Abstract:
A fabrication method of a metal shell to be installed on a park plug is provided which is made up of a small-diameter portion, a large-diameter portion, and a wrapping portion. The wrapping portion is to be wrapped by staking about the spark plug to achieve installation of the metal shell on the spark plug. The method comprises pressing a workpiece with a punch to shape the wrapping portion of the metal shell in a first cold forging process and processing the workpiece to shape the small-diameter portion of the metal shell in a second cold forging process different from the first cold forging process. This produces the metal shell which is less susceptible to cracks when installed on the spark plug and has an increased service life.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field of the Invention  
           [0002]    The present invention relates generally to an improved fabrication method of a metal shell installed on a spark plug which may be employed in automotive internal combustion engines.  
           [0003]    2. Background Art  
           [0004]    Typical plug metal shells are installed on spark plugs by staking an annular wrapping end of the metal shell on a porcelain insulator of the spark plug. The wrapping end of the metal shell is usually made by cold forging. The metal shell also has a hollow cylindrical base portion and a hexagonal boss which are also shaped by the cold forging. The hollow cylindrical base portion has threads formed in an exterior surface thereof by rolling.  
           [0005]    [0005]FIG. 6 illustrates a conventional forging process for fabricating a metal shell of a spark plug, as disclosed in Japanese Patent First Publication No. 7-16693, which forms a wrapping end  11  and a small-diameter base portion  12  of the metal shell in a single process. The formation of the wrapping end  11  is accomplished by striking a large-diameter head portion  13  of a hollow cylindrical workpiece with a cylindrical punch  50  to decrease the diameter of the head portion  13 . An outer wall of the small-diameter base portion  12  is shaped by a die  52 .  
           [0006]    The simultaneous formation of the wrapping end  11  and the small-diameter base portion requires a punch holder  51 . It is impossible for the punch holder  51  to have an outer diameter greater than that of the large-diameter head portion of the workpiece. The punch holder  51  must, therefore, be formed to be thin, so that it has a low strength. Forging the workpiece requires exertion of a large pressure on the punch holder  51 , which will lead to a problem that cracks or physical deformation of the punch holder  51  arise within a short period of time.  
           [0007]    Further, it is difficult to form a large rounded inner wall in an end of the punch holder  51  because it is thin, which results in a drop in fluidity of material of the workpiece around the end of the punch holder  51 . This causes a boundary between inner walls of the wrapping end  11  and the large-diameter head portion  13  to be subjected to shrinkage, which may result in formation of cracks near the boundary between the wrapping end  11  and the large-diameter head portion  13 .  
         SUMMARY OF THE INVENTION  
         [0008]    It is therefore a principal object of the invention to avoid the disadvantages of the prior art.  
           [0009]    It is another object of the invention to provide a fabrication method for fabricating a metal shell which is less susceptible to cracks when installed on a spark plug and has an increased service life.  
           [0010]    According to one aspect of the invention, there is provided an improved fabrication method of a metal shell to be installed on a park plug which may be employed in automotive engines. The metal shell has a given length and is made up of a small-diameter portion, a large-diameter portion, and a wrapping portion. The wrapping portion is to be wrapped by staking about a porcelain insulator of a spark plug to achieve installation of the metal shell on the spark plug. The method comprises the steps of: (a) preparing a cylindrical workpiece which has a given length with a first and a second end opposed to each other; (b) preparing a punch and a die; (c) placing the workpiece in the die and pressing the workpiece with the punch from the second end of the workpiece to shape the wrapping portion of the metal shell on a side of the first end of the workpiece in a first cold forging process; and (d) processing the workpiece to shape the small-diameter portion of the metal shell on a side of the second end of the workpiece in a second cold forging process.  
           [0011]    In the preferred mode of the invention, the method further comprises the step of forming threads on an outer peripheral wall of the small-diameter portion for installation of the spark plug.  
           [0012]    The method further comprises the step of processing the workpiece to form a large-diameter portion on the side of the second end and a small-diameter portion on the side of the first end prior to the first cold forging process in which the wrapping portion is formed.  
           [0013]    In the first cold forging process, a portion of the workpiece on the side of the first end is pressed within the die stepwise to decrease, in sequence, the portion of the workpiece in outer diameter to shape the wrapping portion of the metal shell.  
           [0014]    A hexagonal boss may be formed on the large-diameter portion of the workpiece in the first cold forging process.  
           [0015]    The hexagonal boss may alternatively be formed on the large-diameter portion of the workpiece in a third process different from the first and second cold forging process. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only.  
         [0017]    In the drawings:  
         [0018]    [0018]FIG. 1 is a partially sectional view which shows a metal shell fabricated by cold forging according to the invention;  
         [0019]    [0019]FIG. 2 is a partially longitudinal view which shows a spark plug equipped with the metal shell of FIG. 1;  
         [0020]    FIGS.  3 ( a ),  3 ( b ),  3 ( c ),  3 ( d ),  3 ( e ), and  3 ( f ) illustrate a sequence of cold forging process for making the metal shell of FIG. 1 according to the first embodiment of the invention;  
         [0021]    [0021]FIG. 4 is a partially sectional view which shows a cold forging machine used in the third process in FIG. 3( c );  
         [0022]    FIGS.  5 ( a ),  5 ( b ),  5 ( c ),  5 ( d ),  5 ( e ), and  5 ( f ) illustrate a sequence of cold forging process for making the metal shell of FIG. 1 according to the second embodiment of the invention; and  
         [0023]    [0023]FIG. 6 is a partially sectional view which shows a conventional forging process for making a spark plug shell. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to FIG. 1 there is shown a metal shell  10  to be installed on a spark plug  1  for use in, for example, automotive internal combustion engines which is made by a method of the first embodiment of the invention.  
         [0025]    The metal shell  10  is form by a hollow cylindrical member made of a conductive metal such as a low carbon steel. The metal shell  10  consists essentially of a wrapping end  11 , a small-diameter base portion  12 , and a large-diameter head portion  13  formed between the wrapping end  11  and the small-diameter base portion  12 . The small-diameter base portion  12  has formed on an exterior surface thereof threads  14  which mesh with a threaded hole formed in a cylinder head of the engine (not shown). The large-diameter head portion  13  has formed on an outer wall thereof a generally hexagonal boss  15  used for grasping and turning thereof using a suitable tool such as a conventional spark plug socket.  
         [0026]    [0026]FIG. 2 shows a spark plug  1  on which the metal shell  10  of FIG. 1 is installed. The spark plug  1  includes a hollow cylindrical porcelain insulator  2  made of an alumina ceramic (Al 2 O 3 ). The porcelain insulator  2  is partially retained within the metal shell  10  and has opposed ends exposed out of the metal shell  10 . The retaining of the porcelain insulator  2  in the metal shell  10  is accomplished by inserting the porcelain insulator  2  into the metal shell  10  and elastically bending or staking the wrapping end  11  inward.  
         [0027]    The spark plug  1  also includes a cylindrical center electrode  3 , a stem  4 , and a ground electrode  5 . The center electrode  4  and the stem  4  are disposed within a longitudinal chamber  2   a  of the porcelain insulator  2 . The center electrode  4  has a tip  3   a  exposed outside the porcelain insulator  2  and a rear end thereof joined electrically to the stem  4 . The ground electrode  5  is welded to an end of the metal shell  10 . The ground electrode  5  is bent to an L-shape to define an air gap  6  (also called a spark gap) between a tip thereof and the tip  3   a  of the center electrode  3 .  
         [0028]    A cold forging fabrication method of the metal shell  10  will be described below with reference to FIGS.  3 ( a ) to  4 .  
       First Process  
       [0029]    First, a metal cylinder which is made of, for example, a low carbon steel and cut to a given length is placed in a first station (i.e., a die cavity) of a cold forging machine (not shown) and swaged to form a first forged cylindrical workpiece  110 , as shown in FIG. 3( a ), with a sloping shoulder. The forged cylindrical workpiece  110  is made up of a head portion  111  and a base portion  112  smaller in diameter than the head portion  111 . The forged cylindrical workpiece  110  also has a large-diameter bore  113  and a small-diameter bore  114  formed on opposed ends thereof.  
       Second Process  
       [0030]    The forged cylindrical workpiece  110  is placed in a second station (not shown) of the cold forging machine and subjected to extrusion molding to form a second forged workpiece  120 , as shown in FIG. 3( b ). The second forged workpiece  120  has a substantially horizontal shoulder to define a large cylindrical head preform  121  and a small cylindrical base preform  122 . The large cylindrical head preform  121  has formed in an end thereof a bore  123  deeper than the bore  113  of the first forged cylindrical workpiece  110 . Similarly, the small cylindrical base preform  122  has formed in an end thereof a bore  124  which is deeper than the bore  114  of the first forged cylindrical workpiece  110  and smaller in diameter than the bore  123 .  
       Third Process  
       [0031]    The second forged workpiece  120  is placed in a third station (not shown) of the cold forging machine and subjected to extrusion molding to form a third forged workpiece  130 , as shown in FIG. 3( c ). In the third process, only the large cylindrical head preform  121  is extrusion molded. Specifically, the outer wall of the large cylindrical head preform  121  is machined to form three parts: a tapered wall  131   a , a cylindrical wall  131   b , and an annular projecting wall  131   c . The tapered wall  131   a  forms the wrapping end  11  of the metal shell  10  and is smallest in outer diameter of the three. The annular projecting wall  131   c  is greatest in outer diameter of the three.  
         [0032]    [0032]FIG. 4 shows an internal structure of the third station of the cold forging machine at which the third forged workpiece  130  is made in the third process, as described above. A left half of the drawing illustrates the second forged workpiece  120  before machined in the third process. A right half illustrates the third forged workpiece  130  after machined in the third process.  
         [0033]    Employed in the third process is an extrusion molding machine  20  which includes an upper die  22  and a lower die  23  disposed in a die holder  21 . The upper die  22  has formed therein a cylindrical bore  22   a  which is substantially equivalent in diameter and shape to the large cylindrical head preform  121 . The lower die  23  has three cylindrical bores  23   a ,  23   b , and  23   c  formed coaxially with the cylindrical bore  22   a  of the upper die  22 . The first bore  23   a  leads directly to the bore  22   a  of the upper die  22  and has the same diameter (e.g., φ 19 ) as that of the bore  22   a . The second bore  23   b  formed beneath the first bore  23   a  has an inner diameter (e.g., φ 118 ) that is smaller than that of the first bore  23   a . The third bore  23   c  formed beneath the second bore  23   b  has an inner diameter (e.g., φ 16 ) that is smaller than that of the second bore  23   b.    
         [0034]    Formed between the first and second bores  23   a  and  23   b  is a rounded wall having a radius R of, for example, 1 mm. Similarly, formed between the second and third bores  23   b  and  23   c  is a rounded wall having a radius R of, for example, 2 to 2.5 mm. Each of the upper and lower dies  22  and  23  is made of, for example, cemented carbide. The bore  22   a  of the upper die  22  and the first to third bores  23   a  to  23   c  of the lower die  23  are coated with, for example, titanium nitride using CVD coating techniques.  
         [0035]    The extrusion molding machine  20  also includes a punch  24 , a sleeve  25 , and mandrel  26 . The punch  24  has an outer diameter substantially identical with the inner diameter of the bore  124  of the second forged workpiece  120  and is held to be slidable in a vertical direction, as viewed in the drawing, to press the second forged workpiece  120  in direct contact with the bottom of the bore  124  in a longitudinal direction (i.e., a downward direction as viewed in the drawing).  
         [0036]    The sleeve  25  is made of a hollow cylindrical member and encompasses the punch  24 . The sleeve  25  has an outer diameter substantially identical with the inner diameter of the bore  22   a  of the upper die  22  and an inner diameter substantially identical with the outer diameter of the base preform  122  of the second forged workpiece  120 . The sleeve  25  is held to be slidable vertically, as viewed in the drawing, together with the punch  25  and configured so that the tip of the sleeve  25  is located at a given interval away from the shoulder formed between the head preform  121  and the base preform  122  of the second forged workpiece  120  when the punch  24  is at the tip thereof in direct contact with the bottom of the bore  124 . Specifically, a gap  30  is formed between the tip of the sleeve  25  and the shoulder of the second forged workpiece  120  when the punch  24  abuts to the bottom of the bore  124 .  
         [0037]    During the third process, the second forged workpiece  120  is held by the mandrel  26  within the upper and lower dies  22  and  23 . After completion of the third process, it is removed from the dies  22  and  23  through a kickout sleeve  27 . The mandrel  26  is urged upward, as viewed in the drawing, by a coil spring  28  against the downward pressure of the punch  24 . Similarly, the upper and lower dies  22  and  23  are urged upward by springs  29 .  
         [0038]    In operation of the extrusion molding machine  20 , the second forged workpiece  120  is first retained by the mandrel  26  within the upper and lower dies  22  and  23 . The punch  24  is pressed downward to slide the second forged workpiece  120  within the upper and lower dies  22  and  23 . This causes the tip of the head preform  121  of the second forged workpiece  120  to abut on the rounded wall between the first and second bores  23   a  and  23   b  of the lower die  23 . A further downward movement of the punch  24  causes the second forged workpiece  120  to be deformed plastically, so that the outer wall of a tip portion of the head preform  121  is shaped by the second bore  23   b  to have a decreased outer diameter substantially identical with the inner diameter of the second bore  23   b.    
         [0039]    A further downward movement of the punch  24  causes the tip of the head preform  121  of the second forged workpiece  120  to abut on the rounded wall between the second and third bores  23   b  and  23   c  of the lower die  23  and be deformed along the inner wall of the third bore  23   c , so that the outer wall of the tip of the head preform  121  is shaped to have a decreased outer diameter substantially identical with the inner diameter of the third bore  23   c.    
         [0040]    In the manner, as described above, the cylindrical wall  131   b  of the third forged workpiece  131  is finished by the second bore  23   b , and the tapered wall  131   a  (i.e., the wrapping end  11 ) is completed by the third bore  23   c.    
         [0041]    If the resistance of the material of the second forged workpiece  120  to deformation thereof when the head preform  121  is decreased in diameter is great, it becomes impossible for the material of the second forged workpiece  120  to have the fluidity required for desired deformation of the head preform  121 . The structure of the extrusion molding machine  20  is, however, so designed as to allow the upper and lower dies  22  and  23  to move against the springs  29  for allowing the material of the second forged workpiece  120  to flow when the deformation resistance of the second forged workpiece  120  exceeds a preselected critical value, thereby avoiding the shrinkage.  
         [0042]    The tapered wall  131   a  of the third forged workpiece  130  which forms the wrapping end  11  is formed by decreasing the diameter of the tip portion of the head preform  121  of the second forged workpiece  120  a plurality of times (two times in this embodiment) by the second and third bores  23   b  and  23   c , thus enabling the tapered wall  131   a  to be formed with a relative small resistance to deformation thereof.  
         [0043]    The bore  22   a  of the upper die  22  and the first to third bores  23   a  to  23   c  of the lower die  23  are, as described above, coated with, for example, titanium nitride using CVD coating techniques, thus, resulting in a decrease friction between the second forged workpiece  120  and the upper and lower dies  22  and  23 , which leads to a decrease in resistance of the material of the second forged workpiece  120  to deformation thereof.  
       Fourth Process  
       [0044]    The third forged workpiece  130  is placed in a fourth station of the cold forging machine and subjected to extrusion molding to form a fourth forged workpiece  140 , as shown in FIG. 3( d ). The cylindrical wall  131   b  of the third forged workpiece  130  is shaped by to form the hexagonal boss  15 .  
       Fifth Process  
       [0045]    The fourth forged workpiece  140  is placed in a fifth station (not shown) of the cold forging machine and extrusion molded to form a fifth forged workpiece  150 , as shown in FIG. 3( e ). This process employs a punch tool consisting of larger and smaller punches (not shown). The larger punch has an outer diameter substantially equal to the inner diameter of the bore  123  of the fourth forged workpiece  140 . The smaller punch is joined to the tip of the larger punch and has an outer diameter smaller than that of the base preform  122  of the fourth forged workpiece  140 .  
         [0046]    In the fourth process, only the base portion  12  of the fourth forged workpiece  140  is machined by inserting the punch tool into the bore  123  and pressing the bottom of the bore  123  to extend the base preform  122  in the longitudinal direction thereof, thereby forming a desired length of a base portion  152 . The pressing of the punch tool also results in formation a bottom bore  155  in the bottom of the bore  123  which is smaller in diameter than the bore  123 .  
       Sixth Process  
       [0047]    The fifth forged workpiece  150  is placed in a sixth station (not shown) of the cold forging machine and punched to form a sixth forged workpiece  160  which has a bore  166  communicating between the bores  155  and  124  of the fifth forged workpiece  150 . The peripheral surface and corners of the tapered wall  131   a  and peripheral surfaces of ends of the a base portion  152  are finish machined. The threads  14  are cut in the periphery of the base portion  152  by rolling, thereby forming an end product of the metal shell  10 . The ground electrode  5  is, as described above, welded to the metal shell  10 . The porcelain insulator  2  and the center electrode  3  are inserted into the metal shell  10 , after which the tapered wall  131   a  is bent inward to joint the metal shell  10  to the porcelain insulator  2  firmly, thereby making the spark plug  1 .  
         [0048]    As apparent from the above discussion, the fabrication method of the metal shell  10  forms the tapered wall  131   a  (i.e., the wrapping end  11 ) and the base portions  122  and  152  in independent processes, respectively. This allows the peripheral surface of the tapered wall  131   a  to be formed without use of a thin-walled punch as used in a conventional system and also permits the lower die  23  to have an increased thickness, which will result in an increased useful life of the cold forging machine.  
         [0049]    The increased thickness of the lower die  23  also allows the great rounded wall to be formed between the first and second bores  23   a  and  23   b  and between the second and third bores  23   b  and  23   c , thus ensuring desired fluidity of the material of the workpiece  120 , which minimizes the undesirable shrinkage thereof to avoid cracks formed in staking the tapered wall  131   a  to join the metal shell  10  to the porcelain insulator  2 .  
         [0050]    FIGS.  5 ( a ) to  5 ( f ) illustrate a sequence of cold forging processes for making the metal shell  10  according to the second embodiment of the invention. The same reference numbers as employed in the first embodiment will refer to the same parts, and explanation thereof in detail will be omitted here.  
       First Process  
       [0051]    First, a metal cylinder which is made of, for example, a low carbon steel and cut to a given length is placed in a first station (not shown) of a cold forging machine and swaged to form a first forged workpiece  210 , as shown in FIG. 5( a ), which is of cylindrical shape.  
       Second Process  
       [0052]    The first forged workpiece  210  is placed in a second station (not shown) of the cold forging machine and swaged to form a second forged workpiece  220 , as shown in FIG. 5( b ), with a sloping shoulder which is substantially identical in shape with the first forged workpiece  110  in the first embodiment.  
       Thid Process  
       [0053]    The second forged workpiece  220  is placed in a third station (not shown) of the cold forging machine and extrusion molded to form a third forged workpiece  230 , as shown in FIG. 5( c ), which is substantially identical in shape with the second forged workpiece  120  in the first embodiment.  
       Fourth Process  
       [0054]    The third forged workpiece  230  is placed in a fourth station (not shown) of the cold forging machine and subjected to extrusion molding to form a fourth forged workpiece  240 , as shown in FIG. 5( d ). In the fourth process, only the large cylindrical head preform  121  is extrusion molded. Specifically, the outer wall of the large cylindrical head preform  121  is machined to form three parts: a tapered wall  131   a , a hexagonal boss  15 , and an annular projecting wall  131   c . The fourth forged workpiece  240  is substantially identical in shape with the fourth forged workpiece  140  in the first embodiment.  
         [0055]    The fourth process employs the same extrusion molding machine as the one shown in FIG. 4 except that the second bore  23   b  of the lower die  23  is of hexagonal shape for making the hexagonal boss  15 .  
         [0056]    The fifth and sixth processes are identical with those in the first embodiment, and explanation thereof in detail will be omitted here.  
         [0057]    While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims.