Some years ago I found that the lithium battery pack for my Milwaukee drill had died and I was considering repacking the battery myself. I soon realised that the terminals on the replacement lithium batteries had to be spot welded instead of soft soldered. That got me interested in making my own spotwelder from the ubiquitous microwave oven transformer. When I started to research the rewinding of the transformer I realised that this would be the easy part! Making the associated clamps, electrodes, electrode arms and chassis was going to be the real challenge. Most of the designs that I had seen on YouTube were cobbled together from bits of plywood and scrap and although they worked I wanted the finished machine to look, well, finished! I was just starting out in home metal casting and I had a 3D printer, a laser cutter and a mill and lathe so I figured I would like to engineer a proper spot welder which was capable of welding sheet metals and wire. I soon forgot about welding battery tabs. During my career as a technology teacher I had used a small portable spotwelder made by Aston here in Australia. It was a great little machine with lots of accessories including a foot pedal and a variety of electrodes and arms. It was heavy but could be taken to the job rather than being bolted to the bench. I based my design on the Aston welder but had to work from memory because by the time I got around to this project I was retired and no longer had access to the original machine.
The design that I came up with worked right from the get go and despite some upgrades that were less than successful, the latest iteration works reliably and is capable of welding material up to 2mm thick. I recently made some stainless steel filter frames for my two rainwater tanks and it was a joy to be able to spot weld the flat sheet parts together rather than relying on pop rivets which invariable rot out after a few years.
The playlist for this welder is on YouTube and it includes the engineering, the design considerations, the electrode design and a failed attempt to make a better quality secondary winding for the transformer. However you should at least watch the last episode in the series which shows the latest iteration of the secondary winding which has turned out to be the best.
If you are interested in the design, I put all the 3D models for the parts and the circuit diagram on Thingiverse here: https://www.thingiverse.com/thing:2368142
Feel free to use those resources and share them as you see fit.