Precise locations of spot welded parts is a cost-related process and should be considered during the design. Part positioning involves either extensive fixturing or, preferably, selfalignment through built-in stamped features like holes and tabs. With the latter method, the location is predetermined by the accuracy of alignment features. The most preferred and most easily achieved method for accurately self-fixturing parts is the half sheared or extruded cylindrical button and matching hole in the mating part. One mating hole should be 0.003 in. (0.08 mm) larger in diameter than the extrusion and the second hole should be slotted by 0.040 in. (1.02 mm) minimum to allow for normal fabrication tolerances as shown in the drawing. Another alternative is to produce a lanced tab in a punching process. Mating parts can then be brought up to it and located in position.
Position the welding torch with the wire in the center of the hole contacting the back sheet of metal. It is important to arc against this back sheet rather than on the edge of the hole, otherwise the weld might not penetrate into the back sheet. The torch should ideally be pointing directly into the hole rather than at the angle in the photograph. Start welding in this position and don’t move the welder until the hole is almost full of weld. Then move the welder outwards in ever increasing circles until the weld is completed.
Materials Appropriate for Spot Welding: Due to its lower thermal conductivity and higher electrical resistance, steel is comparatively easy to spot weld, with low carbon steel being most suited to spot welding. However, high carbon content steels (Carbon equivalence > 0.4wt%) are prone to poor fracture toughness or cracking in the welds as they tend to form hard and brittle microstructures. Galvanised steel (zinc coated) requires slightly higher welding currents to weld than uncoated steels. Also, with zinc alloys, the copper electrodes rapidly degrade the surface and lead to a loss of weld quality. When spot welding zinc coated steels, electrodes must either be frequently exchanged or the electrode tip surface should be ‘dressed’, where a cutter removes contaminated material to expose a clean copper surface and reshapes the electrode. See extra info on Tecna Spot Welder Parts.
What type of sheets can be welded? Rust-free, non-painted sheets of the same or different metals can be welded provided they are compatible alloys with a very similar melting point. Metals such as stainless steel, aluminium, steel alloys and galvanized steels can be spot welded, subject to operating adjustments (current, welding time, intensity of compression). Note that the coating on galvanized metals tends to clog the electrodes – which must be cleaned regularly!