Tecna cranked electrodes

If possible, spot welding of sheet metal components should be restricted to joining flat, coplanar surfaces. Spot welding for assembly of mating parts in multiple planes should be limited to parts smaller than a “bread box” that are easy to handle. With large, heavy parts, another fastening method, welding process or possibly a redesign should be considered unless production quantities support the initial expense of specialized spot welding equipment. Although single- and double-bend electrode tips are available to reach confined weld locations, a small flange dimension may restrict access, and thereby prevent a successful spot weld. Such is the case with C-shaped parts or U-shaped channels with short flanges (Figure 7). For instance, specifying a 1/4 in. (6 mm) diameter weld on a 3/8 in. (10 mm) flange not only violates spacing considerations, but also makes it very difficult for the operator to access the weld location.

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).

How Does Spot Welding Work? A form of resistance welding, spot welding is one of the oldest welding processes whereby two or more sheets of metal are welded together without the use of any filler material. The process involves applying pressure and heat to the weld area using shaped alloy copper electrodes which convey an electrical current through the weld pieces. The material melts, fusing the parts together at which point the current is turned off, pressure from the electrodes is maintained and the molten “nugget” solidifies to form the joint. See more info on Auto Body Spot Welder.

As is often the case with machine tools, there are two types: portable (for ease of use but with limited performance); and stationary (better suited to intensive work and thicker metal sheet).