Spot Welding

Questions and Answers

Spot welding electrodes have a large influence on the resistance welding process. Their alloy and face size contribute to the development of the proper weld nugget. All of this assumes that the electrodes have been installed properly. This means proper water cooling and water tube location and good alignment. Alignment means that the face of the electrode is square and parallel to the part being welded. In addition the upper and lower electrodes faces should line up vertically with each other. The goal is to apply all forces on the same central axis. This prevents the electrodes from imparting and twist or distortion into the part during the welding operation.

Improper weld current selection can cause problems during resistance welding. The problems normally show up in the form of expulsion, indentation, electrode sticking, mushrooming or low weld strength. There is an ideal current range for every resistance weld set up. This will produce good weld nuggets with little or no expulsion, modest indentation and minimal electrode wear or sticking. Whenever you push the extremes of the current in the weld window either high or low you risk problems.

Electrode face mushrooming is the normal wear mechanism of a spot welding electrode. To slow this process or prevent excessive mushrooming one must control several factors in the welding process.

 Mushroomed electrode

EXTREME MUSHROOMING

Back up electrodes are used for projection welding electrodes. They are normally used on the lower side and are flat faced. There is no need for a defined weld face since the projection concentrates the weld current and heat into a desired spot for nugget creation.

Some resistance welding applications call out forces above the limits of tapered electrodes. The RWMA Manual Fig 18.3 shows force vs taper. It shows that above 2400 Lbs force tapers are not viable. Above this force specially designed High Pressure/High Force Electrodes must be used. These electrodes can be straight shanks, threaded or flanged electrodes. They are available from several manufacturers. Contact your electrode supplier for assistance.

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