Recommended spacing between welds and distances from a spot weld to component edges and other part features should be followed to obtain optimum weld quality and strength. Weld-to-weld spacing should be a minimum of 10 material thicknesses. For 0.060 sheet steel, that’s about 0.6 in. (15 mm). Ideally, 20 times material thickness is recommended to reduce shunting effects with a minimum spacing of 1/2 in. (13 mm). See Table I for minimum and recommended weld spacing for various material thickness of aluminum, carbon and stainless steel. Weld-to-edge distance should also conform to a minimum dimension that is a function of the weld diameter. Generally, the center of a spot weld (its location point) is positioned one to two diameters away from the edge of the part being welded or from a feature in the part being welded depending on thickness of material.

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 extra info on https://www.weldingsuppliesdirect.co.uk/welding-equipment/spot-welding.html.

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).