Projection welding schedules can be developed using standard spot welding schedules. For a single projection the weld time, current and force can be based upon available spot weld schedules. When welding multiple projections at one time and they are relatively far apart multiply the current times the number of projections. If the projections are close together and aligned, the total current will be reduced. Weld force is determined in the same manner. Weld time is the same no matter how many projections. Remember that in set up you always start on the cold side of power and work up to where you think you need to be to avoid damage to product and for the safety of people.
Weld schedules are available in the references below for some standard materials.
AWS and RWMA both publish spot welding schedules that can be used to develop a projection welding schedule. They also have published projection welding schedules for many materials.
Reference: AWS C1.1 Recommended Practices for Resistance Welding
RWMA Manual Section 3
Tuffaloy Product Inc. Catalog
CMW Inc. Resistance Welding Products Catalog
Follow up describes the ability of the welding equipment to maintain full force while the projection weld collapses. This is very critical because any variation will change the contact resistance and thus the heat being generated in that projection. Generally this would result in more heat and the projection might overheat and start to expel molten material. If the material expels there may not be anything left to form a nugget. This weld nugget collapse occurs very quickly and some large press welders have excess weight to move causing inertia or a faulty gun cylinder may stick and not be able to follow up properly.
Oversized projections can cause issues in projection welding. All processes are set up based upon design criteria. Meaning projections will be a given size and shape with some tolerance for variation and the process will accommodate this variation. What happens when the stamping tooling wears and the projections go out of specification or a punch breaks and some parts are missing a projection? The process is now out of control.
Undersized projections can cause issues in projection welding. All processes are set up based upon design criteria. Meaning projections will be a given size and shape with some tolerance for variation and the process will accommodate this variation. What happens when the stamping tooling wears and the projections go out of specification or a punch breaks and some parts are missing a projection? The process is now out of control.
Unequal shape or size projections can cause issues in projection welding. If one or more or the projections are not the same shape or size, current will flow through the part differently. The current will flow through all projections making contact at the end of the squeeze portion of the weld cycle. The larger cross section projections will not heat as fast as smaller ones. The smaller mass projections in contact will heat faster and start to collapse sooner than the larger ones in contact. If a projection height is short it may not even be in contact at current initiation. It won’t make contact until the other projections start to collapse. When contact does occur arcing or expulsion is likely from this short projection. Quality could be compromised. Before it makes contact the other projections are carry its current so they are overheated. Again the process is not in control.
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