Abstract:
A method for producing a spark plug for internal combustion engines, in particular for gas-powered internal combustion engines, having a metallic shell, a ceramic insulator held in the shell, a center electrode embedded in the insulator, at least one ground electrode attached to a front end of the shell, an end piece made of a precious metal and/or alloy attached to a front end of the center electrode, and a counterpart made of a precious metal and/or alloy attached to the ground electrode opposite the end piece, between which is a spark gap with a nominal width. The spark gap is produced initially with a width that is smaller, at least in places, than the nominal width, and is subsequently brought to its nominal width by removing material from at least one of the two surfaces of the end piece and the counterpart that delimit the spark gap.

Description:
This Application claims the benefit of German Application No. 10 2015 112 038.0, filed on Jul. 23, 2015 and German Application No. 10 2015 118 935.6 filed Nov. 4, 2015, the contents of which are hereby incorporated by reference in their entirety. 
     FIELD 
     The invention generally relates to a method for producing a spark plug with a center electrode that is tipped with an end piece made of a precious metal. Opposite the center electrode, a counterpart made of precious metal is welded onto the at least one ground electrode. The end piece and the counterpart delimit the spark gap. 
     BACKGROUND 
     Spark plugs for use in gas engines, especially in stationary gas engines are subject to special requirements; in particular, a long service life is demanded. In order to achieve this, it is important for the width and position of the spark gap to have the smallest possible dimensional tolerances. The cause of dimensional variations may reside in the fusing of the center electrode into the insulator, in the shrink-fitting of the insulator in the spark plug shell, in the process of crimping the back end of the spark plug shell, in the welding of the ground electrodes onto the front edge of the spark plug shell, and in the welding of the precious metal pieces onto the ground electrode and onto the center electrode, and bring about dimensional variations in the spark gap, deviations from parallelism of the surfaces bordering the spark gap, and deviations in the alignment of the center electrode and the precious metal counterpart on the ground electrode coaxial to the spark plug center line. Keeping these error sources as small as possible requires great manufacturing effort and is partly responsible for a high price of the spark plugs for gas engines. 
     SUMMARY 
     An object of the present application is to disclose a way that spark plugs of this type can be produced with less effort without sacrificing dimensional accuracy. 
     According to one aspect, there is provided a method for producing a spark plug for internal combustion engines, in particular for gas-powered internal combustion engines, having: 
     a metallic shell that has an open front end and an open back end; 
     a ceramic insulator, held in the shell, that has a front end and a back end that projects from the back end of the shell; 
     a center electrode, embedded in the insulator, that has a back end that projects from the back end of the insulator and has a front end that projects from the front end of the insulator; and 
     having a ground electrode that is attached to the front end of the shell; 
     wherein an end piece made of a precious metal or a precious metal alloy is attached to the front end of the center electrode, and a counterpart made of a precious metal or a precious metal alloy is attached to the ground electrode opposite the end piece, between which is formed a spark gap that is set to a nominal width; 
     The method may include the following production steps: 
     the spark gap is produced initially with a width that is smaller, at least in places, than the nominal width; subsequently, the spark gap is brought to its nominal width by the removal of material from at least one of the two surfaces—of the end piece and of the counterpart—delimiting the spark gap. 
     It is preferable that the last of these two steps for creating the spark gap be the final step in manufacturing the spark plug, and at the same time, is the manufacturing step that is important for the accuracy of the width of the spark gap. All other manufacturing steps, which in the prior art affect the location, shape and width of the spark gap, have already been completed when the width of the spark gap is created by material-removing processing of the surfaces delimiting the spark gap—in other words, by trimming of the spark gap—so the preceding manufacturing steps are no longer able to adversely affect the accuracy of the spark gap. This has the further advantageous consequence that manufacturing steps that are performed prior to the trimming of the spark gap need not be performed with the same high accuracy as in the prior art in order to achieve an accurate spark gap, because the accuracy thereof is determined by the final manufacturing step of the method, namely by the trimming of the spark gap. 
     The trimming of the spark gap can be performed with high precision without special effort. In consequence, the use of the method results in improved accuracy while at the same time reducing manufacturing effort. 
     In order to manufacture a spark plug according to the method, it is possible to initially proceed as in the prior art: the shell of the spark plug, the insulator and the center electrode can be prefabricated separately. The ground electrode can be welded to the shell as usual. An end piece made of a precious metal or a precious metal alloy, for example a precious metal disk, can be welded onto the front end of the center electrode as usual. As counterpart thereto, a counterpart made of a precious metal or a precious metal alloy, for example another precious metal disk, can be welded onto the ground electrode. The center electrode is inserted into the insulator and is fused by one of its sections into the insulator. The insulator equipped with the center electrode can then be inserted into the shell of the spark plug from the back end, for instance pushed forward to a stop and secured by crimping the back end of the shell. 
     Advantageously, the spark gap is initially produced with a width that is no greater than the nominal width at any part of the spark gap. In this way, it is possible to ensure that the spark gap really obtains its nominal width at every point due to the trimming. If the spark gap is already no wider than the nominal width at the beginning of the trimming process, then the trimming process also does not make it any wider than the nominal width. However, the trimming process does enlarge the spark gap to the nominal width at the places where it had been narrower than the nominal width before the trimming process. 
     The removal of material from at least one of the two surfaces delimiting the spark gap, that of the end piece on the center electrode and that of the counterpart on the ground electrode, is preferably accomplished through electrical discharge machining. Using electrical discharge machining, the trimming can be accomplished with high accuracy. Moreover, electrical discharge machining is especially well suited for machining precious metal alloys, for example platinum-iridium alloys, which frequently are used for spark plugs and have the disadvantage that they can be mechanically machined only with difficulty because of their very high strength. 
     In spark plugs whose spark gap is located between the end face of the end piece of the center electrode and the end face of the counterpart facing it, which counterpart is attached to a ground electrode implemented as a front electrode, the nominal width of the spark gap preferably is produced by wire erosion. For this purpose, the tensioned wire for the erosion is guided along the surface of the end piece or of the counterpart to be machined at a distance required for producing the erosive sparkover until the desired width of the spark gap is achieved. 
     Instead of using wire erosion, the spark gap in a spark plug of this type with a ground electrode implemented as a front electrode can also be trimmed by laser beam cutting, or by water jet cutting. Just like wire erosion, these methods permit not only spark gaps delimited by flat surfaces, but also spark gaps with predefined width delimited by profiled surfaces. 
     In spark plugs whose spark gap surrounds a lateral surface of the end piece of the center electrode and the ground electrode is an annular electrode that surrounds the lateral surface of the center electrode, the nominal width of the spark gap preferably is produced through sink EDM. Sink EDM can be carried out with an annular electrode that during erosion is moved in the longitudinal direction of the center electrode past its end piece. For this purpose, it is best for the annular electrode required for sink EDM to have a casing with an outer lateral surface and an inner lateral surface, the radial dimensions of which give the casing a thickness that, together with the gap in which the eroding sparks spark over to the surface undergoing erosion of the end piece of the center electrode or of the counterpart on the ground electrode, yields precisely the nominal width of the spark gap. In this way, it is possible to obtain a spark gap that is precisely centered on the longitudinal axis of the center electrode and has the nominal width. 
     It is possible to proceed analogously in the case of a spark plug in which the lateral surface of the center electrode is located opposite the end of one or more ground electrodes, for example in a spark plug that has two diagonally opposing ground electrodes, or in a spark plug that has three ground electrodes that mutually enclose an angle of 120° and whose counterparts are pointed toward the lateral surface (circumferential surface) of the end piece of the center electrode. In this case, as well, the spark gaps can be trimmed simultaneously through sink EDM with an annular electrode. 
     The mutually opposing surfaces delimiting the spark gap can be shaped by removal of material in such a manner that ridges are produced that are opposite valleys, and/or valleys are produced that are opposite ridges, while the predefined width of the spark gap is maintained. In particular, the mutually opposing surfaces of the end piece placed on the center electrode and of the counterpart placed on the ground electrode can be designed with a corrugated or zigzag shape. In this way, the electrode consumption occurring in operation of the spark plug can be distributed over a larger area while maintaining the predefined width of the spark gap, thereby extending the service life of the spark plug. 
     It is useful for the center electrode to be designed as a circular cylinder at its front end in a known manner. The end piece that is welded to the front end of the center electrode also usefully is made in the shape of a circular cylinder and can have the same diameter as the front end of the center electrode, but can also have a different diameter, in particular a smaller diameter. The counterpart that is placed on the ground electrode preferably is also made in the shape of a circular cylinder. A circular cylindrical design of the center electrode, of the end piece attached thereto, and of the counterpart is not a prerequisite for the success sought with the invention, however. 
    
    
     
       DRAWINGS 
       Preferred exemplary embodiments of the invention will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein: 
         FIG. 1  is a side view of a spark plug according to one embodiment; 
         FIG. 2  shows an enlargement of the detail A of the spark plug from  FIG. 1 ; 
         FIG. 3  schematically shows, in a side view, the spark gap of a spark plug of the type shown in  FIG. 1  prior to a trimming process; 
         FIG. 4  shows the spark gap from  FIG. 3  after a trimming process; 
         FIG. 5  shows a side view in a larger scale of the arrangement of the center electrode in a spark plug of the type shown in  FIG. 1  with a preliminary spark gap whose width is far smaller than its nominal width; 
         FIG. 6  shows the arrangement from  FIG. 5  in a side view after a trimming process; 
         FIG. 7  shows a top view of the end face of a center electrode of a spark plug as well as of an annular ground electrode surrounding the center electrode to form an annular spark gap that, because of an incorrect eccentric position of the center electrode relative to the ground electrode, does not have a uniform width smaller than the nominal width; 
         FIG. 8  schematically shows how the spark gap from  FIG. 7  can be trimmed to its nominal width through sink EDM by means of a cup-shaped electrode; 
         FIG. 9  shows a top view of the front end of a spark plug with a preliminary spark gap between an annular ground electrode and a circular cylindrical center electrode, wherein the width of the spark gap is initially much smaller than the nominal width; 
         FIG. 10  shows, in an enlarged scale as compared to  FIG. 9 , a top view of the free end of an annular electrode folded in the circumferential direction for producing a correspondingly folded annular spark gap in the spark plug from  FIG. 9  through sink EDM; and 
         FIG. 11  shows the spark plug from  FIG. 9  with the folded annular spark gap, produced through sink EDM with an annular electrode having the profile shown in  FIG. 10 . 
     
    
    
     Like or corresponding parts are labeled with matching reference numbers in the figures. 
     DESCRIPTION 
     The spark plug shown in  FIG. 1  has a metallic shell  1  with a front end  2  and a back end  3 . Inserted in the shell  1  is an insulator  4 , which has a front end  5  and a back end  6 , which projects out of the back end  3  of the shell  1 . Inserted in the insulator  4  is a center electrode  7 , which has a front end  8  and a back end  9 . The back end  9  of the center electrode  7  projects beyond the back end  6  of the insulator  4  and is implemented as an electrical terminal. For the majority of its length, the center electrode  7  is made of a base metal, for example of a nickel alloy. Welded onto the front end  8  of the center electrode  7 , which projects out of the front end  5  of the insulator  4 , is an end piece  13 , which is made of a precious metal or a precious metal alloy. The end piece  13  can have the same diameter as the adjacent base-metal section of the center electrode  7 . Preferably, the end piece  13  is a body in the form of a circular cylinder. 
     Welded onto the front end  2  of the shell  1  is a ground electrode  10 , which is designed as a front electrode and usefully is made of the same material as the shell  1 . The term “front electrode” is meant to express that the ground electrode  10  is designed with a hook shape and its free end section  10   a  opposes the end face of the end piece  13 . The spark plug shown in  FIG. 1  can be produced as follows, for example: 
     The shell  1 , the ceramic insulator  4 , and the center electrode  7  are prefabricated individually. The end piece  13 , which is made of a precious metal or a precious metal alloy, for example platinum or iridium or a platinum alloy or an iridium alloy, in particular of a platinum-based alloy or an iridium-based alloy, is welded onto the front end  8  of the center electrode  7  and includes a surface  25  . The ground electrode  10  is also prefabricated. The counterpart  14 , which can be made of the same material as the end piece  13 , is welded laterally to the free end section  10   a  of the ground electrode  10  and includes a surface  26  which, together with the surface  25 , delimit the spark gap. 
     In order to assemble the spark plug, the center electrode  7  is pushed from behind into the insulator  4  to a stop and fixed in place therein. This is not shown in  FIG. 1 , but is generally known for spark plugs. 
     Together with the center electrode  7  inserted in it, the insulator  4  is pushed from behind into the shell  1  until its front external shoulder strikes an internal shoulder of the shell  1 . These two shoulders are usefully conical in design, and in this way contribute to centering of the insulator  4  in the shell  1 . In order to fix the insulator  4  in place in the shell  1 , the back end  3  of the shell  1  can be crimped inward against a back external shoulder of the insulator  4 . 
     For the sake of completeness, it is mentioned that an external thread  15  can be provided on the front section of the shell  1 , with which thread the spark plug can be screwed into a matching threaded bore in the cylinder head of an internal combustion engine. A seal ring  19  can be provided adjacent to the external thread  15 . 
     Once the insulator  4  is fixed in place in the shell  1 , the ground electrode  10  is attached by welding to the front end  2  of the shell  1  such that the counterpart  14  opposes the end piece  13  of the center electrode  7 . 
     The spark gap  18  is shown in  FIG. 2  and is supposed to have a nominal width of, for example, 0.8 mm. 
     Inaccuracies in the spark gap  18  can be prevented with the method. For this reason, it is preferred to provisionally create the spark gap  18  such that it is narrower at every point than its nominal width. Moreover, an unwanted, incorrect orientation of the end piece  13  and counterpart  14  relative to one another is shown in  FIG. 4 , so that the preliminary spark gap  18  has a wedge shape. By means of the method, the spark gap can be trimmed to its nominal width, and at the same time the wedge shape of the spark gap  18  shown in  FIG. 3  can be corrected. This can be accomplished through wire erosion.  FIG. 3  shows a wire  16  intended for wire erosion, in cross-section, in its position prior to the start of the erosion process. The wire  16  can be moved at right angles to itself in the direction of the arrow  17 , in contact with one of the two surfaces delimiting the spark gap  18 , through the spark gap  18 , in which process it expands the spark gap through electrical discharge machining.  FIG. 4  shows the wire  16  after the conclusion of the erosion process, which has resulted in a spark gap  18  that is delimited by flat surfaces and that has the nominal width. 
     As is known, electrical discharge machining is brought about by the means that the wire  16  is electrically connected as a cathode, whereas the surface to be machined is connected as an anode. As the wire  16  approaches the surface to be machined, sparks jump from the wire  16  to the surface to be machined and bring about an erosion of the surface to be machined. The gap between the wire  16  and the surface to be machined is typically a few hundredths of a millimeter wide, depending on the level of the voltage applied and on the intensity of the current. The width of the spark gap that was expanded through wire erosion thus is the result of the diameter of the wire  16  plus the width of the gap [between the tool and the workpiece], which can be experimentally determined in advance. 
     During erosion, the wire can either glide along the surface of the end piece  13  or along the surface of the counterpart  14 . The erosion then takes place at the particular surface from which the wire  16  maintains an appropriate distance while forming the gap [between the tool and the workpiece]. 
     The spark gap  18  that is coming into being is delimited by two parallel surfaces, see  FIG. 4 . These surfaces may extend at right angles to the longitudinal axis  11  of the center electrode  7 , but this does not have to be the case. It can even be advantageous for the spark gap  18  to extend obliquely to the longitudinal axis  11  of the center electrode  7 , because then the surfaces delimiting the spark gap  18  are larger than they would be if they were perpendicular to the longitudinal axis  11 , so that the electrode consumption occurring during operation of the spark plug can be distributed over a larger area than if the surfaces delimiting the spark gap  18  were precisely at right angles to the longitudinal axis  11  of the center electrode  7 . This extends the service life of the spark plug. 
     The surfaces delimiting the spark gap  18  need not be flat.  FIGS. 5 and 6  show how a spark gap  18  with a corrugated or zigzag shape (see  FIG. 6 ) can be formed by wire erosion in a spark plug having a spark gap  18  that is initially made with only a very narrow width (see  FIG. 5 ) between the end piece  13  on the center electrode  7  and the counterpart  14  of the ground electrode  10 . As a result of the corrugated shape or zigzag shape, the two surfaces delimiting the spark gap  18  are greatly enlarged while the width of the spark gap  18  remains unchanged, so that the electrode consumption occurring during operation of the spark plug can be distributed over the enlarged areas, thus significantly increasing the service life of the spark plug. 
       FIGS. 7 and 8  show how, in the case of a spark plug in which the ground electrode  10  surrounds the center electrode  7  in an annular shape, the annular spark gap  18 , which, because of an unintentional eccentricity does not have a uniform width—see  FIG. 7 —but instead has a width that varies between a maximum width EAmax and a minimum width EAmin, can be trimmed by sink EDM such that the spark gap  18  has its nominal width everywhere and no longer exhibits any eccentricity. In order to be able to carry out the method, the width of the preliminary spark gap  18  is selected such that the maximum width EAmax≦the nominal width, preferably less than the nominal width. To carry out the method, it is possible to use for the sink EDM a cup-shaped electrode  12  that has a circular cylindrical casing  19 , whose external diameter determines the internal diameter of the annular ground electrode  10  that is present after the sink EDM, whereas the internal diameter of the casing  19  of the electrode  12  determines the external diameter that is present of the end piece  13  of the center electrode  7  after the sink EDM. The nominal width of the spark gap  18  minus the width of the gap [between the tool and the workpiece] required for the electrical discharge machining, which can be determined in advance for the given application by experiment, is selected as the wall thickness  28  of the casing  19  of the electrode  12 . To perform sink EDM, the electrode  12  can be moved coaxially to the longitudinal axis  11  of the ground electrode  10 , gliding along the same in the direction of the arrow  17 , with the gap between the electrode  12  and the end piece  13  of the center electrode that is required for electrical discharge machining being maintained. Alternatively, the internal diameter of the casing  19  of the electrode  12  can be matched to the external diameter of the end piece  13 , and the gap can be formed between the external surface of the casing  19  of the electrode  12  and the surface of the counterpart  14  of the ground electrode  10  facing the end piece  13 . 
       FIG. 9  shows a top view of the front end of a spark plug with a preliminary spark gap  18  between an annular ground electrode  10  and a circular cylindrical center electrode  7 , wherein the width of the spark gap  18  is initially much smaller than its nominal width. The annular ground electrode  10  has three feet  21 , which are welded to the front end  2  of the shell  1 . 
     With an annular electrode  12 , whose contour is shown greatly enlarged in  FIG. 10 , the spark gap  18  can be enlarged to its nominal width through sink EDM. In the example from  FIG. 10 , the annular electrode  12  has a shape folded in a zigzag. It is moved coaxially to the longitudinal axis  11  of the center electrode  7  in the spark gap  18 , which is expanding as a result of electrical discharge machining, so that it removes material both from the outer lateral surface of the center electrode  7  and from the inner lateral surface of the ground electrode  10  by electrical discharge machining, and creates a spark gap  18  that is folded in a zigzag shape corresponding to the shape of the electrode  12 , wherein the width of the spark gap  18  is the same everywhere. Compared to a conventional circular cylindrical spark gap, the zigzag spark gap  18  is distinguished by the fact that the surfaces delimiting it are larger than in a comparable circular cylindrical spark gap. The electrode consumption occurring at the surfaces delimiting the spark gap  18  during operation of the spark plug is thus distributed over a larger area, which results in a longer service life. 
     It is to be understood that the foregoing is a description of one or more preferred exemplary embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the scope of the appended claims. 
     As used in this specification and claims, the terms “for example,” “e.g.,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  shell 
           2  front end of  1   
           3  back end of  1   
           4  insulator 
           5  front end of  4   
           6  back end of  4   
           7  center electrode 
           8  front end of  7   
           9  back end of  7   
           10  ground electrode 
           10   a  end section of  10   
           11  longitudinal axis of  7   
           12  electrode for the EDM process 
           13  end piece 
           14  counterpart 
           15  external thread 
           16  wire 
           17  arrow 
           18  spark gap 
           19  casing of  12   
           20  seal ring 
           21  feet of  19