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.
When welding galvanized and other coated materials a layer of oxides and brass builds up on the face of the electrode. During the first twenty five to fifty welds while this layer builds up and develops equilibrium, the welding results can be inconsistent. This period is called conditioning.
All spot welding electrodes mushroom during use and when this happens the resultant weld eventually is unacceptable. Before this happens most operations either dress or change the electrode to get the face back to original operating conditions.
Spot welding electrodes require dressing or conditioning back to original face geometry whenever the resultant weld nugget is getting smaller and approaching an unacceptable size. This is determined through quality checks of the product or coupons as determined by your quality program. Most checks are made by destructive means. The two sheets of metal are clamped in a vise and chiseled or pulled apart. The average of the measured length and width of the resultant pulled nugget is the value being sought.
The quality system normally will specify a minimum which must be maintained. In some facilities tensile tests are specified and similar samples are pulled and tested at specified intervals to insure quality. The need for dressing the electrode and testing of product applies not only to spot welding but also projection and seam welding.
Dressing an electrode means mechanically machining or forming the face of the worn electrode back to its original geometry. All resistance welding electrodes wear during use. In the case of spot welding this appears in the form of what is referred to as mushrooming. The weld face of the electrode begins to spread out perpendicular to the length of the electrode.
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