Spot Welding

Questions and Answers

A cosmetic weld is a resistance weld on a surface where a typical weld indentation would not be permitted. The middle of the hood of your car is a good example. Welds on medical instruments would be another case. To prevent these indentations from showing most of the heat input must be made from the other side of the part. Generally “C” flat faced electrodes are used on the cosmetic side and care is taken to not over heat the part. Usually the weld nuggets are run in the lower range of the quality specification to reduce the heat affected area.

 

The proper electrode material can be determined from published weld schedules based on the work pieces being spot welded. These weld schedules can be found in AWS Standard C1.1, Recommended Practices For Resistance Welding and RWMA The Resistance Welding Manual. Your electrode supplier is another source. Spot welding requires a few initial decisions to be made to set up a good welding process. Most of these evolve around pressure, current and time.

Cooling is possibly the most important factor in electrode life. Proper water tube location will insure water flow as close as possible to the working face with the proper flow and temperature water. In each resistance welding application after the weld schedule is determined and the proper pressure, current and time (PCT) are set another important factor for electrode life is water cooling. Even with the proper setting every electrode will see very high temperatures and forces on its face. Over time this creates the traditional mushrooming of the weld face. To slow this face wear down we must reduce the time at high temperature that the weld face sees. Proper water cooling of the electrode is critical.

Water for the electrode should always come directly from the water manifold and not be plumbed to use water from another weld machine component. Cooling water is the most important item to insure electrode life. Therefore the amount of water and its temperature are very critical.

To adequately cool the electrodes we need to minimize their time at high temperatures so we need a lot of water flow.  RWMA recommends 1.5 gallons/minute at the coldest temperature water available. Any compromise does not mean you will produce discrepant welds. It only means you will not produce as many good welds before dressing of the electrodes is required. This means down time in your production schedule which means fewer parts out the door each day. The recommended water flow rate is 1.5 gallons of water/minute per electrode. You can successfully produce good welds at 1.0 gal/min and 0.5 gal/min.

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