In applications where space is limited, specifying one weld can produce a stronger bond than two spots, which may be limited in size and integrity because of contraints in positioning, accessibility and shunting effects (current loss). Typically, diameters of spot welds range from about 1/8 in. (3 mm) to l/2 in. (13 mm) depending on the thicknesses of the workpieces and the material. When the size of spot welds is designated, the designer should specify only one size throughout an assembly in the interest of manufacturing economy and total part cost.
Plug welding is an alternative to spot welding used by vehicle manufacturers where there is insufficient access for a spot welder. For DIY car restoration it’s generally used instead of spot welding on panels flanges that would have originally been spot welded. Plug welds when done properly tend to be stronger than the original spot welds. Rally car builders often use the technique, and it is acceptable in a UK MOT test as an alternative to spot welds where repairing older cars (it would not be suitable for modern high tensile steels).
The welding heat is generated by the electric current, which is transferred to the workpiece through copper alloy electrodes. Copper is used for the electrodes as it has a high thermal conductivity and low electrical resistance compared to most other metals, ensuring that the heat is generated preferentially in the work pieces rather than the electrodes. The amount of heat depends on the thermal conductivity and electrical resistance of the metal as well as the amount of time the current is applied. Other materials commonly spot welded include stainless steels (in particular austenitic and ferritic grades), nickel alloys and titanium. See extra info on Tecna Spot Welder Arms.
To make sure your welding settings are correct, you should carry out a test run using metal offcuts, followed by a destructive test where you separate the welded parts with a hammer and a forked chisel. After separation, a hole must be made in one of the two conjoined sheets – a process known as “unbuttoning” in metalworking jargon – as proof that the settings are right for the metals and thicknesses involved. Apologies for the abstruse technical term! When welding radiators, make sure you also carry out a leak test.