Force is the "P" in PCT - Pressure Current and Time.The Force component of a weld schedule is required to hold the two work pieces being welded together during the welding operation. This prevents any arcing due to the parts being loose. If not held tightly molten material would be expelled during the weld operation. It also provides a forging action as the weld solidifies and cools. This forging action greatly improves the final weld strength.
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.
When a job is set up properly and the electrodes and water cooling are maintained it is still possible to have weld variation during a production day. It is very common that the line voltage at most production plants can vary during the day because of the demand on the Utility Company. The problem could be your feed to your plant or the Utility Company's main grid.
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.
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