This video isn’t a how-to, but it doesn’t look all that hard to build. I don’t think that completely open and rewound transformer is too safe. You may want to enclose it a bit better if you try to make one for yourself.

I had a dead microwave oven transformer so I sawed off the secondary and replaced it with 2 turns of very thick wire. I then made a simple yet very effective spot welder.

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Homemade Spot Welder