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.
Many welders are designed with low inertia rams/heads to make them responsive to the projection collapse.
MACHINE WITH FAST-FOLLOW-UP SPRING INSTALLED IN RAM
Cylinders maintenance for the smaller gun type installations will keep them in good operating order. There are also low inertia holders available to adapt standard machine welders. These low inertia holders employ strong die springs to generate the instantaneous force and follow-up necessary for projection welding.
FAST-FOLLOW-UP
LOW INERTIA HOLDER
References: Photo Courtesy T.J. Snow Co.
Tuffaloy Catalog 2017, CMW Catalog 2014
AWS C1.1 Recommended Practices for Resistance Welding
RWMA Manual Section 1, Chapter 3