FIELD OF THE INVENTION 
       [0001]    The present invention relates to a stud and a method of fabricating the same, and more particularly, to a stud and a method of fabricating the same, in which a female screw portion of the stud is formed in a cup shape by means of a reverse drawing process, thereby preventing damage of the female screw portion due to vibration and torque and solving a problem of quality degradation of various electronic products due to generation of burrs. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventionally, a stud is utilized to connect various electronic products to a PCB, and a mechanical machined stud has been chiefly employed for the stud. However, nowadays, various studs such as a forged stud, a stud fabricated by means of a collar drawing process, a stud fabricated by means of a tube drawing process have been used. 
         [0003]    However, the mechanical machined stud has a disadvantage that productivity is degraded remarkably, and the forged stud has lots of limitations depending on a shape and a structure of the stud and has a problem of precision degree caused by a change of the size due to hot formation at the time of the forging. 
         [0004]    In this regard, there has been proposed a prior Korean Patent No. 655954 previously issued to the present inventor, in which a stud is fabricated by a collar drawing process to provide excellent discrimination against the prior art owing to merits such as excellent productivity, reduced weight and the like. 
         [0005]    However, as shown in  FIG. 1 , in case of the stud  1  fabricated by the collar drawing process, which was developed by the present inventor, if a bolt  3  is fastened to a female screw portion  2  which is tap-mechanical machined at the time of mass production, burrs are generated and drops downwardly to cause fatal damages to a circuit of the electronic product, and there is a limitation in absorbing rotation torque and vibration at the time of engaging the screw due to structural fragileness of a collar portion. 
         [0006]    As a result, the fastened portions are damaged due to strong friction force and instant action and reaction between the thread of the female screw portion  2  and the thread of the bolt  3  at the time of the screw engagement, so that they are fastened incompletely, thereby causing the quality degradation of the electronic products, and acting as fatal defect factors to the reliability test. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, the present invention has been devised to solve several problems originated from the conventional studs fabricated by various processes as described above, and it is an object of the present invention to provide a stud and a method of fabricating the same, in which a female screw portion of the stud fabricated by a collar drawing process can be formed in a cup shape by means of a reverse drawing process, thereby stabilizing the female screw portion structurally, ensuring prevention of damages of the female screw portion due to vibration and rotation torque through formation of the strong female screw portion, and preventing fatal defects of the electronic products caused by the burrs. 
         [0008]    To achieve the above object, the present invention provides a method of fabricating a stud, comprising the steps of: performing a deep drawing on a metal plate at a round shaped blank into a cup-shaped blank to thereby form a flange portion; performing a plurality of drawing processes again for reducing a diameter of the cup-shaped blank to thereby complete a body portion; performing a reverse drawing to form a female screw portion at the body portion; and forming a press-fit fastening groove at a contact surface of the body portion and the flange portion by means of a slitting process, and performing a trimming process to fabricate the stud. 
         [0009]    In addition, the present invention may further comprise performing a plurality of embossing processes to form a protrusion portion acting as a guide when assembled with corresponding mating parts after forming the female screw portion and the flange portion or performing a piercing process on a lower end portion of the female screw portion to fabricate the stud formed with a piercing portion. 
         [0010]    Also, according to the present invention, there is provided a stud fabricated according to any one of the above methods. 
         [0011]    Therefore, as described above, the stud and the method of fabricating the same of the present invention has advantageous and remarkable effects in that it is possible to form a female screw portion of the stud fabricated by a collar drawing process in a cup shape by means of a reverse drawing process, thereby stabilizing the female screw portion structurally, ensuring prevention of damages of the female screw portion due to vibration and rotation torque through formation of the strong female screw portion, and preventing fatal defects of the electronic products caused by the burrs at the time of fastening the bolt to the female screw portion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a cross-sectional view showing a use state of a stud fabricated by a conventional method of collar drawing process; 
           [0013]      FIG. 2  is a cross-sectional view showing a use state of a stud fabricated according to a method of fabricating a stud of the present invention; 
           [0014]      FIG. 3  is a view showing a process of fabricating a standard stud made of metal plate according to the present invention; 
           [0015]      FIG. 4  is a view showing a process of fabricating an embossed standard stud according to the present invention; 
           [0016]      FIG. 5  is a view showing a result of CAE analysis of a first deep drawing formation step for fabricating a metal plate stud according to the present invention; 
           [0017]      FIG. 6  is a view showing a result of CAE analysis of a second deep drawing formation step for fabricating a metal plate stud according to the present invention; 
           [0018]      FIG. 7  is a view showing a result of CAE analysis of a third deep drawing formation step for fabricating a metal plate stud according to the present invention; 
           [0019]      FIG. 8  is a view showing a result of CAE analysis of a fourth deep drawing formation step for fabricating a metal plate stud according to the present invention; 
           [0020]      FIG. 9  is a view showing a result of CAE analysis of a fifth deep drawing formation step for fabricating a metal plate stud according to the present invention; 
           [0021]      FIG. 10  is a view showing a result of CAE analysis of a sixth deep drawing formation step for fabricating a metal plate stud according to the present invention; 
           [0022]      FIG. 11  is a view showing a result of CAE analysis of a seventh deep drawing formation step for fabricating a metal plate stud according to the present invention; 
           [0023]      FIG. 12  is a view showing a result of CAE analysis of an eighth deep drawing formation step for fabricating a metal plate stud according to the present invention; 
           [0024]      FIG. 13  is a view showing a result of formation analysis of an assembly state of a metal plate stud according to the present invention; 
           [0025]      FIG. 14  is a view showing a result of formation analysis of an assembly state of a metal plate stud according to the present invention; 
           [0026]      FIG. 15  is a view showing a result of sectional analysis of an assembly state before the press-fit of the metal plate stud according to the present invention; 
           [0027]      FIG. 16  is a view showing a result of sectional analysis of an assembly state after the press-fit of the metal plate stud according to the present invention; 
           [0028]      FIG. 17  is a view showing a result of stress analysis of a metal plate stud according to the present invention; 
           [0029]      FIG. 18  is a view showing a result of planar analysis of an assembly state before the press-fit of the metal plate stud according to the present invention; 
           [0030]      FIG. 19  is a view showing a result of planar analysis of an assembly state after the press-fit of the metal plate stud according to the present invention; 
           [0031]      FIG. 20  is a view showing a result of stress distribution state analysis after the completion of the press-fit of the metal plate stud according to the present invention; 
           [0032]      FIG. 21  is a view showing a result of analysis of tensile test of a conventional mechanical machined stud; 
           [0033]      FIG. 22  is a view showing a result of analysis of tensile test (result of load analysis) of a conventional mechanical machined stud; 
           [0034]      FIG. 23  is a view showing a result of test analysis of stress deformation behavior produced from a stud, a joint base metal, and a bolt, by fastening the bolt into the metal plate stud of the present invention and applying tensile load thereto; 
           [0035]      FIG. 24  is a view showing a result of tensile test analysis (result of load analysis) of a metal plate stud of the present invention; 
           [0036]      FIG. 25  is an actual article photograph of a material test machine for testing a test-piece to fabricate a metal plate stud according to the present invention; 
           [0037]      FIG. 26  is an actual article photograph showing a test-piece before test to fabricate a metal plate stud according to the present invention; 
           [0038]      FIG. 27  is an actual article photograph showing a test-piece after test to fabricate a metal plate stud according to the present invention; 
           [0039]      FIG. 28  is a graph showing a result of tensile test of a metal plate stud and a mechanical machined stud according to the present invention; 
           [0040]      FIG. 29  is a graph showing a result of compression test of a metal plate stud and a mechanical machined stud according to the present invention; 
           [0041]      FIG. 30  is a graph showing a result of side force test of a metal plate stud and a mechanical machined stud according to the present invention; 
           [0042]      FIG. 31  is a table showing a result of test of test-pieces of a metal plate stud according to the present invention; 
           [0043]      FIG. 32  is a view showing a process of fabricating a metal plate pierced stud according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]    Hereinafter, the preferred embodiment of the present invention will be described in detail with reference to the appended drawings. 
         [0045]    In the present invention,  FIG. 2  is a cross-sectional view showing a construction of a stud  1  fabricated according to the present invention, and  FIGS. 3 and 4  are process views of a standard stud  1  and a protruded standard stud  1  among studs  1  according to the present invention. 
         [0046]    The present invention will be described at first with reference to these drawings, in which identical elements are denoted by identical numerals. 
         [0047]    The stud  1  of the present invention is characterized by forming as a cup shape by means of a reverse drawing process to form a female screw portion  2  at the stud  1 , which is fabricated by means of a collar drawing process disclosed in Korean patent No. 655954 issued to the present inventor. 
         [0048]    In other words, a body portion  4  of the stud  1  is formed sequentially by means of the drawing process, and a cup-shaped body portion  5  of the female screw portion  2  side is formed by means of the reverse drawing process, and then a tap-machining is carried out on an inner circumferential portion of the body portion  5  to form a female screw portion  2 . This process will be described in detail with reference to a process diagram. 
         [0049]      FIG. 3  is a fabricating process view of a standard stud among the metal plate studs according to the present invention, in which a primary drawing process is carried out on a round blank of a metal plate state into a cup shape to form a flange portion, and a plurality of drawing processes is performed again to reduce a diameter of the cup-shaped portion, thereby completing a body portion thereof. 
         [0050]    In this instance, a drawing ratio is determined depending on a diameter of the thickness of material, and a diameter of a material, and a process design is carried out according to the determined drawing ratio. 
         [0051]    The cup shaped formation product with completed body portion has a flat plane whose flange portion is vertical with respect to a cylindrical centerline. Thereafter, a process progresses to accomplish the female screw portion of the stud by the formation of a bottom portion thereof. 
         [0052]    The female screw portion is formed by means of the reverse drawing process. 
         [0053]    The drawing ratio at the time of the reverse drawing is determined based on the material, and the thickness of the material. 
         [0054]    The intermediate formation body including completed female screw portion is shaped to form a slitting groove for the press-fit of the flange portion, so that it can be formed as a recessed form with respect to the thickness of the material from the basic surface of the flange. 
         [0055]    Then, a press-fit fastening groove is formed by means of a slitting process, and a trimming process is carried out, and then a final formation body can be accomplished. 
         [0056]      FIG. 4  is a view showing a process of fabricating an embossed standard stud among the studs of the present invention, in which the embossed standard stud is accomplished according to a formation process similar to the standard stud forming process. 
         [0057]    In this regard, the stud is completed by a process to which a plurality of emboss forming processes is added, so as to form protrusions acting as a guide, when the female screw portion and the flange portion are formed and assembled with corresponding mating parts. 
         [0058]      FIGS. 5 through 11  are views showing results of CAE analysis in a deep drawing process of a stud according to the present invention, in which the first formation step ( FIG. 5 ) is a view showing an analysis result for preparing a process of carrying out a primary drawing in the blank. 
         [0059]    As shown in the drawing, a punch is positioned at the upper portion and a die is positioned at the lower portion. In this instance, a size of the punch, a size of the die, a diameter of the punch, and a diameter of the die are identical with those of the actual process design. Then, a shape and a size of the blank to be formed are positioned between the punch and the die. 
         [0060]    Then, the second formation step ( FIG. 6 ) shows an analysis procedure of the primary drawing step. In this procedure, the cup shaped formation body is obtained when the metal plate is moved following the shape of the punch radius and the die shape together with the descending of the punch. According to the result of the formation analysis, it is confirmed that the formation is possible without any badness of the material such as puncture, burst, wrinkles, and the like. 
         [0061]    The third formation step ( FIG. 7 ) is an analysis procedure for completing a cup shaped drawn intermediate formation body. The whole shape shows that formation is carried out without any difficulty. While the thickness of the material at the radius portion of the punch is thinnest and the thickness of the material at the distal end of the opening is thickest, it is shown that the process can be properly completed by the control of the formation speed and the stroke. 
         [0062]    The fourth formation step ( FIG. 8 ) is an analysis procedure of a process of forming the cup shaped formation body by reducing the diameter of the cup shaped formation body. 
         [0063]    What is important in this step is to observe whether side wrinkles are produced or not. As a result of the analysis, it is shown that the procedure can be carried out without any difficulty. 
         [0064]    The fifth formation step ( FIG. 9 ) shows an analysis procedure of a drawing process of the diameter-reduced cup shaped formation body. In this step, an intermediate formation product is completed according to the shape of the punch, and there was not found any problem in the process. 
         [0065]    The sixth formation step ( FIG. 10 ) shows an analysis procedure of a drawing process of reducing the diameter of the cup shaped intermediate formation product again. It was confirmed that formation is smoothly carried out without producing any side wrinkles at the cup shaped intermediate formation body. 
         [0066]    The seventh formation step ( FIG. 11 ) shows an analysis procedure of a reverse drawing process of a bottom portion. The reverse drawing process is to push a plate member of the bottom portion from the inside of the die together with the ascending of the punch. In this procedure, clearances between the diameter of the punch and the diameter of the die, and between the punch and the die are principal process factors, and the drawing ratio is determined based on the material, and the thickness of the material. As a result of the analysis of the procedure, it was confirmed that the stud could be completed without any difficulty. 
         [0067]    The eighth formation step ( FIG. 12 ) shows an analysis result of a reverse drawing process, which is further progressed. It is confirmed that a plastic working can be carried out smoothly according to the shape of the punch and the shape of the die based on the formation material of the bottom portion. 
         [0068]    Meanwhile,  FIG. 13  shows an analysis result of forming the press-fit portion of the lower flange portion of the stud fabricated according to the present invention, in which the press-fit of the slitting groove with the joint base metal is completed. It is shown that stress is concentrated on a portion adjoining the completion portion of the press-fit of the hexagonal flange portion, and the slitting groove with the joint base metal. 
         [0069]      FIG. 14  shows a formation analysis for observing the shape change of the press-fit portion, after the press-fit of the stud fabricated according to the present invention and the joint base metal. 
         [0070]    As a result of the analysis, it is confirmed that there was no problem in the entire joint completeness. In other words, there is not expected any of deformation, twist, damage, and burst phenomenon, and it is confirmed that stress was concentrated on the press-fit portion. 
         [0071]      FIG. 15  is a view showing a result of sectional analysis of an assembly state before the press-fit of the metal plate stud according to the present invention. 
         [0072]    The principle of the press-fit is to insert the metal plate stud into a hole of the base metal to be joined, and pressurize it by means of the action of the punch and the die to join them by the deformation of the slitting groove and the base metal. 
         [0073]      FIG. 16  is a view showing an analysis obtained after the completion of the press-fit of the metal plate stud according to the present invention by means of the application of the load of the punch and the die. 
         [0074]    As a result of the analysis, it can be seen that the body portion of the stud is maintained as it is. It is also confirmed that plastic deformation is produced at the slitting groove and the distal end of the flange, and the thick portion of the flange, stress is produced at the joint base metal surrounding the press-fit portion. 
         [0075]      FIG. 17  is a view showing a result of stress analysis of a metal plate stud according to the present invention after the completion of the press-fit process of the stud. It is shown that stress is concentrated on the flange portion, and it is confirmed that the thickness of the flange portion was deformed thin due to the compression load. 
         [0076]      FIG. 18  is a view showing a result of planar analysis of an assembly state before the press-fit of the stud. It is observed clearly that a rectangular groove was formed at the slitting portion. 
         [0077]      FIG. 19  is a view showing a result of planar analysis of an assembly state after the completion of the press-fit of the stud. It is shown that the rectangular groove is plastic deformed so that it is press-fittingly coined into the joint base metal. The linear portion of the distal end of the flange is plastic deformed by the compression load so that the linear portion is changed into an irregular curve portion. 
         [0078]      FIG. 20  shows an analysis result of the stress distribution state after the press-fit of the stud. Where the maximum stress is produced and distributed with respect to the compression load, is the flange portion. In addition, the stress is chiefly concentrated on the circumference of the flange portion of the joint base metal. 
         [0079]      FIG. 21  is a view showing a result of test analysis of a stress deformation behavior produced from a stud, a joint base metal, and a bolt, by fastening the bolt to the conventional mechanical machined stud and applying the tensile load thereto. According to the result, the maximum load was about 2,400N, and the time consumed for the start of the isolation was about 0.5 seconds. 
         [0080]      FIG. 22  is a view showing a test analysis result of stress deformation behavior produced from a stud, a joint base metal, and a bolt, by fastening the bolt to the metal plate stud of the present invention, and applying the tensile load thereto. According to the result, the maximum load was about 2,300N, and the time consumed for the start of the isolation was about 1 seconds. 
         [0081]    Comparing both results, the maximum load of the metal plate stud was identical with that of the conventional mechanical machined stud shown in  FIG. 21 , and the time consumed for the start of the isolation was twice thereby producing a stable result. 
         [0082]      FIG. 23  is a view showing a result of test analysis of stress deformation behavior of a metal plate stud of the present invention. 
         [0083]      FIG. 24  is a view showing an analysis result (result of load analysis) of tensile test of a metal plate stud of the present invention. 
         [0084]    Furthermore,  FIG. 25  is a photograph showing an actual article of a material test system (MTS) for testing a test-piece of the stud fabricated according to the present invention, in which the material test system (MTS) is called as an ‘MTS 858 TEST FRAME’ (manufacturing company: MTS SYSTEM Corp., Manufacturing country: USA), and the force capacity is 25 kN, the maximum pressure is 70 bar/1,000 psi, and the temperature range falls in a range of −18° C.(0° F.) ˜65° C.(150° F.). 
         [0085]      FIGS. 26 and 27  show photographs of actual articles of the test product obtained before and after the test. Total numbers of 25 test-pieces have been used to carry out the test. The joint base metal was GALVALUME (AZ120 organic coating) by 0.8 t, and the test-pieces were fabricated in the press formation apparatus constructed of punches and dies. The test-piece shown in  FIG. 27  shows a portion of the test-piece completed of the test. The test-piece was completed of the tensile, compression, and side force tests. 
         [0086]      FIGS. 28 ,  29  and  30  are graphs showing comparisons between the results of tensile test, compression test, and side force test of the test-pieces of the metal plate stud of the present invention and the conventional mechanical machined stud. 
         [0087]    It was shown that the maximum load of the metal plate stud was 4.5% higher than that of the mechanical machined stud because the maximum load of the metal plate stud was 183 kgf and the maximum load of the mechanical machined stud was 175 kgf. The isolation distance of the stud from the joint base metal was 1.22 mm for the mechanical machined stud and 5.29 mm for the metal plate stud of the present invention. As a result, it was confirmed that the metal plate stud has a higher value by 433% than that of the mechanical machined stud. 
         [0088]    In addition, as a result of the tensile test, the total work energy obtained from the comparison of the data shown in the graph was 720.63 kgf mm for the metal plate stud, and 153.12 kgf mm for the mechanical machined stud, so that the metal plate stud had a higher value by 370% than the mechanical machined stud. 
         [0089]    Moreover, as shown in the graph of the compression test, the maximum load of the mechanical machined stud was 163 kgf, and the maximum load of the metal plate stud was 175 kgf, so that the maximum load of the metal plate stud was higher than that of the mechanical machined stud by 7.3%. 
         [0090]    The distance for the stud to be separated from the joint base metal was 2.7 mm for the mechanical machined stud and 3.8 mm for the metal plate stud, so it was confirmed that the metal plate stud has a higher value than the mechanical machined stud by 140%. 
         [0091]    As a result of the compression test, the total work energy obtained from the comparison of the data shown in the graph was 507.24 kgf mm for the metal plate stud, and 368.30 kgf mm for the mechanical machined stud, so that the metal plate stud has a higher value by 37% than the mechanical machined stud. 
         [0092]    Also, as can be seen from the graph of the side force test, the maximum load of the mechanical machined stud was 40 kgf, and the maximum load of the metal plate stud was 42 kgf, so that the maximum load of the metal plate stud was higher than that of the mechanical machined stud by 5%. Also, the distance for the stud to be separated from the joint base metal was 3.8 mm for the mechanical machined stud and 5.0 mm for the metal plate stud, so it was confirmed that the metal plate stud has a higher value than the mechanical machined stud by 31%. 
         [0093]    Moreover, as a result of the side force test, the total work energy obtained from the comparison of the data shown in the graph was 170.39 kgf mm for the metal plate stud, and 152.84 kgf mm for the mechanical machined stud, so that the metal plate stud has a higher value by 11% than the mechanical machined stud. 
         [0094]    As described above, according to the metal plate stud fabricated by the present invention, as is apparent from the table representing the test results shown in  FIG. 30 , when we compare the metal plate stud with the mechanical machined stud, a weight of the product was reduced by 80% in comparison with the mechanical machined stud, resulted in a good material cost reduction effect, the tensile strength was increased by 4.5% in comparison with the mechanical machined stud, resulted in the stability, the compression strength was increased by 7.7% in comparison with the mechanical machined stud, resulted in the stability, the shear strength was increased by 4.1% in comparison with the mechanical machined stud, resulted in the more stability, and the torque strength was identical with that of the mechanical machined stud. 
         [0095]    Meanwhile,  FIG. 31  shows another embodiment of the present invention, in which a fabrication process diagram of the metal plate pierced stud is shown. The pierced stud  1  is used in the manufacturing of PDP, LCD, and the like, and constructed by forming a piercing portion  6 , which penetrates through a female screw portion. In case of PDP, and LCD, it is possible to obtain the identical result because a glass plate is disposed at the piercing portion  6  at the following step. 
       INDUSTRIAL APPLICABILITY 
       [0096]    As described above, the stud and the method of fabricating the same according to the present invention has advantageous and remarkable effects in that it is possible form a female screw portion of the stud fabricated by a collar drawing process in a cup shape by means of a reverse drawing process, thereby stabilizing the female screw portion structurally, ensuring prevention of damages of the female screw portion due to vibration and rotation torque through formation of the strong female screw portion, and preventing fatal defects of the electronic products caused by the burrs at the time of fastening the bolt to the female screw portion.
The present invention relates to a stud and a method of fabricating the same, in which a female screw portion of the stud is formed in a cup shape by means of a reverse drawing process, thereby preventing damage of the female screw portion due to vibration and torque and solving a problem of quality degradation of various electronic products due to generation of burrs. The present invention comprises the steps of performing a deep drawing on a metal plate at a round shaped blank into a cup-shaped blank to thereby form a flange portion; performing a plurality of drawing processes again for reducing a diameter of the cup-shaped blank to thereby complete a body portion; performing a reverse drawing to form a female screw portion at the body portion; forming a press-fit fastening groove at a contact surface of the body portion and the flange portion by means of a slitting process, and performing a trimming process to fabricate the stud.