Patent Publication Number: US-9893496-B2

Title: Spark plug having improved ground electrode orientation and method of forming

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This U.S. continuation-in-part patent application claims the benefit of U.S. continuation application Ser. No. 14/875,277, filed Oct. 5, 2015, which claims the benefit of U.S. divisional application Ser. No. 14/518,166, filed Oct. 20, 2014, which claims the benefit of U.S. application Ser. No. 13/350,140, filed Jan. 13, 2012, now U.S. Pat. No. 8,866,369, which claims the benefit of U.S. provisional application Ser. No. 61/432,403, filed Jan. 13, 2011, the contents of which are incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to spark plugs for internal combustion engines, and methods of forming the same. 
     2. Related Art 
     Sparks plugs of internal combustion engines typically include a metal shell threaded into a bore of a cylinder head and extending into a combustion chamber for providing a spark to ignite a combustible mixture of fuel and air in the combustion chamber. The spark is provided between a central electrode and ground electrode, which should be properly positioned in the combustion chamber, in order to provide a reliable and robust ignition of the fuel-air mixture. Without the proper positioning, the spark may not provide a robust ignition, or may not provide any ignition of the fuel-air mixture. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention provides a more accurate and repeatable method of threading a shell for a spark plug of an internal combustion engine. 
     According to one embodiment, the method includes providing a shell extending to a shell lower surface and including a shell outer surface, wherein the shell includes a shell seat presenting a ledge facing the shell lower surface; and providing a ground electrode extending longitudinally from an attachment surface. The attachment surface of the ground electrode is attached to the shell lower surface before disposing the shell and the ground electrode in a thread forming apparatus. The method also includes determining the start position of the threads in the shell outer surface relative to the ledge of the shell seat. The step of determining the start position is based on a desired location of the shell in the cylinder head. The method further includes determining a predetermined rotational position of the threads in the shell outer surface. The method then includes placing the shell and the attached ground electrode between a set of threading dies of the thread forming apparatus so that the ledge of the shell seat is at a specified distance relative to a start position of the threads of the threading dies. The method also includes placing the ground electrode at a known rotational position in relation to a start position of the threads to be formed in the shell outer surface by the threading dies. The method then includes rotating the threading dies to form the threads at the predetermined rotational position in the shell outer surface 
     According to a second embodiment, a method of threading at least one shell includes providing a shell extending to a shell lower surface and including a shell outer surface, the shell including a shell seat presenting a ledge facing the shell lower surface; and providing a ground electrode extending longitudinally from an attachment surface. The attachment surface of the ground electrode is attached to the shell lower surface before disposing the shell and the ground electrode in a thread forming apparatus. The method further includes determining a start position of the threads to be formed by threading dies of the thread forming apparatus, wherein the start position is based on a desired location of the shell in a cylinder head in which the shell will be used. The method next includes disposing the shell and the attached ground electrode between the threading dies of the thread forming apparatus, wherein the step of disposing the shell between the threading dies includes engaging the ledge of the shell seat with a surface disposed at a specified distance relative to the start position of the threads. The method also includes determining a predetermined rotational position of the threads in the shell outer surface in relation to the rotational location of the of the ground electrode. The method then includes rotating the threading dies and forming the threads at the predetermined rotational position in the shell outer surface. 
     According to a third example embodiment, a method of threading at least one shell includes providing a shell extending to a shell lower surface and including a shell outer surface, wherein the shell includes a shell seat presenting a ledge facing the shell lower surface; and providing a ground electrode extending longitudinally from an attachment surface. The method next includes determining the longitudinal location of the ledge of the shell seat, which is the distance between the shell lower surface and the ledge. The method further includes placing the shell and the attached ground electrode between a set of threading dies of the thread forming apparatus so that the ledge of the shell seat is at a specified distance relative to a start position of the threads of the threading dies. The step of placing the ledge of the shell seat at the specified distance relative to the start position of the threads includes disposing the shell lower surface on a solid adjustment feature located between the dies, and adjusting the longitudinal position of the solid adjustment feature relative to the start position of the threads of the dies. The method also includes placing the attached ground electrode at a known rotational position in relation to a starting position of the threads of the threading dies. The method next includes rotating the threading dies to form the threads at the predetermined rotational position in the shell outer surface. 
     Another aspect of the invention includes a method of manufacturing at least one spark plug for an internal combustion engine and including the threaded shell manufactured according to the method of the first, second, or third embodiment. Yet another aspect of the invention provides a method of manufacturing an internal combustion engine including a spark plug with the threaded shell manufactured according to the first, second, or third embodiment. Other aspects of the invention provide a threaded shell manufactured according to the method of the first, second, or third embodiment; a spark plug including a threaded shell manufactured according to the method of the first, second, or third example embodiment; and an internal combustion engine including a threaded shell manufactured according to the method of the first, second, or third example embodiment. 
     When the shell is threaded into the cylinder head, the ground electrode of the spark plug is oriented in a desired position in the combustion chamber relative to the cylinder head and other components in the combustion chamber. The position of the ground electrode allows the spark plug to provide a more reliable and efficient ignition of the fuel-air mixture. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: 
         FIG. 1  is a cross sectional view of a spark plug threaded in a cylinder head according to one embodiment of the invention; 
         FIG. 1A  is a side view of a portion of a shell including threads and an attached ground electrode with the threads disposed at a predetermined angle relative to the ground electrode according to one embodiment of the invention; 
         FIG. 2  is a cross-sectional view of a shell and ground electrode according to one embodiment of the invention before forming threads in the shell; 
         FIG. 3  is an illustration of an orientation tool according to one embodiment of the invention; 
         FIG. 4  is a perspective view of an orientation tool according to another embodiment of the invention; 
         FIG. 4A  is a side view of the orientation tool of  FIG. 4 ; 
         FIG. 4B  is a cross sectional view of the orientation tool of  FIG. 4 ; 
         FIG. 5  is a perspective view of the orientation tool of  FIG. 3  disposed in a thread forming apparatus according to one embodiment of the invention; 
         FIG. 6  is a perspective view of the shell and attached ground electrode disposed on the orientation tool of  FIG. 5  before locating the ground electrode and forming the threads; 
         FIG. 7  is a perspective view of the shell and attached ground electrode disposed on the orientation tool of  FIG. 5  after locating the ground electrode and before forming the thread; 
         FIG. 8  is a side view of an example threaded shell and ground electrode formed according to a first, second, or third alternate method; 
         FIG. 9  is a side view of an example threaded spark plug and ground electrode formed according to the first, second, or third alternate method; 
         FIG. 10  is a side view of an example threaded shell and ground electrode disposed adjacent a threading die used in the first alternate method; 
         FIG. 11  is a side view of an example threaded shell and ground electrode disposed adjacent a threading die used in the second alternate method; and 
         FIG. 12  is a side view of an example threaded shell and ground electrode disposed adjacent a threading die used in the third alternate method. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     One aspect of the invention provides a spark plug  20  for providing a spark to ignite a combustible mixture of fuel and air of combustion chamber  22 . The spark plug  20  includes a metal shell  24  with threads  26  attached to a component having mating threads, typically a cylinder head  28  of an internal combustion engine. The shell  24  of the spark plug  20  surrounds an insulator  30  and a central electrode  32 . A ground electrode  34  is attached to a shell lower surface  36 , as shown in  FIG. 1 . The threads  26  are formed in a predetermined location and at a predetermined angle α relative to the ground electrode  34 . By forming the threads  26  of the shell  24  in the predetermined location relative to the ground electrode  34 , the spark plug  20  can be oriented in a desired position relative to the cylinder head  28  and other components in the combustion chamber, such as the fuel injector, allowing the spark plug  20  to provide a more reliable and efficient ignition of the fuel-air mixture. Another aspect of the invention provides a method of forming the spark plug  20  using an orientation tool  38  to locate the ground electrode  34  and align the shell  24  such that the threads  26  are formed in the predetermined location relative to the ground electrode  34 . 
     The central electrode  32  is formed of an electrically conductive material extending longitudinally along an igniter central axis a i  from an electrode terminal end  40  to a central firing end  42 . In one embodiment, the electrically conductive material of the central electrode  32  is a nickel-based material including nickel in an amount of at least 60.0 wt. %, based on the total weight of the nickel-based material. The central electrode  32  can also include a central firing tip  44  formed of a precious metal alloy disposed on the central firing end  42 , as shown in  FIGS. 1 and 8 , to provide the spark. 
     An insulator  30  formed of an electrically insulating material, such as alumina, surrounds the central electrode  32  and extends longitudinally along the igniter central axis a i  from an insulator upper end (not shown) to an insulator nose end  48  such that the central firing end  42  is disposed outwardly of the insulator nose end  48 . The insulator  30  includes an insulator bore  50  extending along the igniter central axis a i  for receiving the central electrode  32 . 
     The spark plug  20  also includes a terminal  52  formed of an electrically conductive material received in the insulator  30  and extending longitudinally along the igniter central axis a i  from a first terminal end (not shown), which is electrically connected ultimately to a power source, to a second terminal end  56 , which is electrically connected to the electrode terminal end  40 . A resistor layer  58  is disposed between and electrically connects the second terminal end  56  and the electrode terminal end  40  for transmitting energy from the terminal  52  to the central electrode  32 . The resistor layer  58  is formed of an electrically resistive material, such as a glass seal. 
     The metal shell  24 , typically formed of steel, surrounds the insulator  30  and extends longitudinally along the igniter central axis a i  from a shell upper surface  60  to the shell lower surface  36  such that the insulator nose end  48  extends outwardly of the shell lower surface  36 , as shown in  FIG. 1 . In one preferred embodiment, the shell lower surface  36  is planar and presents a shell thickness t s  extending perpendicular to the igniter central axis a i . The shell lower surface  36  also extends annularly around the insulator  30 . 
     The shell  24  includes a shell inner surface  62  facing the insulator  30  and a shell outer surface  64  facing opposite the shell inner surface  62 . The shell inner surface  62  and shell outer surface  64  extend circumferentially around the igniter central axis a i  and longitudinally between the shell upper surface  60  and the shell lower surface  36 . The shell inner surface  62  presents a shell inner diameter D i  and the shell outer surface  64  presents a shell outer diameter D o , each extending across the igniter central axis a i . 
     The shell outer surface  64  presents the plurality of threads  26  extending circumferentially around the igniter central axis a i  between the shell upper surface  60  and the shell lower surface  36  for engaging mating threads of the cylinder head  28  or another component maintaining the spark plug  20  in position in the end application. The threads  26  are formed after attaching the ground electrode  34  to the shell  24  such that the ground electrode  34  is disposed in the predetermined location relative to the threads  26  of the shell  24  and the threads  26  are disposed in the predetermined location relative to the ground electrode  34 . 
     Each of the threads  26  present a thread diameter D thread  across the igniter central axis a i . The peak of each thread  26  is spaced from the peak of an adjacent thread  26 . The peaks of the threads  26  are oriented in the predetermined location relative to the ground electrode  34 , for example at a predetermined angle α relative to the side surface  66  of the ground electrode  34  adjacent the attachment surface  68 , as shown in  FIG. 1A . The angle α of the threads  26  can be determined by indexing methods. For example, the angle α can be determined by first locating the desired position of the shell  24  and ground electrode  34  when the spark plug  20  is disposed in the combustion chamber  22 , which is typically the position providing the most effective combustion of the fuel-air mixture, and then determining an angle α of the threads  26  that can provide that desired position. In one embodiment, the peaks of the threads  26  are formed at an angle α plus or minus a certain degree from the side surface  66  of the ground electrode  34 , as shown in  FIG. 1A . The peaks of the threads  26  can also be formed at an angle a plus or minus a certain degree from a plane perpendicular to the igniter central axis a i  and extending through a predetermined point P along the shell outer surface  64 , for example the point P shown in the spark plug of  FIGS. 8 and 8A . The threads  26  can also be formed at a predetermined distance from the attachment surface  68  of the ground electrode  34 . 
     The ground electrode  34  is formed of an electrically conductive material, such as a nickel alloy, and extends from an attachment surface  68  to a ground firing surface  70  with a side surface  66  between the attachment surface  68  and the ground firing surface  70 . The attachment surface  68  and firing surface  70  are planar and present an electrode thickness t e  between the side surface  66 . The electrode thickness t e  is typically not greater than the shell thickness t s . In one embodiment, the ground electrode  34  is initially provided as extending straight from the attachment surface  68  to the ground firing surface  70 , as shown in  FIG. 2 . The attachment surface  68  is attached to the shell lower surface  36 , typically by welding. The attachment surface  68  is disposed at a predetermined circumferential location along the shell lower surface  36  relative to the threads  26 . 
     Typically after the threads  26  are formed in the shell outer surface  64 , the ground electrode  34  is bent inwardly such that the ground electrode  34  curves and the ground firing surface  70  extends past the igniter central axis a i . The ground firing surface  70  is spaced from the central firing end  42 , such that the side surface  66  of the ground electrode  34  and the central firing end  42  provide a spark gap  72  therebetween. However, the ground electrode  34  can comprise another design while still being disposed at a predetermined angle α relative to the threads  26 . In one embodiment, the ground electrode  34  includes a ground firing tip  74  formed of a precious metal alloy disposed on the ground firing surface  70  for providing the spark. The ground firing tip  74  is spaced from the central firing tip  44  to provide a spark gap  72  therebetween. 
     Another aspect of the invention provides a method of forming the spark plug  20  including the ground electrode  34  and shell  24  disposed in the predetermined location relative to one another, so that the spark plug  20  can be oriented in a desired position relative to the cylinder head  28  and other components of the internal combustion engine, allowing the spark plug  20  to provide a more reliable and efficient or optimal combustion of the fuel-air mixture. Before forming the spark plug  20 , the method includes determining a location of threads  26  to be formed in the shell outer surface  64  relative to the ground electrode  34 , such that when the spark plug  20  is threaded to the cylinder head  28 , the ground electrode  34  is disposed in an optimal position for ignition. In one embodiment, the threads  26  are oriented at the predetermined angle α relative to the side surface  66  of the ground electrode  34  adjacent the attachment surface  68 , as shown in  FIG. 1A . The angle α of the threads  26  can be determined by indexing methods. 
     A thread forming apparatus  102  is used to form the threads  26  in the predetermined location, for example a thread roller including a plurality of thread dies  76 , as shown in  FIGS. 5-7 . The thread forming apparatus  102  is designed to form the threads  26  in the predetermined location relative to the ground electrode  34  when the ground electrode  34  is disposed in a predetermined position relative to the thread forming apparatus  102 , for example when the ground electrode  34  is disposed in a predetermined position relative to the opposing thread dies  76 . The orientation tool  38  is preferably used to dispose the ground electrode  34  in the predetermined position relative to the thread forming apparatus  102 . 
     The method of forming the spark plug  20  first includes providing the shell  24 , ground electrode  34 , and other components of the spark plug  20 . The ground electrode  34  is initially provided as extending longitudinally and straight from the attachment surface  68  to the ground firing surface  70 , as shown in  FIG. 2 . Before forming the threads  26  in the shell outer surface  64 , the method includes attaching the attachment surface  68  of the ground electrode  34  to the shell lower surface  36  at a predetermined circumferential location along the shell lower surface  36 . 
     Once the ground electrode  34  is attached to the shell  24 , the orientation tool  38  is used to locate the ground electrode  34  and position the ground electrode  34  and the shell  24  in the thread forming apparatus  102 . The orientation tool  38  may be mechanically coupled to the thread forming apparatus  102 , as shown in  FIGS. 5-7 . Alternatively, the orientation tool  38  may be separate from the thread forming apparatus  102  and then placed along the thread forming apparatus  102  after locating the position of the ground electrode  34 . 
     The orientation tool  38  typically extends longitudinally along a tool central axis a t  from a first end  78  to a second end  80 . The orientation tool  38  includes a tool outer surface  82  between the first end  78  and the second end  80  with a thread orientation feature  84  disposed in a predetermined location along the tool outer surface  82  and extending transverse to the tool outer surface  82 . The orientation tool  38  presents a tool diameter D t  that is no greater than the shell inner diameter D i . In one embodiment, shown in  FIG. 3 , the orientation tool  38  includes a mandrel and the tool outer surface  82  presents a cylindrical shape. In this embodiment, the thread orientation feature  84  is a lip extending transversely from the tool outer surface  82 . The mandrel is typically placed in a bore of a receptacle  88  and extends perpendicular to the thread dies  76 , as shown in  FIG. 5 . 
     In an alternate embodiment, shown in  FIG. 4-4B , the orientation tool  38  includes a receptacle  88  extending longitudinally from a support surface  90  along a tool central axis a t  to a base surface  92 , wherein the support surface  90  is planar and extends annularly around the tool central axis a t . In this embodiment, the orientation tool  38  also includes mandrel with a tool outer surface  82  that can be disposed in a bore of the receptacle  88  and presents a cylindrical shape. The mandrel presenting the tool outer surface  82  includes a flat disposed in a slot along the tool bore. The thread orientation feature  84  is provided by a surface of the slot extending from the support surface  90  toward the base surface  92  of the receptacle  88  and the flat of the mandrel. The slot surface is located in a predetermined location along the tool outer surface  82  and extends transverse to the tool outer surface  82 . 
     The method also includes disposing the thread orientation feature  84  of the orientation tool  38  in a predetermined position relative to the thread forming apparatus  102 , such that when the ground electrode  34  contacts the thread orientation feature  84  the thread forming apparatus  102  can form the threads  26  in the shell outer surface  64  in the predetermined location relative to the ground electrode  34 . In the embodiment of  FIGS. 5-7 , the orientation tool  38  is mechanically attached to the thread forming apparatus  102 . Thus, when the ground electrode  34  is maintained in contact with the thread orientation feature  84  of the orientation tool  38 , the ground electrode  34  will be disposed in a predetermined position relative to the thread forming apparatus  102 , allowing the thread forming apparatus  102  to form the threads  26  in the shell outer surface  64  in the desired location relative to the ground electrode  34 . In another embodiment, the orientation tool  38  is separate from the thread forming apparatus  102 , and the orientation tool  38  is transferred to the thread forming apparatus  102  with the shell  24  and ground electrode  34  maintained along the thread orientation feature  84 . 
     To dispose the ground electrode  34  in the desired position, the method includes aligning the tool central axis a t  of the orientation tool  38  with the igniter central axis a i  of the shell  24  and disposing the shell  24  on the first end  78  of the orientation tool  38  such that the ground electrode  34  engages the tool outer surface  82 , as shown in  FIG. 6 . In the alternate embodiment using the orientation tool  38  of  FIG. 4 , the ground firing surface  70  of the ground electrode  34  is disposed on the support surface  90  of the receptacle  88 . 
     Once the shell  24  is disposed on the orientation tool  38 , the method includes locating the ground electrode  34  by rotating the shell  24  relative to the orientation tool  38  such that the ground firing surface  70  slides along the tool outer surface  82  circumferentially around the central axes a i , a t  until the side surface  66  of the ground electrode  34  contacts the thread orientation feature  84  and is disposed in a predetermined position relative to the thread orientation feature  84 , as shown in  FIG. 7 . In the alternate embodiment using the orientation tool  38  of  FIG. 4 , the ground firing surface  70  slides along the support surface  90  of the receptacle  88  until sliding into the slot and engaging the thread orientation feature  84 , which is the slot surface. 
     Once the ground electrode  34  is positioned correctly in the thread forming apparatus  102 , the method includes forming the threads  26  in the shell outer surface  64  in the predetermined location relative to the ground electrode  34 , for example using the thread dies  76 . The side surface  66  of the ground electrode  34  is maintained in contact with the thread orientation feature  84  until the thread forming apparatus  102  begins to form the threads  26  in the shell  24 . Next, the method includes forming the threads  26  in the shell  24  at the predetermined angle α relative to the ground electrode  34 . The thread forming apparatus  102  is programmed to form the threads  26  at the predetermined angle α. 
     The method next includes disengaging the threaded shell  24  and ground electrode  34  from the orientation tool  38 , and proceeding to form the remainder of the spark plug  20 . In one embodiment, the further steps include bending the ground firing surface  70  of the ground electrode  34  inwardly toward the igniter central axis a i , sliding the insulator  30  into the shell  24 , sliding the central electrode  32  into the insulator  30 , disposing the resistor layer  58  in the insulator  30  along the central electrode  32 , and disposing the terminal  52  in the insulator  30  on the resistor layer  58 . 
     After forming the spark plug  20 , the method includes threading the spark plug  20  into the cylinder head  28  or another component maintaining the spark plug  20  in position during the end application. The cylinder head  28  includes threads  26  mating the threads  26  of the shell  24 . The method includes engaging the threads  26  of the shell  24  and the threads  26  of the cylinder head  28 , and rotating the shell  24  relative to the cylinder head  28  to screw the shell  24  into the cylinder head  28 . When the shell  24  is threaded into the cylinder head  28 , the ground electrode  34  will be disposed in the predetermined location relative to the threads  26  of the shell  24  and thus in an optimal location relative to the cylinder head  28 , fuel injector, and other components of the combustion chamber of the internal combustion engine, allowing the spark plug  20  to provide a more reliable and efficient ignition of the fuel-air mixture in the combustion chamber  22 . 
     Three alternate methods of forming the threads  26  in the shell outer surface  64  are also provided. The alternate methods are capable of reliably and repeatedly orienting the threads  26  at the desired, predetermined rotational angle α and in a desired start position s, which is especially advantageous when manufacturing multiple spark plugs  20  of the same design. Examples of the threaded shell  24  and ground electrode  34  formed according to these alternate methods are generally shown in  FIGS. 8 and 9 .  FIG. 10  illustrates an example of the shell  24  and ground electrode  34  relative to one of the dies  76  of the thread forming apparatus  102  according to the first alternate method.  FIG. 11  illustrates an example of the shell  24  and ground electrode  34  relative to one of the dies  76  of the thread forming apparatus  102  according to the second alternate method.  FIG. 12  illustrates an example of the shell  24  and ground electrode  34  relative to one of the dies  76  of the thread forming apparatus  102  according to the third alternate method. In addition, it is noted that individual or multiple steps of the methods of the three embodiments could be combined to create another embodiment of the method of orienting the threads  26  at the desired rotational position α and in the desired start position s. These methods provide for improved thread indexing accuracy, so that the threads  26  of the multiple shells  24  can be repeatedly located in an optional location relative to the cylinder head  28 , fuel injector, and other components of the internal combustion engine. 
     The alternate methods begin by positioning the ground electrode  34  in a desired position outside of the thread forming apparatus  102 , i.e. before the shell  24  and ground electrode  34  are disposed in the thread forming apparatus  102 . Typically, the attachment surface  68  of the ground electrode  34  is already attached to the shell lower surface  36  along the shell lower surface  36  and so that the ground electrode  34  extends longitudinally from the attachment surface  68 . However, the method can include attaching the attachment surface  68  of the ground electrode  34  to the shell lower surface  36  at a predetermined circumferential location along the shell lower surface  36  and so that the ground electrode  34  extends longitudinally from the attachment surface  68  before disposing the shell  24  between the threading dies  76 . The predetermined circumferential location of the ground electrode  34  is selected so that the ground electrode  34  will be disposed in a desired position in the thread forming apparatus  102  which helps to maintain a consistent relationship between the known rotational position of the ground electrode  34 , the start position s of the threads, and the predetermined rotational position α of the threads  26  to create a ground electrode  34  capable of repeating its rotation location inside a combustion chamber, for example a position providing effective combustion. Once the ground electrode  34  is positioned, the improved thread indexing method begins. 
     According to the first alternate method, after the ground electrode  34  is oriented, the method includes determining a location of a ledge  88  of a shell seat  86  which extends perpendicular to the center axis A of the shell  24 , faces the shell lower surface  36 , and rests on the gasket or on a surface within the combustion chamber of the engine. If the spark plug  20  being manufactured will be used with the gasket, the ledge  88  of the shell seat  86  comes into contact with the gasket, which typically contacts the mating surface of the cylinder head  28 . If the spark plug  20  being manufactured is not used with the gasket, then the ledge  88  of the shell seat  86  typically comes in contact with the mating surface of the cylinder head  28 . 
     The method of the first embodiment next includes determining the start position s of the threads  26  to be formed in the shell outer surface  64  relative to the ledge  88  of the shell seat  86 . The start position of the threads  26  is also based on a desired location of the shell  24  in the cylinder head  28 . The method further includes determining the predetermined rotational position α of the threads  26  in the shell outer surface  64  and determining the known rotational position of the ground electrode  34  relative to the start position s of the threads  26  to be formed in the shell outer surface  64 . These steps can be conducted by determining the location of a gage point g of the shell  24  in relation to a stating location of the top of the threading dies  76 . The gage point g can be a radial diameter reference point, as shown in  FIGS. 8 and 9 , or a reference point anywhere else on the shell  24  that relates to the contact point of the mating surface of the final assembly position of the spark plug application. Whether or not the spark plug  20  is used with the gasket, the gage point g can be determined by creating a datum line at a specified diameter on the ledge  88 , related to the contact position of the mating surface in the application. The gage point g can be located outside of the thread forming apparatus  102  off a hard contact point located at a known relative distance to the stating location of the top of the threading dies  76  by a known distance, vision or other measurement system. Alternatively, the location of the gage point g can be determined fully by vision or other measurement system inside, or outside, the threading apparatus  102 . The entire shell  24  or spark plug  20  can be designed based on the desired location of the ground electrode  34  rotational position, gage point g, and thread start position s relative to the cylinder head  28  of the engine in which the spark plug  20  will be used. In addition, the start position s and predetermined rotational position α of the threads  26  is designed so that the ground electrode  34  is disposed in a desired position when threaded into the cylinder head  28  of the combustion chamber, for example a position providing effective combustion. The gage point g, starting location of the threads of the threading dies  76 , and the ground electrode  34  rotational placement can be referenced from the thread start position s. 
     After the position of the ledge gage point g is determined, the first alternate method includes picking up the shell  24  with the ground electrode  34  oriented, and holding the shell  24  while placing the shell  24  between the threading dies  76  of the thread forming apparatus  102 .  FIG. 10  illustrates an example of the shell  24  disposed adjacent one of the threading dies  76  of the thread forming apparatus  102  according to the first alternate embodiment. This step includes placing the shell  24  and the attached ground electrode  34  between the set of threading dies  76  of the thread forming apparatus  102  so that the ledge  88  of the shell seat  86  is at a specified distance relative to the starting location of the threads of the threading dies  76 , and clamping the shell  24  with the threading dies  76 . The step of placing the shell  24  and the attached ground electrode  34  between the set of threading dies  76  also includes placing the shell  24  and the attached ground electrode  34  at the known rotational position in relation to the start position s of the threads  26  to be formed in the shell outer surface  64 . The method can further include disposing the ground electrode  34  rotational position and the gage point g at a specified distance d 1  relative to the start position s of the threads  26  to be formed by threading dies  76 . The start position s is important as it relates to the contact point of the shell  24  with the cylinder head  28 , which controls the indexing position of the spark plug  20  in the engine. The specified distance d 1  is determined based on the design of the cylinder head  28  in which the spark plug  20  will be used. For example, the specified distance d 1  in relation to the ground electrode rotational position can be replicated onto the threads of the cylinder head  28  to position the placement of the ground electrode  34  in the combustion chamber. The threading dies  76  should not be too high relative to the shell seat ledge  88 , otherwise there is the possibility of scratching the shell outer surface  64 , which can lead to leakage of combustion gases. Also, the dies  76  are positioned and set to rotate at a predetermined rotational position and speed so that when multiple spark plugs  20  of the same design are manufactured, the predetermined rotational position α of the threads  26  on the dies  76  is in the same repeated position. 
     The step of determining the predetermined rotational position α of the threads  76  in the shell outer surface  64  and thus the rotational position of the threads of the dies  76  can be done theoretically by calculating the distance d 1  from the gage point g on the ledge  88  to the threads  26  in relation to the rotational position of the ground electrode  34 . Alternatively, once the threads  26  are located at the start position s, i.e. the desired height in the thread relief, this step can include measuring the degree, or the circumferential location, of the ground electrode  34  in relation to the gage point g and rotational position α of the threads  76  in the shell outer surface  64  with a coordinate measuring machine (cmm), hard gage tool, or vision measurement system, and adjusting the position of the dies  76  accordingly. Once the predetermined rotational position α of the threads  26  is determined, the method also typically includes forming the threads  26  in the cylinder head  38  in which the spark plug  20  will be used at a rotational position corresponding to the predetermined rotational position α of the threads  26  in the shell outer surface  64  so that the ground electrode  34  is ultimately located at the correct radial position when the shell  24  is threaded in the cylinder head  38  of the engine. 
     The method next includes clamping the shell  24  with the dies  76  to lock in the start position s of the threads  26  relative to the ledge  88  of the shell  24 . Next, the method includes rotating the dies  76  to form the threads  26  at the predetermined rotational position α in the shell outer surface  64 . The method can also include moving the threading dies  76  in the longitudinal direction while they are rotating, for example towards the center of the shell  24 , to form the correct thread parameters. Once the threads  26  are formed, the threaded shell  24  is removed from the thread forming apparatus  102  and then combined with the other components of the spark plug  20 . After the threading step, the dies  76  return to a specified initial position, so that they are ready to thread another shell  24 . The specified initial position of the dies  76  is repeated to form multiple shells  24  and/or spark plugs  20  having the same design. 
     The method of the second embodiment also includes determining the start position s of the threads  26  in the shell outer surface  64 . The second alternate method further includes determining the predetermined rotational position α of the threads  26  in the shell outer surface  64 , and thus the rotational position of the threads of the dies  76  used to form the threads  26  in the shell outer surface  64 . The dies  76  are positioned and set to rotate at a predetermined rotational position and speed so that when multiple spark plugs  20  of the same design are manufactured, the rotational position of the threads  26  on the dies  76  is in the same repeated position. The step of determining the predetermined rotational position α of the threads  76  in the shell outer surface  64  and thus rotational position of the threads in the dies  76  can be done theoretically by calculating the distance dl from the gage point g to the threads  26  in relation to the rotational position of the ground electrode  34 . Alternatively, once the threads  26  are located at the start position s, i.e. the desired height in the thread relief, this step can include measuring the degree of the ground electrode  34  in relation to the gage point g and rotational position α of the threads  76  in the shell outer surface  64  with a coordinate measuring machine (cmm), hard gage tool, or vision measurement system, and adjusting the position of the dies  76  accordingly. Once the predetermined rotational position α of the threads  26  is determined, the method also typically includes forming the threads  26  in the cylinder head  38  in which the spark plug  20  will be used at the correct rotational position so that the ground electrode  34  is ultimately located at the correct radial position inside the cylinder head  38  of the engine. 
     After locating the ground electrode  34 , the method includes picking up the shell  24  with the ground electrode  34  oriented in a predetermined circumferential location, and holding the shell  24  while placing the shell  24  between the threading dies  76  of the thread forming apparatus  102 .  FIG. 11  illustrates an example of the shell  24  disposed adjacent one of the threading dies  76  of the thread forming apparatus  102  according to the second alternate method. 
     Unlike the method of the first embodiment, the step of disposing the shell  24  and the attached ground electrode  34  between the threading dies  76  according to the second embodiment includes engaging the ledge  88  of the shell seat  86  with a surface  94  between the dies  76  which is disposed at a specified distance d 2  relative to the start position s of the threads  26 . This surface  94  contacts the gage point g on the shell seat ledge  88 . The specified distance d 2  depends on the design of the cylinder head  38  in which the spark plug  20  is used. The step of determining the start position s is based on a desired location of the shell  24  in the cylinder head  28 . The start position s is again important as it relates to the contact point of the shell  24  with the cylinder head  38 , which controls the indexing position of the spark plug  20  in the engine. This step includes making sure that the threads  26  are high enough into the thread relief area on the shell  24  so that the shell  24  fully threads into the cylinder head  28 . The surface  94  can be provided by an interchangeable insert  96 , as shown in  FIG. 11 , capable of holding the gasket or the ledge  88  of the shell seat  86 , which can be tapered. Alternatively, the surface  94  can be provided by another solid surface capable of maintaining the shell  24  at the specified distance d 2  relative to the start position s of the threads  26 . For example, the top of one of the threading dies  76  or another material located on top of the dies  76  could be used to provide the surface  94 . 
     The surface  94  can remain in position during the threading step, and thus is typically formed from a material resistant to scratching and scarring the gasket or the ledge  88  of the shell seat  86 . Alternatively, the surface  94  can be moved to a lower position spaced from the ledge  88  prior to the threading step. Scratching and scarring should be avoided, as scratches and scars could prevent sealing of the spark plug  20  in relation to the gasket or the ledge  88  and thus could cause combustion gases to escape the combustion chamber. 
     The method further includes clamping the shell  24  with the dies  76  to lock in the start position s of the threads  26  relative to the ledge  88  of the shell  24  and the rotational position of the ground electrode  34 . Next, the method includes rotating the dies  76  and forming the threads  26  at the predetermined rotational position α in the shell outer surface  64 . Once the threads  26  are formed, the threaded shell  24  is removed from the thread forming apparatus  102  and then combined with the other components of the spark plug  20 . After the threading step, the dies  76  return to a specified initial position, and the surface  94  is brought back to its specified initial position, if moved, so that the thread forming apparatus  102  is ready to thread another shell  24 . The specified initial position of the surface  94  and the dies  76  is repeated to forming multiple shells  24  and/or spark plugs  20  having the same design. 
     The third example embodiment also includes providing the shell  24  with the ledge  88  of the shell seat  86  facing the shell lower surface  36 , and providing the ground electrode  34  extending longitudinally from the attachment surface  68 . The method of the third embodiment further includes determining the longitudinal location of the ledge  88  of the shell seat  86 , which is the distance between the shell lower surface  36  and the ledge  88 . This can be done outside or inside the threading forming apparatus  102  by vision or other measurement system. The method also includes placing the attached ground electrode  34  at the known rotational position in relation to the start position s of the threads  26  to be formed in the shell outer surface  64  before disposing the shell  24  between the dies  76 . 
     The method next includes placing the shell  24  and the attached ground electrode  34  between the threading dies  76  of the thread forming apparatus  102  so that the ledge  88  of the shell seat  86  is at a specified distance relative to the starting position of the threads of the threading dies  76 . The step of placing the ledge  88  of the shell seat  86  at the specified distance relative to the starting position of the threads of the threading dies  76  includes disposing the shell lower surface  36  on a solid adjustment feature  104  located between the dies  76 , and adjusting the location of the solid adjustment feature  104  relative to the starting position of the threads of the dies  76 . For example, a mechanism can be used to adjust the position of the solid adjustment feature  104  in the longitudinal direction, i.e. move the solid adjustment feature  104  up or down, to a specific distance to position the shell seat ledge  88  at the correct distance from the start of the dies  76 . The top surface of the solid adjustment feature  104  can either have a cutout to clear the ground electrode  34  or it can have a slot cut into it to help locate the ground electrode  34  at a tighter rotational angle. 
     As in the other embodiments, the third embodiment includes clamping the shell  24 , and rotating the threading dies  76  to form the threads  26  at the predetermined rotational position α in the shell outer surface  64 . The dies  76  are at a specific repeatable rotational position, and the solid adjustment feature  104  is lowered out of the way of the rotating shell  24  or rotates freely while the shell  24  rotates during the threading operation. The threaded shell  24  is then ejected and the process is started over again. The processing of the third embodiment can be the same as the other embodiments, besides determining the height location of the shell seat  88  and the use of the solid adjustment feature  104  between the dies  76  that the shell lower surface  36  contacts to maintain the correct distance from the shell seat ledge  88  to the starting position of the threads of the dies  76 . 
     As indicated above, the main components of the improved alternate methods are the position of the ledge  88 , gage point g, orientation of the ground electrode  34 , start position s of the threads  26  on the shell  24  and the starting position of the threads on the dies  76 , the specified distance d 1 , the specified distance d 2  of the surface  94 , and the clamping position. In summary, the method includes locating the ground electrode  34  outside of the thread forming apparatus  102 , rather than internally, starting the threads of the dies  76  at the repeated start position s along the shell outer surface  64 , and clamping the shell  24  between the dies  76  in relation to a set distance from the ledge  88  gage point g. The factures, which are typically determined before the threading step, accurately control the index threading position. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the appended claims. In addition, the reference numerals in the claims are merely for convenience and are not to be read in any way as limiting.