method for mechanically sizing a weld tool for forming a resistance spot weld to fasten a joint having at least two sheet metal members is provided. The method includes the steps of providing a plurality of weld force curves, determining a governing metal thickness, determining a total thickness of the sheet metal members, determining a plurality of joint parameters, including a weld gap parameter, a thickness parameter, a strength parameter, an equalization parameter and a sheet stiffness parameter, using the plurality of joint parameters to select one of the plurality of weld force curves, and using the governing metal thickness and the selected weld force curve to calculate a weld force parameter. 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a device being welded with a conventional resistance spot weld tool; 
     FIGS. 2 a  and  2   b  are schematic illustrations of a resistance weld tool having equalizing capability but whose actuation causes deflection in the joint to be welded; 
     FIG. 3 is a schematic illustration of a resistance weld tool having no equalizing capability; 
     FIGS. 4 a  through  4   d  are schematic diagrams of a method according to the teachings of the present invention in flowchart form; 
     FIG. 5 is a schematic diagram of a method for calculating a stiffness component for a pair of abutting joint members illustrated in flowchart form; and 
     FIG. 6 is a chart showing various weld curves. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, a conventional weld tool is generally indicated by reference numeral  10 . Weld tool  10  is illustrated as having a generally C-shaped structural frame  12 , a first electrode  14 , a transport mechanism  16  and a second electrode  18 . Although a particular type of weld tool is illustrated as a “C” type weld gun, it will be understood that the teachings of the present invention have applicability to other types of weld tools, including but not limited to pinch guns and scissors guns. Mechanically, first electrode  14  is fixedly but removably coupled to frame  12 . First electrode  14  is also electrically coupled to frame  12 . Transport mechanism  16  is fixedly coupled to frame  12  but electrically isolated therefrom. Second electrode  18  is fixedly but removably coupled to transport mechanism  16  but is electrically isolated therefrom. Transport mechanism  16  includes a pneumatic cylinder or clamp (not specifically shown) which permits second electrode  18  to be moved relative to first electrode  14 . As is well known in the art, transport mechanism  16  may alternatively include an electrically or hydraulically operated clamp or cylinder. 
     Weld tool  10  is operable for resistance spot welding a joint  30  comprised of two or more sheet metal members  32  together. With the particular weld tool illustrated, first electrode  14  is typically positioned so as to abut sheet metal member  32   c  and transport mechanism  16  is actuated to extend second electrode  18  toward first electrode  14  to exert a clamping force through joint  30 . A high current electric charge is then passed through the first and second electrodes  14 ,  18  which causes the formation of a resistance spot weld. Of important note in this example is the fact that first electrode  14  is brought into abutment with sheet metal member  32   c  without causing deflection in one or more of the sheet metal members  32  forming joint  30  in excess of a predetermined amount. Weld tool  10  is said to have perfect equalizing capability. 
     Referring to FIGS. 2 a  and  2   b , an alternate weld tool  10 ′ is illustrated. When a locating force is exerted to the first electrode  14 ′ to bring it into abutment with sheet metal member  32   b ′, the locating force is shown to cause deflection in the sheet metal members  32 ′ in excess of a predetermined amount. Weld tool  10 ′ is said to have an equalizing capability which causes deflection in excess of a predetermined amount. The same type of condition may also result when the spring constant associated with the transport mechanism  16 ′ and its associated second electrode  18 ′ is greater than the spring constant of the joint  30 ′, causing the transport mechanism  16 ′ to bend the sheet metal members  32 ′ toward the first electrode  14 ′ rather than drawing first electrode  14 ′ into abutment with the sheet metal members  32 ′. 
     Referring to FIG. 3, a second alternate weld tool  10 ″ is illustrated. First electrode  14 ″ is fixed and cannot be brought into abutment with sheet metal member  32   b ″. Rather, a force must be exerted through second electrode  18 ″ which is sufficient in magnitude to include not only the desired clamping force for performing the resistance spot weld, but also a sufficient locating force to deflect the sheet metal members  32 ″ of joint  30 ″ to bring sheet metal member  32   b ″ into abutment with fixed electrode  14 ″. Weld tool  10 ″ is said to have no equalizing capability. 
     In FIGS. 4 a  through  4   d , a method according to the teachings of the present invention is shown in flowchart form. The methodology is entered at bubble  200  and begins to determine a gap parameter. In the particular embodiment illustrated, the gap parameter includes three components: a design gap parameter, a maximum gap due to panel variation parameter and a gap sequence parameter. Each of these parameters will be discussed in detail below. 
     The methodology progresses to block  204  where it determines the design gap magnitude (DGM) and the total thickness of all the sheet metal members  32  which make up joint  30 . The DGM is the nominally designed distance between the sheet metal members  32 . In cases where each of the sheet metal members  32  is designed to abut one another, the DGM is zero. The total thickness of all the sheet metal members  32  is simply the sum of the thicknesses for each sheet metal member. In the example provided above, the thickness of the first sheet metal member  32   a  is 1.25 mm, the thickness of the second sheet metal member  32   b  is 1.75 mm and the thickness of the third sheet metal member  32   c  is 1.50 mm and as such, the total thickness is 4.50 mm. 
     The methodology next proceeds to decision block  208  where the magnitude of the DGM is evaluated. If the DGM is not greater than 0.9 mm, the methodology proceeds to block  212  where the design gap parameter is set to a predetermined first value. The methodology then proceeds to block  228 . Returning to decision block  208 , if the DGM is greater than 0.9 mm, the methodology advances to decision block  216  where the methodology determines if the DGM exceeds 2.0 mm. It the DDGM does not exceed 2.0 mm, the methodology progresses to block  220  where the design gap parameter is set to a second value. The methodology then proceeds to block  228 . Returning to decision block  216 , it the DGM exceeds 2.0 mm, the methodology proceeds to block  224  where the design gap parameter is set to a third value. The methodology then advances to block  228 . 
     In block  228  the methodology determines the maximum gap due to panel variation (GVM). The GVM is the maximum actual gap permitted. The methodology proceeds to block  232  where the methodology evaluates the GVM. If the GVM is not greater than 1.5 mm, the methodology proceeds to block  236  where the maximum gap due to panel variation parameter is set to a first value. The methodology then proceeds to block  252 . In decision block  232 , if the GVM is greater than 1.5 mm, the methodology proceeds to decision block  240  where the methodology determines if the GVM is greater than 3.5 mm. If the GVM is not greater than 3.5 mm, the methodology proceeds to block  244  and sets the maximum gap due to panel variation parameter equal to a second value. The methodology then proceeds to block  252 . Returning to decision block  240 , if the GVM is greater than 3.5 mm, the methodology proceeds to block  248  and sets the maximum gap due to panel variation parameter to a third value. The methodology proceeds to block  252 . 
     In block  252 , the methodology determines the effect of the weld sequence on the gap creation (GSV). Weld sequence may negatively affect gap creation if one or more of the sheet metal members  32  buckles or otherwise deforms during the welding process. The methodology then progresses to decision block  256  where the impact on GSV is quantified. If the welding process has a low effect on GSV, the methodology proceeds to block  260  and sets the gap sequence parameter equal to a first value. The methodology then proceeds to decision block  276 . Returning to decision block  256 , if the welding process does not have a low effect on GSV, the methodology proceeds to block  264  where the methodology determines if the effect on GSV is high. If the effect on GSV is not high in decision block  264 , the methodology proceeds to block  268  and sets the gap sequence parameter equal to a second value. The methodology then proceeds to decision block  276 . In decision block  264 , if the welding process does have a high effect on GSV, the methodology proceeds to block  272  and sets the gap sequence parameter to a third value. The methodology then proceeds to decision block  276 . 
     In decision block  276  the methodology determines the quantity of sheet metal members  32  which comprise joint  30 . If joint  30  is comprised of three sheet metal members  32 , the methodology proceeds to block  280  and determines the stiffness component for the second pair of abutting sheet metal members  282  (i.e., sheet metal members  32   b  and  32   c . In the example provided, the stiffness component for each pair of abutting sheet metal members  32  is obtained through a stiffness subroutine that is schematically illustrated in flowchart form in FIG.  5 . 
     In FIG. 5, the subroutine is entered at bubble  1000  and progresses to decision block  1002  where the methodology determines if the first sheet in the pair of abutting sheet metal members is flexible. The term “flexible” is used where about 5 pounds of force (i.e., finger pressure) is adequate to push the particular sheet metal member  32  into abutment with the opposed sheet metal member  32 . If the first sheet is flexible, the methodology proceeds to decision block  1004  where the methodology determines whether the second sheet in the pair of abutting sheet metal members is flexible. If the second sheet is also flexible, the methodology proceeds to block  1006  and sets a stiffness value equal to 1. The methodology then proceeds to decision block  1050 . 
     Returning to decision block  1004 , if the second sheet is not flexible, the methodology proceeds to decision block  1008  where the methodology determines if the second sheet is rigid. The term “firm” is used where moderate clamping force, such as a force which does not exceed one tenth (10%) of the recommended resistance spot weld clamping force, is required to push the particular sheet metal member  32  into abutment with the opposed sheet metal member  32 . The term “rigid” is used where a clamping force in excess of a moderate clamping force is required to bring the particular sheet metal member  32  in to abutment with the opposed sheet metal member  32 . In the embodiment of the invention illustrated, if a sheet metal member is neither flexible nor firm, it will be classified as rigid. If the second sheet is not rigid, the methodology proceeds to block  1010  and sets the stiffness value equal to 2. The methodology then proceeds to decision block  1050 . If, however, the second sheet is rigid in decision block  1008 , the methodology proceeds to block  1012  and sets the stiffness value equal to 3. The methodology then proceeds to decision block  1050 . 
     Returning to decision block  1002 , if the first sheet is not flexible, the methodology proceeds to decision block  1020  where the methodology determines if the first sheet is firm. If the first sheet is firm, the methodology proceeds to decision block  1022  where it determines whether the second sheet is flexible. If the second sheet is flexible in decision block  1022 , the methodology proceeds to block  1010  and sets the stiffness value as discussed above. If, however, the second sheet is not flexible in decision block  1022 , the methodology proceeds to decision block  1024  where it determines whether the second sheet is rigid. If the second sheet is not rigid in block  1024 , the methodology proceeds to block  1026  where the stiffness value is set to 4. The methodology then proceeds to decision block  1050 . If the second sheet is rigid in decision block  1024 , the methodology proceeds to block  1028  where the stiffness value is set to 6. The methodology then proceeds to decision block  1050 . 
     Returning to decision block  1020 , if the first sheet is not firm, the methodology proceeds to decision block  1030  and determines whether the second sheet is flexible. If the second sheet is flexible, the methodology proceeds to block  1032  and sets the stiffness value to 3. The methodology then proceeds to decision block  1050 . If, however, the methodology determines that the second sheet is not flexible in decision block  1030 , the methodology proceeds to decision block  1034  where it determines whether the second sheet is rigid. 
     If the second sheet is not rigid in decision block  1034 , the methodology proceeds to block  1036  where the stiffness value is set to 6. The methodology then proceeds to decision block  1050 . If the second sheet is rigid in decision block  1034 , the methodology proceeds to block  1038  where the stiffness value is set to 9. The methodology proceeds to decision block  1050 . 
     In decision block  1050 , the methodology determines if the stiffness value is less than a first predetermined value. If the stiffness value is less than a first predetermined value, such as 3, the methodology proceeds to block  1052  where the stiffness parameter is set to a first value. The methodology then proceeds to bubble  1060  where the subroutine terminates. Returning to decision block  1050 , if the stiffness value is not less than the first predetermined value, the methodology proceeds to block  1054  and determines if the stiffness number is less than a second predetermined value. If the stiffness value is less than a second predetermined value, such as 6, the methodology proceeds to block  1056  where the stiffness parameter is set to a second value. The methodology then proceeds to bubble  1060 . If the stiffness value is not less than the second predetermined value, the methodology proceeds to block  1058  where the stiffness parameter is set to a third value. The methodology then proceeds to bubble  1060  and terminates where it reenters the main methodology. 
     The methodology next proceeds to decision block  284  where the total thickness of the sheet metal members  32  is evaluated. If the total thickness is less than or equal to 4.5 mm, the methodology proceeds to block  288  where the thickness parameter is set to a second value. The methodology then proceeds to decision block  296 . Returning to decision block  284 , if the total thickness is not less than or equal to 4.5 mm, the methodology proceeds to block  292  and sets the thickness parameter to a third value. The methodology then proceeds to decision block  296 . 
     In decision block  296  the methodology next evaluates the tensile strength of the third sheet metal member  32   c . If the tensile strength of the third sheet metal member is less than 35,000 lbs. (35 ksi), the methodology proceeds to block  300  where the third strength component is set to a first value. The methodology then proceeds to block  356 . Returning to decision block  296 , if the tensile strength of the third sheet metal member  32   c  is greater than 35,000 lbs, the methodology proceeds to decision block  304 . 
     In decision block  304 , if the tensile strength of the third sheet metal member  32   c  is not greater than 50,000 lbs (50 ksi), the methodology proceeds to block  308  where the third strength component is set to a second value. The methodology then proceeds to block  356 . If in decision block  304  the tensile strength of the third sheet metal member  32   c  is greater than 50,000 lbs, the methodology proceeds to block  312  where the third strength component is set to a third value. The methodology then proceeds to block  356 . 
     Returning to decision block  276 , if the joint  30  is not comprised of three sheet metal members  32 , the methodology proceeds to decision block  320  and determines if the joint is comprised of two sheet metal members  32 . If the joint is not comprised of two sheet metal members  32 , the methodology proceeds to bubble  600  where the methodology terminates. If the joint is comprised of two sheet metal members  32  in decision block  320 , the methodology proceeds to block  328  where the third strength component and the stiffness component for the second pair of abutting joint members are both set to zero (0). The methodology proceeds to decision block  336 . 
     In decision block  336  the methodology evaluates the total thickness of the sheet metal members  32  forming the joint  30 . If the total thickness is less than 2.0 mm, the methodology proceeds to block  340  where the thickness parameter is set to a first value. The methodology then proceeds to block  356 . If the total thickness is not less than 2.0 in decision block  336 , the methodology proceeds to decision block  344 . If the total thickness is not greater than 4.0 mm in decision block  344 , the methodology proceeds to block  348  where the thickness parameter is set to a second value. The methodology then proceeds to block  356 . In block  344 , if the total thickness is greater than 4.0 mm, the methodology proceeds to block  352  where the thickness parameter is set to a third value. The methodology then proceeds to block  356 . 
     In block  356  the methodology employs the stiffness subroutine that is discussed in detail above to determine the stiffness component for the first pair of abutting joint members  358  (i.e., sheet metal members  32   a  and  32   b ). The methodology also determines a stiffness parameter for the joint  30 . The stiffness parameter for the joint  30  is calculated by adding the stiffness components for the first and second pair of abutting joint members. The methodology next proceeds to decision block  360  where the tensile strength of the second sheet metal member  32   b  is evaluated. 
     If the tensile strength of the second sheet metal member  32   b  is less than 35,000 lbs. (35 ksi), the methodology proceeds to block  364  where the second strength component is set to a first value. The methodology then proceeds to decision block  380 . Returning to decision block  360 , if the tensile strength of the second sheet metal member  32   b  is greater than 35,000 lbs., the methodology proceeds to decision block  368 . 
     In decision block  368 , if the tensile strength of the second sheet metal member  32   b  is not greater than 50,000 lbs. (50 ksi), the methodology proceeds to block  372  where the second strength component is set to a second value. The methodology then proceeds to block  380 . If in decision block  368  the tensile strength of the second sheet metal member  32   b  is greater than 50,000 lbs., the methodology proceeds to block  376  where the second strength component is set to a third value. The methodology then proceeds to block  380 . 
     In decision block  380  the methodology next evaluates the tensile strength of the first sheet metal member  32   a . If the tensile strength of the first sheet metal member is less than 35,000 lbs. (35 ksi), the methodology proceeds to block  384  where the first strength component is set to a first value. The methodology then proceeds to block  400 . Returning to decision block  380 , if the tensile strength of the first sheet metal member  32   a  is greater than 35,000 lbs., the methodology proceeds to decision block  388 . 
     In decision block  388 , if the tensile strength of the first sheet metal member  32   a  is not greater than 50,000 lbs. (50 ksi), the methodology proceeds to block  392  where the first strength component is set to a second value. The methodology then proceeds to block  400 . If in decision block  388  the tensile strength of the first sheet metal member  32   a  is greater than 50,000 lbs., the methodology proceeds to block  396  where the first strength component is set to a third value. The methodology then proceeds to block  400 . 
     In block  400  the strength parameter is calculated. In the particular embodiment illustrated, the strength parameter is simply the sum of the first, second and third strength components. The methodology then proceeds to decision block  404  where the positioning or equalizing capability of the weld tool  10  is evaluated. 
     If the weld tool  10  does not include an equalizing capability, such that neither the first or second electrodes  14  and  18  are brought into abutment with one of the sheet metal members  32  while actuating the weld tool  10  to exert a clamping force to the sheet metal members  32 , the methodology proceeds to block  408  where the equalization parameter is set to a third value. The methodology next proceeds to decision block  424 . Returning to decision block  404 , if the weld tool  10  does include an equalizing capability such that one of the first and second electrodes  14  and  18  are brought into abutment with one of the sheet metal members while actuating the weld tool  10  to exert a clamping force to the sheet metal members  32 , the methodology proceeds to decision block  412 . 
     In decision block  412 , the methodology determines the extent to which a locating force for causing the first electrode to abut one of the sheet metal members  32  impacts the joint  30 . If the locating force applied through first electrode  14  causes deflection in one or more of the sheet metal members  32  in excess of a predetermined amount while actuating the weld tool  10  to exert a clamping force to the sheet metal members  32 , the methodology proceeds to block  416  and sets the equalization parameter to a second value. The methodology then proceeds to decision block  424 . Returning to decision block  412 , if the locating force applied through first electrode  14  does not cause deflection in one or more of the sheet metal members  32  in excess of a predetermined amount prior to actuating the weld tool  10  to exert a clamping force to the sheet metal members  32 , the methodology proceeds to block  420  and sets the equalization parameter to a first value. The methodology then proceeds to decision block  424 . 
     In decision block  424  the methodology evaluates the tip shank diameter of the electrodes. If the tip shank diameter of any of the electrodes is less than or equal to about ⅝-inch diameter, the methodology proceeds to block  428  where the tip parameter is set to a first value. The methodology then proceeds to decision block  444 . Referring back to decision block  424 , if neither of the electrodes has a tip shank diameter which is less than or equal to ⅝ inch, the methodology proceeds to decision block  432 . In decision block  432 , if the tip shank diameter of both of the electrodes does not exceed ¾ inch, the methodology proceeds to block  436  where the tip parameter is set to a second value. The methodology then proceeds to decision block  444 . Returning to decision block  432 , if the tip shank diameter of both of the electrodes exceeds ¾ inch in diameter, the methodology proceeds to block  440  where the tip parameter is set to a third value. The methodology next proceeds to decision block  444 . 
     In decision block  444 , the methodology determines whether any of the sheet metal members  32  has been coated with a zinc coating. If none of the sheet metal members has been coated with a zinc coating, the methodology proceeds to block  448  where the coating parameter is set to a first value. The methodology then proceeds to block  468 . 
     Returning to decision block  444 , if any of the sheet metal members  32  has been coated with a zinc coating, the methodology proceeds to decision block  452  where the methodology determines if any of the sheet metal members  32  have a zinc coating with a weight/area in excess of a first predetermined threshold. In the particular example disclosed, the first predetermined threshold is 45 grams per square meter. If none of the sheet metal members  32  have been coated with a zinc coating having a weight/area in excess of the first predetermined threshold, the methodology proceeds to block  448 . 
     If any of the sheet metal members has a zinc coating with a weight/area in excess of the first predetermined threshold in decision block  452 , the methodology proceeds to decision block  456  where the methodology determines if any of the sheet metal members  32  have a zinc coating with a weight/area in excess of a second predetermined threshold. In the particular example disclosed, the second predetermined threshold is 55 grams per square meter. If none of the sheet metal members  32  has been coated with a zinc coating having a weight/area in excess of the second predetermined threshold, the methodology proceeds to block  460  where the coating parameter is set to a second value. The methodology then proceeds to block  468 . 
     If any of the sheet metal members has a zinc coating with a weight/area in excess of the second predetermined threshold in decision block  452 , the methodology proceeds to block  464  where the coating parameter is set to a third value. The methodology then proceeds to block  468 . 
     In block  468 , the methodology determines the gross force index and the governing metal thickness. In the example provided, the gross force is the numeric total of each of the parameters. In this regard, various parameters may be weighted equally to have equal impact or weighted differently to provide selected parameters with more importance relative to other parameters. Differential weighting may therefore be employed to completely factor out parameters such as the coating parameter or the tip parameter, which although are strongly related to the difficulty in welding a joint, but only weakly related to the sizing of a weld tool  10 . In the particular embodiment illustrated, each of the parameters is weighted equally, with the first value being equal to one (1), the second value being equal to two (2) and the third value being equal to three (3). 
     In a joint formed by two sheet metal members, the governing metal thickness (GMT) is simply the thinnest sheet metal member  32  which forms the joint. In a joint formed by three sheet metal members, the GMT is the intermediate thickness of the sheet metal members. In the particular example provided, 1.50 mm is the governing metal thickness because the joint  30  is formed by three sheet metal members  32  and the third sheet metal member  32   c  is the sheet metal member having the intermediate thickness. The methodology next proceeds to decision block  472 . 
     If all of the sheet metal members  32  forming the joint  30  are low carbon steel in decision block  472 , the methodology proceeds to decision block  476  where the methodology determines if the GFI is less than or equal to 12. If the GFI is less than or equal to 12, the methodology proceeds to block  480  and selects weld curve “A”. The methodology then proceeds to decision block  540 . If the GFI is greater than 12 in decision block  476 , the methodology proceeds to decision block  484 . If the GFI is not greater than 20 in decision block  484 , the methodology proceeds to block  488  and selects weld curve “B”. The methodology then proceeds to decision block  540 . If the GFI is greater than 20 in decision block  484 , the methodology proceeds to block  492  and selects weld curve “C”. The methodology then proceeds to decision block  540 . 
     Returning to decision block  472 , if all of the sheet metal members  32  forming joint  30  are not low carbon steel, the methodology proceeds to block  496  and determines if all of the sheet metal members  32  are high strength steel. If all of the sheet metal members are high strength steel, the methodology proceeds to decision block  500  and determines whether the GFI is less than or equal to 12. If the GFI is less than or equal to 12, the methodology proceeds to block  504  and selects weld curve “C”. The methodology then proceeds to decision block  540 . If the GFI is greater than 12 in decision block  500 , the methodology proceeds to decision block  508 . If the GFI is not greater than 20 in decision block  508 , the methodology proceeds to block  512  and selects weld curve “D”. The methodology then proceeds to decision block  540 . If the GFI is greater than 20 in decision block  508 , the methodology proceeds to block  516  and selects weld curve “E”. The methodology then proceeds to decision block  540 . 
     Referring back to decision block  496 , if all of the sheet metal members  32  forming joint  30  are not high strength steel, the methodology proceeds to decision block  520 . In decision block  520 , the methodology determines whether the GFI is less than or equal to 12. If the GFI is less than or equal to 12, the methodology proceeds to block  524  and selects weld curve “B”. The methodology then proceeds to decision block  540 . If the GFI is greater than 12 in decision block  520 , the methodology proceeds to decision block  528 . If the GFI is not greater than 20 in decision block  528 , the methodology proceeds to block  532  and selects weld curve “C”. The methodology then proceeds to decision block  540 . If the GFI is greater than 20 in decision block  528 , the methodology proceeds to block  536  and selects weld curve “D”. The methodology then proceeds to decision block  540 . 
     In block  540  the methodology determines if any of the sheet metal members  32  forming the joint  30  are coated with a free zinc coating (i.e., a non-alloyed zinc coating). If none of the sheet metal members  32  are coated with a free zinc coating (e.g., the sheet metal members may be uncoated or “bare”, or may be coated with an iron-zinc coating), the methodology proceeds to block  544  and selects a first set of weld curves, such first weld curve set  546  illustrated in FIG.  6 . The methodology then proceeds to block  552 . If any of the sheet metal members  32  are coated with a free zinc coating in decision block  540 , the methodology proceeds to block  548  and selects a second set of weld curves, such as second weld curve set  550 , also illustrated in FIG.  6 . The methodology then proceeds to block  552 . 
     In block  552 , the methodology uses GMT, the selected set of weld curves and the selected weld curve to interpolate a weld force index. The weld force index is to be used to size a weld tool  10  for resistance spot welding the joint  30  analyzed and ensures that the weld tool  10  will be sized in a robust manner to provide a desired level of reliability yet will not be grossly oversized so as to interfere with the design or manufacture of the joint  30 . Accordingly, it is presently preferred that the weld force index be calculated as a gross weld force value, having units in pounds (force) or Newtons. The methodology then proceeds to bubble  600  and terminates. 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the description of the appended claims.