GB2343398A

GB2343398A - Welding electrode assembly

Info

Publication number: GB2343398A
Application number: GB9925888A
Authority: GB
Inventors: Brian Duffy
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1998-11-06
Filing date: 1999-11-03
Publication date: 2000-05-10
Anticipated expiration: 2019-11-03
Also published as: GB2343398B; GB9824301D0; GB9925888D0

Abstract

A weld gun assembly comprises an aluminium gun arm (2) and a copper electrode/adapter holder (4) in which the copper electrode/adapter holder is a shrink fit in the aluminium gun arm. The ingress of air or moisture between the surfaces is inhibited and, as a result, the possibility of arcing between the surfaces is greatly reduced.

Description

Weldine Apparats The invention relates to an improved welding apparatus, and, in particular, relates to an improved weld gun assembly for use in resistance spot welding.

Known resistance spot welding gun assemblies, e. g. large scissor types, comprise large aluminium gun arms with a copper electrode/adapter holder mounted at one end of each gun arm. The copper electrode/adapter holder has an interface separator plate. The plate is a bimetallic of copper and aluminium. The copper electrode/adapter holder and the interface plate are a press fit or screwed into the end of the gun arm.

In such large lightweight scissor type resistance spot welding gun assemblies arcing and surface corruption can occur across aluminium/copper joints. Such arcing and surface corruption can significantly reduce the working life of the welding gun assembly and as such is highly undesirable. In particular, there will be arcing at the aluminium to aluminium interface between the interface separator plate, causing major surface disruption to the aluminium gun arm. This eventually loosens the aluminium-copper joint, leading to increased arcing. Water may now ingress into the now loose joint. The effect of the water accelerates the corrosive action to a position where current is unable to flow through the interface surfaces.

The present invention has as an advantage that such defects are eliminated or substantially reduced.

According to a first aspect of the present invention a weld gun assembly comprises an aluminium gun arm and a copper electrodeladapter holder wherein the copper electrode/adapter holder is a shrink fit to the aluminium gun arm.

According to a second aspect of the present invention a method of assembly of a weld gun assembly comprises the steps of providing an aluminium gun arm having a recess, providing a copper electrode/adapter holder, heating the aluminium gun arm and cooling the copper electrode/adapter holder, inserting the cool copper electrode/adapter holder in the recess of the aluminium gun arm and allowing the gun arm to cool and the electrode/adapter holder to warm such that the electrode/adapter holder is a shrink fit within the recess of the gun arm.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 shows, in schematic form, components of a weld gun assembly in accordance with the present invention prior to assembly; Fig. 2 is a view similar to Fig. 1 showing the components in a condition for assembly; and Fig. 3 shows the components of Fig. 1 in an assembled condition.

Each of the Figures shows, in cross-section, an end portion of an aluminium gun arm 2, together with a copper electrode/adapter holder 4 for manufacturing a weld gun assembly 6. Like reference numerals are used to refer to like parts throughout. The aluminium gun arm 2 is provided with a recess in the form of a bore 8 having a continuous bore surface 10 and a step 12. A passageway 13 extends from the bore 8 through the aluminium gun arm 2.

The copper electrode/adapter holder 4 is substantially cylindrical in form, having a continuous side wall surface 14 and front and rear end surfaces 16,18.

Pipe connections 20 for a liquid coolant are shown extending from the rear end surface 18 of the electrode/adapter holder.

When considered prior to assembly, and within a range of normal ambient temperatures, a diameter d of the bore 8 is less than a outer diameter D of the copper electrode/adapter holder 4-(Fig. 1).

To assemble the aluminium gun arm 2 and the copper electrode/adapter holder 4 together, it is necessary to cryogenically cool the copper electrode/adapter holder and to heat the aluminium gun arm. Due to the nature of the respective thermal expansion coefficients of the materials, the bore 8 of the aluminium gun arm will expand while the copper electrode/adapter holder 4 will shrink. The pipe connections 20 are fed into the passageway 13 as the copper electrode/adapter holder is placed inside the bore 8 of the aluminium gun arm or may be connected subsequently.

As the components return to normal ambient temperature, the gun arm and the electrode/adapter holder 4 will attempt to revert to their respective sizes and in doing so expel any air from between the surfaces 10,14; 12,16 of the bore 8 and the electrode/adapter holder 4. The bore surface now tightly grips the outer surface of the electrode/adapter holder to the extent that the ingress of air or moisture between the surfaces is inhibited and a good electrical connection is maintained, even when the aluminium arm is at a working temperature and the greater coefficient of expansion of aluminium over copper has reduced the grip slightly. Moreover, the exclusion of air and water is such that even in damp conditions, there is substantially no electrolytic corrosion due to the use of different metals. Hence, the possibility of arcing between the surfaces is greatly reduced.

It will be understood that the term"aluminium"also includes aluminium alloys.

Claims

CLAIMS 1. A weld gun assembly comprising an aluminium gun arm and a copper electrode/adapter holder wherein the copper electrode/adapter holder is a shrink fit to the aluminium gun arm.
2. A weld gun assembly substantially as described herein with reference to and as illustrated in the accompanying drawings.
3. A method of assembly of a weld gun assembly comprising the steps of providing an aluminium gun arm having a recess, providing a copper electrode/adapter holder, heating the aluminium gun arm and cooling the copper electrode/adapter holder, inserting the cool copper electrode/adapter holder in the recess of the aluminium gun arm and allowing the gun arm to cool and the electrode/adapter holder to warm such that the electrode/adapter holder is a shrink fit within the recess of the gun arm.
4. A method of assembly of a weld gun assembly substantially as described herein with reference to and as illustrated in the accompanying drawings.