GB2036608A - Method of manufacturing centre electrodes for spark plugs - Google Patents

Abstract

A centre electrode for a spark plug is formed by capping one end of a composite rod 10 having a core 11 of thermally conducting metal, e.g. copper, and a tubular sheath 12 of a refractory metal, e.g. a nickel- chromium-iron alloy, with a cap 20 of a refractory metal using electrical pressure welding. <IMAGE>

Description

SPECIFICATION Method of manufacturing centre electrodes for spark plugs This invention relates to methods of manufacturing centre electrodes for spark plugs.

One known type of spark plug for an internal combustion engine incorporates a centre electrode with comprises a core of thermally conducting metal, such as copper, and a sheath of refractory metal, such as a nickel-iron-or nickel-chromium-iron alloy. In use, the thermally conducting metal conducts heat from the tip of the electrode, thereby improving the performance of the electrode, whilst the sheath protects the core from corrosion.

Such electrodes are conventionally formed by cold-forming from individual blanks of the two different metals.

This method suffers from the disadvantage that the individual blanks must first be cut to size, and then superimposed accurately upon each other before the final forming of the electrode. In an alternative process, the electrodes are formed from a rod of material having a core of the thermally-conducting metal and a jacket of the refractory metal, the rod having been formed by extrusion. When the rod has been cut to a desired length, the refractory metal must be sealed around the end of the rod which will eventually project from the spark plug. The development of a process for sealing the end of the rod has given rise to a number of difficulties because the process must not only produce a satisfactory seal but must also be adaptable to cheap high-speed operation so that the technique can be used in mass production of the electrodes.

According to the present invention, we provide a method of manufacturing a centre electrode for a spark plug comprising the step of capping one end of a rod having a core of thermally conducting metal and a tubular sheath of refractory meta' by securing thereto a body of a refractory metal by electrical pressure welding.

The expression "electrical pressure welding" refers to the process whereby two metal parts are held in abutment with each other under pressure and an electrical current is passed through the two parts such that the heat generated by the passage of current therebetween is sufficient to cause fusion of the parts at their region of contact. Suitable processes for use in the present invention therefore include spot welding, butt welding and projection welding.

The electrode produced by the process of the invention therefore incorporates an end cap of refractory metal. The end cap may be formed prior to the electrical pressure welding process, for example by cropping from a wire or rod of the refractory metal. Alternatively, the end cap may be formed after the welding process, for example, by electrical pressure welding a wire or rod of the refractory metal to the rod which forms the body of the electrode, and severing the rod of refractory metal adjacent the welded joint.

The end faces of the rod and the body of the refractory material will usually be flat, prior to welding. In some cases however, one or both these faces may define an axially extending circumferential lip which is held in abutment with the other face during the electrical pressure welding.

If desired, the body from which the end cap is formed may have a diameter smaller than the external diameter of the sheath, but larger than the internal diameter thereof so that any "spew" which forms around the body during the welding process does not project outwardly beyond the edge of the rod. The presence of such outwardly-projecting deposits of material could interfere with the installation of the centre electrode in the spark plug. Preferably however, the body from which the end cap is formed is similar in diameter to the external diameter of the sheath. Any spew produced during welding, or any bulging of the rod in the region of the joint with the end cap may be removed, if necessary, by a separate operation such as rolling or grinding.

The core of the rod may be formed form any highly thermally conductive metal such as copper, brass or silver and the end cap and sheath may be made from any refractory metal such as stainless steel, nickel, nickel-chromium alloys or molybdenum alloys. Other suitable materials will be know to those skilled in the art.

Since the end cap is a separate component of the electrode from the sheath, the material from which the end cap is made need not be the same as the material from which the sheath of the rod is made. This is of particular advantage because, in use, the conditions to which the tip of the central electrode is subjected are different from and generally more severe than those to which the shank of the electrode is subjected. The materials of the end cap and sheath can therefore be chosen to match the different conditions to which they are subjected.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing in which: Figure 1 is a partial axial cross-section through a spark plug incorporating an electrode in accordance with the invention; Figure 2 is a side view of the central electrode of the spark plug of Figure 1; Figure 3 is an axial cross-section through the electrode of Figure 2 prior to assembly; and Figure 4 is an axial cross-section through an alternative end cap for an electrode in accordance with the invention.

Referring to the drawings, the spark plug shown in Figure 1 comprises a hollow metal body 1 adapted to be screwed into an opening in an engine block and carrying a fixed electrode. The body 1 receives an insulator 3 of ceramic material which is itself hollow. A conductor 4 extends through the centre of the insulator 3 from an exterior terminal 5 to a central chamber 6 which is packed with a particular conductive material 6a, such as a mixture of copper and glass powders. An axially elongated centre electrode 7 is mounted in the opposite end of the insulator 3 from the terminal 5 in the electrical contact with the material in the central chamber 6. The centre electrode projects from the end of the insulator 3 and its exterior end is spaced from the end of the fixed electrode 2 by a spark gap 8.

Referring to Figures 2 to 4, the centre electrode 7 comprises a body portion 10 which is formed from a rod having a cylindrical core 11 of electrically conductive material such as coppoer, and a tubular sheath 12 of a refractory metal such as the nickel-chromium alloy sold under the trade name "Inconel". Such alloys typically have the following composition by weight: Nickel: at least 75% Chromium: 14to16% Iron: 6to8% Manganese: 0.1 to 0.5% Silicon: 0.1 to 0.5% Carbon: 0.04to 0.15% The rod may be formed by an extrusion process and, in the present embodiment has an external diameter of about 2.75mm, the internal diameter of the sheath 12 being about 1.7 mm. The rod is the cropped into lengths of about 25 mm. A head 14 is then formed on one end of each of the cropped lengths using a conventional head die.The terminal part of the head 14 is then crimped to form a flat projection 16 having indentations 18 in each face.

A disc-shaped end cap 20 is then formed from the rod by stamping from a sheet of refractory metal or by chopping from a rod of the refractory metal. If desired, the metal from which the end cap is formed may be the same as that from which the sheath of the body portion 10 is formed. Alternatively, a different metal, for example an alloy of greater temperature resistance, may be selected. In the embodiment illustrated, the end cap is equal in diameter to the sheath 12. In an alternative embodiment (not shown), the diameter of the end cap 20 is smaller than the external diameter of the sheath 12, but larger than the internal diameter thereof.

The end cap 20 is the positoned on the tip of the body portion 10 with its axis aligned with the axis of the body portion 10 so that the end cap 20 overlies the tip of the core 11. The body portion 10 and the end cap 20 are then connected into an electrical circuit and a current is passed through the body portion and the end cap whilst they are held in engagement under an axial compressive force. The electric current is selected to be sufficient to generate enough heat to fuse the material of the end cap and the body portion at their point of contact. The current is then switched off and the fused material is allowed to solidify whilst the end cap and body are held in engagement so that the two components become welded together.This electrical pressure welding process may be effected in an automatic welding machine to which the end caps and body portions are supplied in the correct relative orientations from suitable handling equipment, e.g. magazine bowl feeders.

Figure 4 illustrates an alternative end cap 20 in which one face of the end cap defines an upstanding lip 24 which projects axially from the face of the end cap. In use the end cap is positioned in abutment with the tip of a body portion with the lip 24 engaging the end wall of the sheath 12. The end cap is then secured to the body portion by an electrical pressure welding process such as described above. The lip 24 localises the contact between the end cap and the sheath and concentrates the passage of electrical current between the end cap and the body at the line along which it is desired to join the components, thus facilitating the formation of the welded joint between the end cap and the sheath. If desired, a similar lip may be formed on the end face of the body portion 10, for example by counter-boring the end face.

Claims (9)

1. A method of manufacturing a centre electrode for a spark plug comprising the steps of capping one end of a rod having a core of thermally conducting metal and a tubular sheath of refractory metal, by securing thereto a body of refractory metal by electrical pressure welding.

2. A method according to Claim 2 wherein the body of refractory material has a diameter smaller than the external diameter of the sheath, but larger than the internal diameter of the sheath.

3. A method according to Claim 1 or Claim 2 wherein the body of refractory material is composed of a different refractory metal from the sheath.

4. A method according to any one of Claims 1 to 3 wherein one of the end faces of the body or the rod which are held in abutment during the electrical pressure welding defines an axially extending circumferential lip.

5. A method according to Claim 4 wherein the said lip is formed on the end face of the body of refractory material.

6. A method according to any one of Claim 1 to 5 wherein the body of refractory metal comprises an end cap formed prior to the electrical pressure welding.

7. A method of manufacturing a centre electrode for a spark plug substantially as hereinbefore described with reference to the drawings.

8. A centre electrode for a spark plug produced by a method according to any one of the preceding claims.

9. A spark plug for an internal combustion engine incorporating a centre electrode according to Claim 8.