Abstract:
A weld nut electrode for differentiating between a properly and an improperly oriented weld nut. The electrode has a conductive portion for passing weld current through the weld nut and an insulated portion electrically isolated from the conductive portion. The conductive portion makes electrical contact with a properly oriented weld nut, and the insulated portion is arranged to prevent physical contact between the conductive portion and an improperly oriented weld nut.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application Serial No.  6 0/295,685, filed Jun. 4, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates generally to an apparatus for detecting whether a weld nut is properly oriented while being resistance welded to a workpiece.  
         DESCRIPTION OF THE RELATED ART  
         [0003]    Weld nuts are often used in applications where the nut portion of a nut and bolt fastening arrangement is inaccessible after completed assembly of a production article. For example, weld nuts are often used in hollow automobile cross members to facilitate bolting to the cross member. Once the cross member is assembled, the weld nut is inside of the hollow member and is inaccessible. If the weld nut were welded upside down to the interior surface of the cross member, the entire cross member may have to be discarded or, preferably, recycled, because the weld nut is inaccessible and unserviceable.  
           [0004]    To reduce the probability of assembling articles with upside down weld nuts, manufacturers often use a method of inspecting the weld nuts. One method of inspecting the weld nuts is to use an automated visual inspection system. This method is often expensive to put in place, and requires computer programming to accommodate parts with different appearances and profiles. Another method of inspecting the weld nuts is to employ an inspector who is assigned the task of making sure the weld nuts are properly oriented. While this method generally produces satisfactory results in the inspection process, it can be costly and subject to human error.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    Accordingly, one aspect of this invention is to provide a relatively simple and reliable method of determining whether a misoriented weld nut has been welded to a workpiece.  
           [0006]    Another aspect of this invention is to provide a weld electrode that cooperates with a weld nut in such a way that the electrical properties of the weld event are substantially different between properly oriented and misoriented weld nuts.  
           [0007]    Yet another aspect of this invention is to provide an inexpensive method of reliably and automatically determining whether a weld nut is properly oriented.  
           [0008]    In accordance with these aspects, the present invention provides a weld nut electrode for differentiating between a properly and an improperly oriented weld nut. The electrode has a conductive portion for passing weld current through the weld nut and an insulated portion electrically isolated from the conductive portion. The conductive portion makes electrical contact with a properly oriented weld nut and the insulated portion makes exclusive contact with an improperly oriented weld nut. Sensing of the conductive or nonconductive condition can provide an indication as to nut orientation.  
           [0009]    Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0010]    [0010]FIG. 1A is a side view of a weld electrode with a cross section of A-A in FIG. 1B;  
         [0011]    [0011]FIG. 1B is an end view of the weld electrode;  
         [0012]    [0012]FIG. 2 is a partial cross section of the weld electrode in connection with a properly oriented weld nut for welding to a workpiece;  
         [0013]    [0013]FIG. 3 is a partial cross section of a weld electrode in cooperation with a misoriented weld nut;  
         [0014]    [0014]FIG. 4 shows weld current and voltage waveforms during a weld event of a properly oriented weld nut; and  
         [0015]    [0015]FIG. 5 shows weld current and voltage waveforms during a weld event of a misoriented weld nut. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    [0016]FIG. 1A shows a partial-cutaway view of weld electrode  10 . The cutaway portion is taken along section line A-A of FIG. 1 B. The weld electrode  10  has a conductive body  16  with an electrode face  22 . An insulated portion  12  is set in the electrode face and, as described below, is electrically insulated from the body  16 . In an exemplary embodiment, the insulated portion  12  has a face  38  flush with the electrode face  22 , and a pin  39  protruding from the insulated face  38 . The insulated portion  12  can be made of a non-ferrous metal that can withstand the heat of resistance welding and contributes little heat via inductive heating. Non-magnetic stainless steel is a suitable material. An insulator  14  electrically isolates the body  16  from the insulated portion  12 . A phenolic material may be used for the insulator  14 . Alternatively, portion  12  could be formed from a nonconductive, thermal insulating material, such as a suitable ceramic material. In such an alternative embodiment, insulator  14  would not be necessary.  
         [0017]    A screw  26  extends through an axial bore in the body  16  and secures the insulated portion  12  within the body  16 . The insulated portion  12  has a threaded hole for receiving and retaining the screw  26 . The screw shank is insulated from the body  16  by an insulating bushing  32  that extends the length of the axial bore in the body  16 . Screw  26  can be made of non-magnetic stainless steel to prevent inductive heating and chemical, galvanic or electrical interaction with the insulated portion  12 . Insulating bushing  32  can be made of a phenolic material. An insulating washer  30  electrically isolates the screw head from the body  16  and a flat washer  28  operates to keep the screw  26  secure. The foregoing description of the screw  26 , washer  28 , insulating washer  30 , and insulating bushing  32  simply describes an arrangement for retaining the insulated portion  12  within the body  16  and is not intended to be limiting. The insulated portion may be retained with other arrangements such as by pressing it into the body  16  or securing it with an adhesive.  
         [0018]    A portion of the outer surface of the body  16  preferably has threads  24  to facilitate securing the weld electrode  10  within a weld fixture. A hexagonal portion  20  may be incorporated to apply torque for threading the electrode  10  into the weld fixture.  
         [0019]    [0019]FIG. 1B shows an end view of the weld electrode looking into the electrode face  22 . The insulated face  38  is located at the outer periphery of the insulated portion  12 , and the electrode face  22  is located radially outward from the insulated face  38 . The insulated portion  12  electrically isolated from the body  16  by the insulator  14 . Insulator  14  may be omitted if the insulated portion  12  is made from a nonconductive material as discussed above. Threaded portion  24  is shown in hidden lines as its view is obscured by the electrode face  22 .  
         [0020]    Turning now to FIG. 2, the weld electrode  10  is shown in a cooperating arrangement with a properly oriented weld nut  34 . The insulated pin  39  extends in a direction normal to the insulated face  38  and is centered on the axial centerline of body  16 . The diameter of insulated pin  39  is slightly less than the inner diameter of the mating portion of the weld nut screw hole  42 . The mating of insulated pin  39  and screw hole  42  keeps the weld nut  34  aligned upon the electrode  10 . The flat portion  44  of the weld nut  34  abuts the electrode face  22  and may contact the insulated face  38 . The surface area in contact between the flat portion  44  and the electrode face  22  creates a low electrical resistance through which welding current can flow.  
         [0021]    During a weld event  46  (shown in FIGS. 4 and 5), a weld voltage V is dropped across the workpiece  50  and conductive body  16 . The mating area of the flat portion  44  of the weld nut  34  and the electrode face  22  has a resistance low enough to allow a weld current I to flow through the workpiece  50 , weld nut  34  and electrode body  16  when the weld voltage V is applied.  
         [0022]    [0022]FIG. 3 shows weld electrode  10  mated with a misoriented weld nut  34 . A weld nut pilot  52 , provided on the weld nut  34 , mates with the insulated pin  39  and insulated face  38 . The diameter of the insulated portion  38  is greater than or equal to the outside diameter of the pilot  52 , thereby preventing the weld nut  34  from making electrical contact with electrode face  22 . An electrical resistance is thereby established between the workpiece  50  and conductive body  16  that is measurably higher than when the weld nut is properly oriented.  
         [0023]    By monitoring the current or voltage, or both, during the weld event  46 , the high electrical resistance may be used to facilitate detecting that the weld nut is misoriented. Because of the high resistance, the current flow I is substantially lower when the weld nut is misoriented than when it is properly oriented. In an ideal case, the current I remains at zero since the insulator  14  prevents current flow. In practice however, current I may arc from weld nut  34  to the electrode face  22 , bypassing insulator  14 . This arc path also has a higher resistance and lower current I than when the weld nut  34  is properly oriented. A weld system monitoring the value of weld voltage or current, or both, determines whether the weld nut is properly oriented by comparing the monitored values to predetermined values.  
         [0024]    [0024]FIG. 4 shows an example of weld current and voltage waveforms with a properly oriented weld nut. During the weld event  46 , the weld current I attains a value greater than a predetermined minimum, I MIN , yet less than a predetermined maximum, I MAX . In one aspect of the invention, I MIN  is equal to 36 kiloamperes and I MAX  is equal to 42 kiloamperes. In addition, during the weld event  46 , the weld voltage attains a value between predetermined minimum V MIN  and a predetermined maximum weld voltage V MAX . In one aspect of the invention, these are 4.5 volts and 6.75 volts, respectively.  
         [0025]    [0025]FIG. 5 shows an example of the weld current and weld voltage when the weld nut is misoriented. The high resistance between the pilot  52  and the electrode face  22  causes the current I to be lower than with a properly oriented weld nut  34 . Also, the high resistance causes the weld voltage V to be higher than when the weld nut  34  is properly oriented and the resistance between the flat portion  44  of the weld nut  34  flat and the electrode face  22  is low.  
         [0026]    When the weld electrode  10  is used in a welding system capable of monitoring the weld current and voltage, the welding system detects a misoriented weld nut  34  by comparing the current and voltage to predetermined values. As shown in FIG. 5, a predetermined current limit I MIN  is chosen such that it is greater than weld current I 2  flowing when a weld nut is misoriented and less than the weld current I flowing when the weld nut is properly oriented. Similarly, a predetermined voltage V MAX  is chosen such that it is less than weld voltage V measured when a weld nut is misoriented, yet greater than weld voltage V measured when the weld nut is properly oriented.  
         [0027]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.