Projection Welding
Questions and Answers
In projection welding the design or shape of the part is used to make discreet individual point or contacts to concentrate the current during the welding process. In some applications a full ring projection or part edge is used to form a complete seal. The TO-5 transistor can is an example of a hermetic sealed device. The full edge of the bottom comes in contact with the lid and a full hermetic ring weld is formed. The actual current flow is controlled by the part design. The electrode is there to deliver force and current into the two work pieces and provide follow up force as the weld is made. Frequently Class 2 & 3 electrodes are used. In nut welding applications Elkonite facing electrodes are commonly used on the sheet steel side. The use of dissimilar electrode materials in projection welding is common. Normally it is done to improve mechanical wear or to resist the heat being generated.
Projection welding traditionally has been performed using AC controls and power supplies. The welders frequently have been traditional press welders and recently fixed station weld guns. Both provide good alignment and can be designed for low inertia fast follow up systems. Rocker arm machines are not a good choice because they apply force in an arc and do not hold good alignment. In all cases fast follow up cylinders or servos are necessary for good quality.
With the advent of MFDC (Mid Frequency) controls and their accuracy these units are a logical player in projection welding. To make them suitable in this application, fast rise time technology has been developed because standard MFDC has a slow current rise. Another system that has shown some application for nut and stud welding is CD (capacitive discharge welders). This technology has been around for years. In the recent years CD has evolved into higher energy machines capable of nut and stud welding.
Welding Journal, July 2017
Reference: AWS C1.1 Recommended Practices for Resistance Welding
RWMA Manual Section 3
Tuffaloy Product Inc. Catalog
CMW Inc. Resistance Welding Products Catalog
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.
For information on settin up a projection weldin g process in this blog see:
How do you develop a projection welding process?
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.
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