Anyone can build this simple to use light duty spot welder. I found all the parts I needed in my shop. I looked at some other plans on the Internet but they all seemed to require something I didn't already have.
Step 1: The transformer (core)
I had three MOTs (Microwave Oven Transformers) collecting dust in a box with a bunch of adapters and transformers. The one was exceptional for the fact that the primary and secondary were separated by a shunt. Placed on it's base the secondary was on top and the primary on the bottom (pictured).
I used a pipe cutting blade (hacksaw blade) on a reciprocating saw to cut the secondary off the transformer core. Near the end of the cut I had to use extreme caution as I did not want to damage the primary coil. The primary and secondary can be identified by the number of turns and the gauge of wire. The secondary has many thousands of turns and uses hair thin wire. The primary is more like 18 gauge.
Once the secondary coil was cut away I used a short metal bar and a hammer to pound the trapped portion from the core. It was wedged in there pretty good. I cleaned out everything except the shunt and the primary.