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Weld nuts and weld studs are commonly used to provide a means for subsequent fastening of additional components and assemblies, or for periodic removal of service parts for maintenance and repair. When specifying welded fasteners, care should be taken not to tightly tolerance concentricity or perpendicularity to a datum plane, since this drastically increases cost. Weld fasteners located by holes punched by prior stamping operations are an accurate and generally preferred location method. For maximum cost-effectiveness, select weld nuts and studs of one size that will be used throughout the assembly. This helps to keep set-ups to a minimum and increases manufacturing throughput. Nuts located by holes are typically within ±0.006 (0.15 mm) of the original hole location. Studs can be located to ±0.020 (0.51 mm) with simple fixturing. Closer tolerances require more sophisticated and costly fixtures.

Start off by drilling 7.5mm holes in the front sheet of metal at a spacing of normally 25mm to 40mm (or whatever the original spot weld spacing was). Then clamp this sheet onto the back sheet. 7.5mm is a reasonably good hole size for 0.8 or 1.0mm sheet. Thicker sheet might require a slightly larger hole size. Try a little test piece out like this one before welding a whole sill onto a car and check the weld has penetrated through both sheets.

Spot welding is a resistance welding process that is used primarily for welding two or more metal sheets together by applying pressure and heat to the weld area. It works by contacting copper alloy electrodes to the sheet surfaces, whereby pressure and electric current are applied and heat is generated by the passage of current through resistive materials such as low carbon steels. See more info at 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.