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.
If the sheets are the same thickness then the power setting used for plug welding would be the same as you would use for 1.5 times the thickness of one of the sheets. This is the sort of penetration you would expect from a plug weld. The molten pool is just breaking out of the reverse of the back sheet. The heat marks indicate the weld has arced against the back sheet rather than at the side of the hole. If you don’t get these marks then consider a little seam welding just to be sure. There is a special clamp designed for plug welding that makes life really easy. The parts you see in the photograph are attached to a normal mole grip. This clamp came in a set of three random welding clamps all of which are extremely useful.
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. Read extra info on Tecna Spot Welder.
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.