Projection Welding
Questions and Answers
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.
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.
Force is used for two main reasons. One is to contain the weld nugget and prevent expulsion and the other is to strengthen the weld by forging the solidifying weld nugget. It is easy to see that if welding force is changed these actions will be affected to some degree. Expulsion could increase or decrease and the resultant weld strength could also increase or decrease.
A weld schedule is an instruction of how to set up a resistance welder for a specific job. It includes all of the specific settings of the machine, control settings, tools, electrodes, forces, times, and rates and all other pertinent data related to a job. This information is stored and used to set the job up the next time you run it. It can be used as a ground zero if parameters have changed and you want to go back to where you started.
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