The heart of this spot welder is made out of the two microwave oven transformers mounted inside the two ATX power supply cases. The price of this project was cca 15€ however you can find most of the needed parts on the electro-junkyard for free.
The transformer itself can be used for projects like MOT Resonant Arcs. However, for a spot welder, we need much more current at lower voltage.
My MCBs didn't really like the current that two coils have drawn while magnetizing their cores (when starting), so I have added a softstart circuit made out of a delay relay and a power resistor.