Spot Welding

Questions and Answers

Resistance welding tapers are precision tapers for attachment of electrodes to resistance welding holders. They are meant for quick and easy attachment and removal. If the weld forces used in the application are excessive for the size of the taper, the male taper can be driven into the female taper and ruin the tooling. Electrode removal may become difficult or impossible and further use of the holder is compromised. RWMA Fig 18.3 in the RWMA Manual 4th Edition, shows the relationship of electrode weld face size, applied force and maximum capability.

Heat balancing is adjusting heat input so that the nugget forms at the center or more importantly the faying surface. Resistance welding two different work piece materials will often cause the heat to build up in one before the other. The same is true if the thicknesses are unequal. This frequently leads to the nugget forming mostly in the hottest side of the part instead of at the faying surface.

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.

   Cosmetic Weld Nugget 1

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.

Reference RWMA Resistance Welding Manual 4th Edition

 

 

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.

Have a Question?

Do you have a question that is not covered in our knowledgebase? Do you have questions regarding the above article? Click here to ask the professor.