Ideally, equal thicknesses of two sheet metal parts to be joined produces an evenly distributed weld nugget within the two layers. When this is not practical, materials of different thicknesses can also be joined and produce a centered weld nugget by using a larger electrode on the thicker member. At a ratio above about 3-to-1 (thickest to thinnest member), spot welding becomes difficult. At this point, another joining method should be considered–for example, projection welding. Note that weld deformation is always greater on the thinner member. For this reason, stiffeners and brackets spot welded to cosmetic parts should be thinner than or equal in thickness to the exposed surface 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.
Where is Spot Welding Used? Spot welding has applications in a number of industries, including automotive, aerospace, rail, white goods, metal furniture, electronics, medical building and construction. Given the ease with which spot welding can be automated when combined with robots and manipulation systems, it is the most common joining process in high volume manufacturing lines and has in particular been the main joining process in the construction of steel cars for over 100 years. Read more details on https://www.weldingsuppliesdirect.co.uk/welding-equipment/spot-welding.html.
Spot welding machines represent a significant long-term investment that can decline in value. Their high performance has led to widespread use in automotive manufacturing; their use can be automated by combining them with extremely fast, precise robots. Radiators can be welded by the same process except using rollers to obtain a continuous, watertight weld. Industrially made furniture and other functional metal objects such as lampshade frames are also spot-welded. The process gives an impressive quality of weld without deforming the components it is used on.