Squeeze-Type Resistance Spot Welding (STRSW)
STRSW is the OEM's primary structural joining method in unitized bodies. The exam tests process variables, the destructive tests that prove a weld, and the weld-count rules that make a replacement panel equivalent to the original.
Process variables
A spot weld is made by squeeze force, current, and time. Force is applied before current (squeeze) and held after it (hold), so the nugget solidifies under pressure. Medium-frequency DC equipment with closed-loop control is now common, and transformer-based units are not interchangeable with it. Tips must be aligned, dressed, and correctly sized - a mushroomed tip spreads current and shrinks the nugget.
Proving the weld
Nugget diameter is the acceptance measure, commonly about 4√t where t is the thinner sheet thickness. Destructive checks are the peel, chisel, and plug tests; a sound weld tears a button from parent metal rather than separating at the interface. Weld-through primer and structural adhesive must stay within OEM allowance, and shunting through existing nearby welds steals current from the new nugget.
Worked example
An OEM procedure for a quarter-panel replacement calls for 24 welds at a stated spacing plus structural adhesive. The tech installs 12 welds and skips the adhesive because the panel "fit tight." That repair fails procedure compliance: weld count, spacing, and adhesive form one validated system, and half the welds roughly halve joint fatigue life. The correct action is to follow the procedure and document it.
Traps
Judging a weld by its appearance or the size of the heat mark; ignoring shunting when welds sit close together; not dressing tips between panels; assuming all STRSW machines deliver the same waveform.