Weld size (nugget diameter) is typically slightly less than the diameter of the impression the electrode creates on the material. These dimensions and other spot welding parameters are given in Table I for aluminum, carbon and stainless steel. For simplicity, such standards can be specified by the designer as the controlling print information on spot welds. Base metal strength and spot weld strength are interrelated. Table I gives realistic strength expectations for design purposes. For economy, avoid over-specification of welds.
Position the welding torch with the wire in the center of the hole contacting the back sheet of metal. It is important to arc against this back sheet rather than on the edge of the hole, otherwise the weld might not penetrate into the back sheet. The torch should ideally be pointing directly into the hole rather than at the angle in the photograph. Start welding in this position and don’t move the welder until the hole is almost full of weld. Then move the welder outwards in ever increasing circles until the weld is completed.
Copper and its alloys can also be joined by resistance spot welding, although spot welding copper cannot be easily achieved with conventional copper alloy spot welding electrodes, as heat generation in the electrodes and work piece are very similar. The solution to welding copper is to use an electrode made of an alloy with a high electrical resistance and a melting temperature far in excess of the melting point of copper (much greater than 1080°C). Electrode materials typically used for spot welding copper include molybdenum and tungsten. See extra details at Auto Body Spot Welder.
In the same category you also find generators, always operating on direct current, intended for car bodywork including repairing dented panels. The machine consists of a holder which lets you position your welding gear precisely in front of the deformed zone, and uses mechanical or hydraulic force to restore it to its original shape. All you have to do is crack out your hammer to fine-tune the finish. The “gougeonneuse”, as it’s known in France, can be used to mend tow rings and other specific parts. The huge advantage of being able to return a panel or component to its original shape without needing to disassemble the car body makes this machine a must-have tool for bodywork!