In resistance brazing we are not spot welding but we still are passing modest currents so an adequate force is necessary to avoid arcing. The main goal of the force is to hold the parts together for current flow and avoid arcing so the braze alloy is not arced out of the joint. This is usually a modest force developed by trial with set up parts. Joule Heating H= I2rt will then generate the energy necessary for the braze material to melt. When molten braze metal is visible in the filet the current can be turned off and allowed to cool under pressure until solidified. Then the electrode can open and the part is ready to be ejected or removed. The temperature that the part reached at the joint is lower than it would have been if spot welded but it will be hot so caution is required.
There is one aspect of force that must be addressed. If too much force is used when the braze material is molten, it can be forced out of the joint. The ideal brazed joint thickness is about 0.0015-0.003”. It is imperative that the force be adequate to hold the parts together without forcing the braze material out of the joint. In some applications this may mean dialing in a force well under 100 pounds. Some machines may not be capable of forces this low. There are adjustable holders available that use spring force that can be adjusted to forces as low as 10 pounds that can be used on most resistance welders.
Low Inertia Low Force Adjustable Holder
Reference: CMW Products Inc Catalog
Tuffaloy Products Catalog
RWMA, Resistance Welding Manual, Section 1, Chapter 6