Automatic welding machine manufacturers have summarized the following pre-welding preparations for spot welding machines to achieve the ideal welding effect:
1. The grease and dirt on the upper and lower electrodes must be removed before work. Power on and check the electrical equipment, operating mechanism, cooling system, air circuit system and body shell for leakage.
2. The indoor temperature should not be lower than 15°C.
3. Before starting, turn on the control circuit changeover switch and the small welding current switch, insert the position of the pole number adjustment switch, turn on the water source, the air source, and the adjustment knobs on the control box. The electrode contacts are kept smooth and clean.
The source of heat required for welding to form a nugget is the same as that of spot welding, which uses the heat generated by the electric current through the resistance of the welding zone. Given other conditions, the size of the welding current determines the penetration rate and overlap of the nugget. When welding low carbon steel, the average penetration rate of the nugget is 30-70% of the thickness of the steel plate, preferably 45-50%. In order to obtain gas-tight seam welding nugget overlap should not be less than 15-20%.
When the welding current exceeds a certain value, continuing to increase the current can only increase the penetration rate and overlap of the nugget without increasing the strength of the joint, which is uneconomical. If the current is too large, defects such as deep indentation and welding burn-through will also occur.
Due to the overlap of the nuggets during the welding seam, a large shunt is caused. Therefore, the welding current is usually 15-40% higher than that in the spot welding. Thick plates or hardened materials sometimes cannot tear the round holes and bosses, but the diameter of the nugget can be judged by the cut fracture. When necessary, low-power measurement, stretch test and X-ray inspection are also required to determine the penetration rate, shear strength, and whether there are shrinkage holes, cracks, etc.