Page 25 - MetalForming July 2014

welds and rejected parts.
While metalformers can select from
several types of tip dressers, the best method of maintaining the proper weld-face diameter is to remove the electrodes from the machine after a predetermined number of welds, long before they’ve mushroomed out of con- trol. Dress the electrode tips offline with a lathe or drill press equipped with the proper cutter blades.
8) Design for resistance welding.
Metalformers can reference charts listing minimum contacting overlap, and designers of spot-welded sheet- metal parts should include adequate flange widths to avoid expulsion, which lessens weld strength. Additionally, carefully consider weld spacing. Plac- ing spot welds too closely together can result in shunting. Here, some of the welding energy is lost through adjoin- ing welds, possibly resulting in sub- standard weld strength.
Springback of the parts being weld- ed also can plague the RSW process. Using a portion of the welding machine’s available weld pressure to force the parts together can compro- mise proper forging of the weld.
9) Purchase steel of known quality.
RSW problems sometimes can be traced back to a coil of steel purchased at an attractive discount, but of dubious quality. High carbon content can cause serious welding problems, as can inconsistent coating thickness. If the welding machine is not adjusted prop- erly to accommodate these variations, welds that look good may fall apart.
10) Schedule regular preventive maintenance.
RSW machines typically don’t see the regular preventive maintenance (PM) they deserve. Machine perform- ance degrades over time, due to corro- sion and/or arcing of the numerous
Since most RSW quality issues can be traced directly to electrode wear, metal- formers cannot afford to neglect elec- trode tips. Dressing or exchanging tips on a regular basis will pay big dividends.
copper-to-copper secondary connec- tions carrying current from the trans- former to the tips. Therefore, metal- formers should disassemble, clean and tighten the entire secondary loop at least once per year. They also should regularly examine the force-delivery system and repair any air leaks and mechanical wear. MF
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