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