There are many reasons that could cause welds to fail specifications. The components and factors and set ups that must be evaluated to correct these situations have been discussed in depth in this blog. Some of the items that my need to be examined are:
Machine design or function
Control
Transformer
Conductors
Connections
Bad Weld Schedule
Inconsistent weld schedule
Bad pressure
Electrodes overheating
Lack of proper water cooling anywhere
There are many pages already written about each of these items in this blog. I will not boar you with a repeat here. Do a search and read about the importance of each item and how it can affect the weld nugget.
This was not a complete list. Plant air, water and power were not mentioned. Other items also apply.
A WEAK OR NO WELD
To start a general evaluation of failing spot welds one might start with the basics by turning to this blog:
WHY DO MY WELD NUGGETS GET TOO SMALL AFTER AN HOUR OF WELDING?
This is a good starting point for spot welds and can lead to many of the other topics listed above which can be found by doing a search by subject or phrase in this blog.
If you are interested in projection welding then I would suggest one starts with the following article:
WHAT ARE THE COMMON FAILURE MODES FOR PROJECTION WELDING?
This is a good place to start an investigation which can potentially lead to a solution for your situation.
Additional searches in this blog may be needed to learn about the control, transformer, other components and how they make resistance welding possible.
Weld schedules may be needed from Catalog or AWS sources. Examples and data is here in this blog.
References: RWMA – Resistance Welding Manual 4th Edition
AWS C1.1 Recommended Practices for Resistance Welding
AWS J1.2 Guide to the Installation and Maintenance of Resistance Welding Machines
Tuffaloy Resistance Welding Products Catalog
CMW Welding Products Catalog