Several points must be observed when employing resistance welding (spot welding).
- Due to the improved electrical conductivity of galvanized sheet in comparison to blank steel sheet^ the current intensity will have to be increased to attain the required welding temperature of 1300 C/ 2372° F.
- The welding time should be as brief as possible to keep the melting zone around the welding spot small. This will also mean that less zinc will stick to the electrodes and thus longer operating times are assured before maintenance becomes necessary.
- Hard copper (copper-chrome-zirconium alloys) is the best material for electrodes. It has high heat physical properties (above 400 C/752 F); maximum service life up to 15, 000 spots.
- The electrode welding surface shape can be as in fig. 1 or fig. 2 (see sketch).
- Depending on the thickness of the material, the max. welding spot diameter should be 4 to 5 mm/ 5/32 to 3/16" depending on pressure applied to the electrode holder.
- Butt weld spots are not acceptable because these welding spots do not have sufficient strength.
- Cool-off periods must be scheduled if welding equipment without water cooling is employed, so that the electrodes are not deformed.
- Never make spot welds on edges of sheet metal (spatter formation).
- Clean and dress electrodes on their flanks and tip surface only if there is a heavy coat of deposits. The tip surface of the electrodes becomes harder from welding; the layer underneath however becomes softer.
- The distance between spots should be about 20 mm (3/4"). If the spots are too close together, the welding current will reduce the strength of the welding spots.
- Electrode arms should be as short as possible to increase the pressure at the weld point.
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