Spot welding machines can be categorized by application (universal/general-purpose, special-purpose); by the number of weld points simultaneously (single-point, double-point, multi-point); by the method of conduction (single-sided, double-sided); by the transmission method of the pressure mechanism (foot-operated, electric motor-cam, pneumatic, hydraulic, and combined pneumatic-hydraulic); by operating characteristics (non-automatic, automatic); by installation method (fixed, mobile, or portable (suspended); by the direction of movement of the moving electrode (usually the upper electrode) (vertical stroke, linear motion, circular stroke); and by power supply method (power frequency welding machines (using 50Hz AC power), pulse welding machines (DC pulse welding machines, energy storage welding machines, etc.), and variable frequency welding machines (such as low-frequency welding machines).
When the workpiece and electrodes are constant, the workpiece's resistance depends on its resistivity. Therefore, resistivity is a crucial property of the material being welded. Metals with high resistivity have poor conductivity (e.g., stainless steel), while metals with low resistivity have good conductivity (e.g., aluminum alloys). Thus, spot welding stainless steel is easier to generate heat but harder to dissipate, while spot welding aluminum alloys is harder to generate heat but easier to dissipate. During spot welding, the former can use a smaller current (several thousand amperes), while the latter requires a much larger current (tens of thousands of amperes). Resistivity depends not only on the type of metal but also on its heat treatment state, processing method, and temperature.
To ensure the weld nugget size and weld strength, welding time and welding current can complement each other within a certain range. To obtain a weld of a certain strength, a large current and short time (strong condition, also known as a hard specification) or a small current and long time (weak condition, also known as a soft specification) can be used. The choice between a hard and soft specification depends on the metal's properties, thickness, and the power of the welding machine used. There are upper and lower limits for the required current and time for metals with different properties and thicknesses; these should be used as references during application.
Electrode pressure has a significant impact on the total resistance R between the two electrodes. As the electrode pressure increases, R decreases significantly, while the increase in welding current is not substantial enough to offset the reduction in heat generation caused by the decrease in R. Therefore, the weld strength always decreases with increasing welding pressure. The solution is to increase both the welding pressure and the welding current.