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.
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.
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. Read more details at Tecna Spot Welder Parts.
Stationary spot welders are mounted on a column with a jack and mobile arms. The frame is heavy and bulky and the machine has a welding capacity of 0.5–10mm. The body of the machine often equipped with a 380V generator and two arms with a mechanical, pneumatic or hydraulic jack to adjust operating height, as well as heat-resistant electrodes made of copper or copper-chromium alloy. No electric arc is produced, and the arms are water-cooled in closed circuit. The arms and electrodes are interchangeable to vary spot size and form across different types of weld.
