This invention relates to resistance welding. More particularly, this invention relates to a method of manufacturing a welding tip for resistance welding, to a welding tip for resistance welding, to a method of manufacturing stock for a welding tip for resistance welding and to stock for a welding tip for resistance welding.

According to a first aspect of the invention, there is provided a method of manufacturing a welding tip for resistance welding, the method including the steps of: forming stock having at least two concentric metallic layers; parting the stock into desired lengths, with each length having the concentric metallic layers; and applying a forming process to each length of stock so that a front end of each length defines a contact surface for resistance welding.

The stock may be formed by drawing a circular cylindrical tube of a first metallic material over a circular cylindrical rod of a second metallic material. At least one of the rod and the tube may be deformed in the drawing to retain the rod in the tube once the tube has been drawn over the rod. Thus dimensions of the rod and the tube may be preselected so that deformation of one of the rod and the tube occurs to retain the rod in

the tube once the tube has been drawn over the rod.

Instead, the stock may be formed by extruding a circular cylindrical tube of a first metallic material over a circular cylindrical rod of a second metallic material. Further instead, the stock may be formed by extruding a circular cylindrical tube of a first metallic material and simultaneously extruding a rod of a second metallic material, the tube being extruded over the rod and concentrically therewith.

The method may include the step of shaping each length of stock so that a rear end of each length defines a mounting for mounting the welding tip on to a resistance welding tool.

The forming process applied to each length of stock for defining the contact surface may include cold forging. The shaping of each length of stock to define the mounting may also be by means of cold forging.

A cavity may be formed in each length of stock, the cavity opening at the rear end of the length of stock. Then, an inner, cavity- defining surface of the length of stock may be shaped to define the mounting for mounting the welding tip on to a resistance welding tool. The cavity-defining surface may be shaped by means of cold forging.

The contact surface of each length of stock may be formed in a first operation, the cavity may be formed in a second operation, and the inner cavity-defining surface may be shaped in a third operation. Instead, the contact surface and the cavity may be formed and the cavity-defining surface shaped in a single operation.

The method may include drawing the tube over the rod in the absence of a lubricating medium between the rod and the tube.

According to a second aspect of the invention, there is provided a welding tip for resistance welding, the welding tip including a metallic body having a passage defined therein; and a discreet metallic core, the core being secured to the body, in the passage, with the front end of the body and a corresponding front end of the core being shaped to define a contact surface for resistance welding, and with at least the body defining a mounting formation to permit the body and the core to be mounted on a welding tool, the body and the core having been formed from a preselected length parted from a length of stock having at least two concentric metallic layers.

One of the body and the core may be of a material having a higher electrical conductivity than the material of the other. Further, one of the body and the core may be of a material having a higher tensile strength than the material of the other. Still further, one of the body and

the core may be of a material having a higher hardness than the material of the other.

The mounting formation may be defined by the body and the core.

The core may project from the front end of the body. Instead, the front end of the core may be substantially flush with the front end of the body.

The mounting formation may be in the form of a cavity defined in the core and opening at a rear end of the core and the body. Instead, the mounting formation may be in the form of a cavity defined in the body and opening at a rear end of the body. Then, an inner cavity-defining surface of the body may frusto-conical in shape, tapering inwardly from the rear end of the body. It is to be appreciated that the mounting formation can be of any suitable shape to suit the welding machine on which the welding tip is to be mounted.

The body of the welding tip may be of substantially pure copper.

The core of the welding tip may be of an alloy of copper. The alloy of copper may be one of the following:

(a) Copper-Chromium-Zirconium; (b) Copper-Zirconium; (c) Copper-Chromium; (d) Copper-Nickel-Tin-Chromium ; (e) Copper-Nickel-Cobalt-Beryllium; (f) Copper-Beryllium; (g) Sintered Copper-Tungsten; (h) Copper-Silver.

Instead, the core of the welding tip may be of an alloy of silver.

In a preferred embodiment of the invention, for use in resistance welding of aluminium, the body may be of substantially pure copper and the core may be of sintered copper-tungsten.

According to a third aspect of the invention, there is provided a method of manufacturing stock for the fabrication of resistance welding tips, the method including the step of drawing a circular cylindrical tube of a first metallic material over a circular cylindrical rod of a second metallic material.

At least one of the rod and the tube may be deformed in the drawing to retain the rod in the tube once the tube has been drawn over the

rod.

According to a fourth aspect of the invention, there is provided a method of manufacturing stock for the fabrication of resistance welding tips, the method including the step of extruding a circular cylindrical tube of a first metallic material over a circular cylindrical rod of a second metallic material.

According to a fifth aspect of the invention, there is provided a method of manufacturing stock for the fabrication of resistance welding tips, the method including the step of extruding a circular cylindrical tube of a first metallic material and simultaneously extruding a rod of a second metallic material, the tube being extruded over the rod and concentrically therewith.

According to a sixth aspect of the invention, there is provided stock for the manufacture of welding tips, the stock including a metallic body having an axially extending passage defined therethrough; and a discreet metallic core coterminous with the body, the core being secured to the body, in the passage.

The invention is now described, by way of examples, with reference to the accompanying drawings.

In the drawings, Figure 1 shows a side sectioned view of a first embodiment of a length of stock, in accordance with the invention, for welding tips for resistance welding; Figure 2 shows a side sectioned view of a welding tip, in accordance with the invention, for resistance welding, fabricated from the stock of Figure 1; Figure 3 shows a side sectioned view of a second embodiment of a length of stock, in accordance with the invention, for welding tips for resistance welding; Figure 4 shows a side sectioned view of a second embodiment of a welding tip, in accordance with the invention, for resistance welding, manufactured from the stock of Figure 3; Figure 5 shows a length of stock to be used in the manufacture of a welding tip for resistance welding according to a method of the invention; Figure 6 shows the length of stock in a first stage of the manufacture of the welding tip; Figure 7 shows the length of stock in a second stage of manufacture of the welding tip; Figure 8 shows the length of stock in a third stage of manufacture of the welding tip; Figure 9 shows a side sectioned view of a third embodiment of the welding tip; Figure 10 shows dimensions of the welding tip of Figure 4; and

Figure 11 shows a side sectioned view of a fourth embodiment of the welding tip.

In Figure 1, reference numeral 10 generally indicates a length of stock, in accordance with the invention, for manufacturing welding tips for resistance welding.

The length of stock 10 includes a circular cylindrical tube 12 and a cylindrical rod 14. The tube 12 is drawn over the rod 14 in a conventional drawing process with the absence of a lubricant between the rod 14 and the tube 12.

The tube 12 is of cathodic copper while the rod 14 is of a copper alloy. The tube 12 is of a more readily deformable than the rod 14.

Thus, as the tube 12 is drawn over the rod 14, the tube 12 is deformed. This, together with the fact that a lubricating medium is not used, results in the rod 14 being fixed within the tube 12.

The copper alloy can be chosen to suit a user's requirements.

Some examples of the copper alloy are: (a) Copper-Chromium-Zirconium (b) Copper-Zirconium (c) Copper-Chromium (d) Copper-Nickel-Tin-Chromium

(e) Copper-Nickel-Cobalt-Beryllium (f) Copper-Beryllium It will be appreciated that, instead, the rod 14 may be of a alloy of silver.

The stock 10 can be supplied in indefinite lengths or in predetermined lengths, depending on a user's requirements. The stock 10 can also be supplied in different diameters to suit a user's requirements.

In Figure 2, reference numeral 20 generally indicates a first embodiment of a welding tip, in accordance with the invention, manufactured from the stock 10.

The tip 20 includes a body 22 having a passage 23 defined therethrough. A core 24 is fixed within the body 22. A front end 26 of the body 22 is flush with a front end 28 of the core 24.

The front ends 26,28 of the body 22 and the rod 28, respectively, are shaped to define a contact surface 30 suitable for resistance welding. In particular, the front ends 26,28 are shaped to define a domed profile. Those skilled in the art will appreciate that the front ends 26,28 can be of any shape to suit a particular application.

A cavity 32 is defined in a rear end 34 of the core 24. The cavity 32 is formed so that a portion 36 of the core 24 is radially deformed to define a wall 38 which defines the cavity 32.

The cavity 32 is formed in a stamping operation while the body 22 is inhibited from moving radially outwardly. This results in a portion 40 of the body 22 surrounding the wall 38 being reduced in thickness.

The cavity 32 facilitates mounting of the tip 20 on to a welding tool lnot shownl.

In Figure 3, reference numeral 50 generally indicates a second embodiment of a length of stock, in accordance with the invention, for the manufacture of welding tips for resistance welding. With reference to Figure 2, like reference numerals refer to like parts, unless otherwise specified.

The tube 12 of the stock 50 has an outer diameter of approximately 16,0 mm. The rod 14 of the stock 50 has a diameter of approximately 6,3 mm.

In Figure 4, reference numeral 60 generally indicates a second embodiment of a welding tip, in accordance with the invention, manufactured from the stock 50. With reference to Figure 2, like reference numerals refer to like parts, unless otherwise specified.

The body 22 of the welding tip 60 defines the cavity 32. The cavity 32 is frusto-conical and tapers inwardly from the rear end 34 of the body 22.

The core 24 of the welding tip 60 is positioned in a passage 62 which extends from a front end 64 of the cavity 32 to open at the front end 26 of the body 22. The core 24 and the passage 62 define corresponding waisted transverse profiles. It follows that the core 24 is effectively locked in the passage 62. The front end 28 of the core 24 is proud of the front end 26 of the body 22 to an extent of approximately 0,2 mm.

In Figures 6 to 8, there are shown three stages of transformation of a length 66 of stock 50, shown in Figure 5, to the welding tip 60 during the manufacture of the welding tip 60. With reference to Figures 1 to 4, like reference numerals refer to like parts, unless otherwise specified.

In a first stage of manufacture, a front end 68 of the length 66 of stock 50 is cold forged so that the front end 68 has a curved axial profile as shown in Figure 6. In a second stage of manufacture, the length 66 is held in a die and a circular cylindrical cavity 70 is punched into a rear end 72 of the length 66. This results in elongation of the length 66 and the formation of the core 24 positioned in the passage 62. This also has the

effect of urging the core 24 forward to stand proud as described above. Further, as a result of the different characteristics of the rod 14 and the tube 12, the core 24 and the tube 12 are deformed so that the core 24 and the passage 62 define the complementary waisted axial profiles described above.

In a third stage of operation, the length 66 is held in a die and a tapering tool is pressed into the cavity 70 to form the frusto-conical cavity 32 described above. This results in further elongation of the tube 12 so that the body 22 reaches a desired length at this stage.

In Figure 9, reference numeral 70 generally indicates a third embodiment of a welding tip, in accordance with the invention, manufactured from the stock 50. With reference to Figures 2 and 4, like reference numerals refer to like parts, unless otherwise specified.

The welding tip 70 has the same general shape as the welding to 60 of Figure 4, and is manufactured in the same general process described above in respect of the tip 60, illustrated in Figures 5 to 8, with the exception that in forming the cavity 32, a slug 72 of the material of the body 22 is urged behind a rear end 74 of the core 24.

The following table give typical dimensions of the tip 60, illustrated in Figure 4, with reference to Figure 10:

A 16. Omm B 20. Omm C 8. Omm D 6. 5mm E 10. 3mm F 12. 7mm G 0. 2mm It will be appreciated that the above dimensions are for example purposes only, and may be varied to suit user requirements.

In Figure 11, reference numeral 80 generally indicates a fourth embodiment of a welding tip, in accordance with the invention, manufactured from the stock 50. With reference to Figures 2,4, and 9, like reference numerals refer to like parts, unless otherwise specified.

Again, the welding tip 80 has the same general. shape as the welding to 60 of Figure 4, and is manufactured in the same general process described above in respect of the tip 60, illustrated in Figures 5 to 8, with the exception that in forming the contact surface 30, a front portion 82 is tapered to provide a more pointed profile than is the case with the tip 60.

It is a problem with welding tips that they wear out at a rate which results in excessive downtime of welding equipment and resultant high costs. The reason for this is that, generally, welding tips are of a

metal which has a high electrical conductivity. Such a metal is cathodic copper. Unfortunately, cathodic copper is of a relatively low tensile strength and relatively low wear resistance. Consequently, it tends to wear out at an unacceptable rate.

It has been found that by using welding tips of a copper alloy the wear rate can be reduced. However, copper alloys do not have the high electrical conductivity of copper and this can result in certain problems.

By having the core 24 of the copper alloy and the body 22 of cathodic copper, the applicant believes it can obtain a suitable electrical conductivity together with adequate strength. Furthermore, the applicant believes that by using the method of this invention, the stock 10 can be produced at a relatively low cost.