Method for producing a spark plug

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.

Claims

1. A method for producing a spark plug for 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 that has a front end that projects from the front end of the insulator; at least one ground electrode that is attached to the front end of the shell; and 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 has a nominal width; the method comprising the steps of: initially producing a spark gap with a width that is smaller, at least in places, than the nominal width; and subsequently bringing the spark gap to its nominal width by the removal of material from at least one of a surface of the end piece and a surface of the counterpart that delimit the spark gap, wherein the removal of material from at least one of the two surfaces delimiting the spark gap is accomplished through sink electrical discharge machining (EDM).

2. The method according to claim 1, wherein the spark gap is initially produced with a width that is no greater than the nominal width at any part of the spark gap.

3. The method according to claim 1, wherein, in spark plugs whose spark gap surrounds a lateral surface of the end piece of the center electrode, the nominal width of the spark gap is produced through sink EDM.

4. The method according to claim 1, wherein the mutually opposing surface of the end piece and the surface of the counterpart that delimit the spark gap are shaped by removal of material in such a manner that one or more ridges and one or more valleys are produced that are opposite one another, while a predefined width of the spark gap is maintained.

5. The method according to claim 4, wherein the surface of the end piece and the surface of the counterpart that delimit the spark gap are designed such that they have a zigzag shape or corrugated shape.

6. A method for producing a spark plug for 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 that has a front end that projects from the front end of the insulator; at least one ground electrode that is attached to the front end of the shell; and 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 has a nominal width; the method comprising the steps of: initially producing a spark gap with a width that is smaller, at least in places, than the nominal width; and subsequently bringing the spark gap to its nominal width by the removal of material from at least one of a surface of the end piece and a surface of the counterpart that delimit the spark gap, wherein the removal of material from at least one of the two surfaces delimiting the spark gap is accomplished through electrical discharge machining (EDM), wherein, in spark plugs whose spark gap surrounds a lateral surface of the end piece of the center electrode, the nominal width of the spark gap is produced through sink EDM, wherein the sink EDM is performed with an annular electrode.

7. A method for producing a spark plug for 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 that has a front end that projects from the front end of the insulator; at least one ground electrode that is attached to the front end of the shell; and 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 has a nominal width; the method comprising the steps of: initially producing a spark gap with a width that is smaller, at least in places, than the nominal width; and subsequently bringing the spark gap to its nominal width by the removal of material from at least one of a surface of the end piece and a surface of the counterpart that delimit the spark gap, wherein the removal of material from at least one of the two surfaces delimiting the spark gap is accomplished through electrical discharge machining (EDM), wherein, in spark plugs that have at least two ground electrodes or one ground electrode with at least two counterparts pointed toward a lateral surface of the end piece of the center electrode, the nominal width of the spark gap is produced through sink EDM.

8. The method according to claim 7, wherein the sink EDM is performed with an annular electrode.

9. The method according to claim 8, wherein during erosion, the annular electrode is moved in a direction of the longitudinal axis of the center electrode that is coaxially to the center electrode.

Description

DRAWINGS

(1) Preferred exemplary embodiments of the invention will hereinafter be described in conjunction with the appended drawings, wherein like designations denote like elements, and wherein:

(2) FIG. 1 is a side view of a spark plug according to one embodiment;

(3) FIG. 2 shows an enlargement of the detail A of the spark plug from FIG. 1;

(4) 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;

(5) FIG. 4 shows the spark gap from FIG. 3 after a trimming process;

(6) 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;

(7) FIG. 6 shows the arrangement from FIG. 5 in a side view after a trimming process;

(8) 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;

(9) 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;

(10) 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;

(11) 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

(12) 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.

(13) Like or corresponding parts are labeled with matching reference numbers in the figures.

DESCRIPTION

(14) 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.

(15) 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 10a opposes the end face of the end piece 13. The spark plug shown in FIG. 1 can be produced as follows, for example:

(16) 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 10a of the ground electrode 10 and includes a surface 26 which, together with the surface 25, delimit the spark gap.

(17) 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.

(18) 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.

(19) 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.

(20) 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.

(21) The spark gap 18 is shown in FIG. 2 and is supposed to have a nominal width of, for example, 0.8 mm.

(22) 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.

(23) 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.

(24) 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].

(25) 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.

(26) 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.

(27) 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 widthsee FIG. 7but 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 EAmaxthe 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.

(28) 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.

(29) 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.

(30) 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.

(31) 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

(32) 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 10a 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