GB2096039A - Spot welding of metal components - Google Patents

Abstract

Two metal sheets (1, 3), at least one of which has a surface pre-coated with paint A or similar insulating material, are attached to one another by initially forming an electrical current path between them at a first location (4), melting a region of the coating at a second location (W) by passing a current through the said current path, and spot welding the sheets together at the said second location. The initial current path may be formed either by means of a shunt (4) or by removing part of the coating and spot welding the sheets in the conventional manner. <IMAGE>

Description

SPECIFICATION The attachment of metal components This invention relates to the attachment of metal components together by spot welding, and is espe call;, though not exclusively concerned, with the fabrication of domestic appliance cabinets such as are used as the enclosing casings for washing machines, dishwashers and clothes dryers. The use for such a purpose of pre-coated steel panels, that is to say panels which have previously been given a coating of paint or other insulating material of a similar nature, has the advantage that subsequent painting of the assembled cabinet is not required, but it has been found difficult to spot weld such panels satisfactorily to other components without first locally removing the coating material from the positions at which the welds are to be made.Similar difficulties arise in the case of different forms of pre-coated components required to be spot welded together orto other metal components.

Where one of the two components to be welded has an outwardly facing surface which is either uncoated or has a conducting coating it may be possible to effect a weld by utilising a pair of electrodes operating adjacent one another against that surface of the component, the weld current travelling through the component and providing sufficient heat where the components come into contact under pressure at a position intermediate the electrodes, for example at a raised projection, to provide the necessary fusion. However such a process is somewhat cumbersome, requiring additional means for maintaining the required pressure and also necessitating accurate location of the electrodes with respect to the pressure point between the components.Another method requires the use of a special 'U'-shaped clip which is first fitted over the adjacent edges of two components to be welded, in order to provide a current path for the welding operation, but this method has the disadvantage of needing an extra element which becomes a permanent part of the welded structure, and the clip has additionally to be accurately positioned for welding purposes.

Our co-pending patent application No. 8110509 provides a welding system which enables the ready spot welding together of two components, at least one of which has a pre-coated surface facing the other component, without the need for additional elements and with the advantages of allowing free location of the spot welds and use of simple electrode techniques.

According to the invention two metal components, at least one of which has an electrically insulating coating abutting the other component, are attached to one another by initially forming an electrical current path between the components at one location, applying spot welding electrodes to the components at a second location, causing an electric current to flow between the electrodes through the said current path to melt or otherwise disperse a region of the said coating at said second location and then spot welding the components together at the said second location.

Thus it has been found that by providing a current path between the components the spot welding at the said second location can be initiated, using this current path, to the point where sufficient heat is generated to remove any coating intermediate the two components, thereby to allow full welding current to flow and a resistance heat welded joint to be obtained without the need for previously removing the coating at the second location.

When one such spot weld has been formed this can provide a current path for subsequent welds and so on.

The initial current path, in accordance with one embodiment of the invention is achieved by means of a temporary shunt adjacent a region where a first spot weld is to be carried out, the shunt, which may be in the form of an electrically conducting clip engaging the outer surfaces of the components, providing a current path between the welding electrodes through the components, to produce sufficient heat within the components to remove the coating between the components at the spot weld ing position, thereby allowing the full welding current to flow and a resistance welded joint to be obtained.

Alternatively, the initial current path may be formed by forming a spot-welded junction between the components in a conventional manner, the coating material being removed prior to the initial welding operation. In some cases the initial welding process may be facilitated by forming a slight projection which bears against the other component at the position at which the initial weld is to be formed to ensure that under the pressure of the welding electrodes the required current path is established. The forming of such a projection may also facilitate removal of the coating at that point.

Having established an initial current path by either of the methods outlined above, or by any other convenient method, subsequent spot welds can be successively formed at one or more further locations by applying the welding electrodes to the outwardly facing surfaces of the abutting components at a said further location in the conventional manner, so that the heat generated by the initial current melts the region of the coating compressed between the welding electrodes. This enables a direct current path to be formed between the components at that location, resulting in a spot weld.

The said subsequent spot welds should of course be sufficiently near the initial current path between the components to enable adequate current to be carried between the welding electrodes, but the spacing and positioning of the welding electrodes is not critical.

As previously mentioned the invention has particular application for the attachment of pre-painted steel panels to one another or to other components in the manufacture of cabinets for washing machines, dish-washers, tumbler dryers and other similar domestic appliances, and use of the invention in the manufacture of a cabinet for a washing machine cabinet, involving the use of pre-painted steel sheet, will now be described by way of example with reference to Figures 1 to 3 of the accompanying schematic drawings, in which Figure 1 shows a perspective view of a part of the cabinet incorporating two sheet metal components welded together, and Figure 2 and 3 show enlarged sectional views of that part of the cabinet under construction, utilising alternative methods of forming an initial current path.

Referring first to Figures 1 and 2 of the drawings, the cabinet has two side panels 1 (only one of which is shown), formed of sheet steel pre-coated with paint layers A (Figure 2) on their external surfaces.

Each panel has an inwardly turned flange 2 at its rear edge and this flange is attached to a further panel 3 of galvanised steel, which provides the rear of the cabinet, by a spot welding process in accordance with the invention.

The initial current path is first established, say at X (Figure 1), by a spot weld having first removed the pre-coated paint layer A over a minimum area. Any metallic protective coating, such as a zinc galvanised finish on the internal surfaces of the side panels 1 will not interfere with the spot welding operation.

Other protective coatings on the internal surfaces of the side panels, if not themselves of metal, may include a metallic component. If the paint coating, typically between 0.003 inch and 0.004 inch in thickness is especially heavy then a slight projection, as at B (Figure 2), may need to be formed at the point of welding to ensure that a current path is established under the normal pressure exerted by the welding electrodes W. Once the initial spot welded junction has been made at X the next spot weld can be carried out at Y1, and subsequently at Y2 and so on to Yn, without the need to remove any more of the painted coating at these locations. Consequently precise positioning of the welding electrodes W when forming these spot welds is not necessary.

In the example described utilising pre-painted panels of 21 gauge sheet steel having a paint thickness of between 0.003 and 0.004 inch, the spacing of the welds is conveniently between 2 and 6 inches, although the spacing is not critical and other spacings may be employed depending upon the particular application of the invention and the weld strength required.

Instead of the initial current path being formed by removal of the pain layer at X it may alternatively be established by means of an electrically conducting clip 4 engaging the outwardly facing surfaces of the panels adjacent the welding electrodes W where the first spot weld is to be effected as shown in Figure 3.

Current between the welding electrodes will initially flow as indicated by the arrows through the panels 1, 3 and the conducting clip 4. This is sufficient to melt the paint on the region of the panel 1 between the welding electrodes W, a conventional spot weld then being produced. Subsequent welds can then be made as above described.

Claims (5)

1. A method of attaching two metal components to one another, at least one of which has an electrically insulating coating abutting the other component, comprising initially forming an electrical current path between the components at one location; applying spot welding electrodes to the components at a second location; causing an electric current to flow between the electrodes through the said current path to melt or otherwise disperse a region of the said coating at said second location; and spot welding the components together at the said second location.

2. A method according to Claim 1 in which a section of the said initial current path is constituted by a shunt urged against respective contact-making surfaces of the said components.

3. A method according to Claim 1 in which a spot weld constitutes a section of the said initial current path.

4. A method of attaching two metal components substantially as described hereinabove with reference to the accompanying drawings.

5. A washing machine cabinet incorporating at least one pre-painted steel sheet which has been welded to another steel sheet by a method according to any preceding Claim.