I began this project because I am in the engineering IV class at my high school. We had to decide on a project or projects to do over the course of our time in engineering IV that would incorporate some of the skills, if not all the skills, we had learned while in engineering's 1, 2, and 3. I chose the spot welder because it allowed me to work with electricity and also gave me a deeper and better understanding of how and why a welder works. The process of making the spot welder isn't too complicated, but has some steps that take a little more time and knowledge to complete.

Step 1: Materials

Materials:

Microwave Oven Transformer (MOT)
Soldering iron
Solder
Wall cord (ground wire)
12 gauge wire
Switch
Wood (Plywood)
Screws
Cabinet knob screws
Nuts

Tools:

Wire strippers
Blow torch
Screwdriver
Soldering iron

Step 2: Soldering

To start the whole process, I had to get all the wires soldered together. Initially, I had to find which wires on the MOT were associated with each other and solder them together. That way the MOT was energized by a single primary coil. Next, I had to find a wall cord that would allow me to connect the MOT to a wall outlet. This would give me a significant and more reliable charge of energy. The cord had to be stripped and then soldered to the appropriate wires and the ground wire needs to be attached to the structure of the MOT and the base being used. The wires on the transformer were very heavy duty wires and to solder the 12 gauge wire to the MOT, I had to torch solder. The second step I took with the wall cord was to split it and strip it again. I did so to incorporate a switch. Breaking the ground wire and neutral wire and soldering them is an unnecessary step and was simply a misstep on my part.

Step 3: Encasing

The case the MOT is protected by is made out of wood. The base of the case acts as a ground for the MOT. I made my base out of plywood just to have a base for testing the MOT.

Step 4: Creating the Contact Points

Once the soldering is complete, the next step is to create the welders base contact point, and the upper contact point. The upper contact point should be able to be moved vertically and equipped with some form of handle to move the contact point up or down. The base contact point is screwed down into the base itself. The screw will act as the actual contact point. I used 10/24 cabinet knob screws as the upper and lower contact points. They fit well through the ring terminals at the end of my wires. The cabinet knob screws are able to be locked down so they don't fall out or move around in the process of welding.

Step 5: Testing

The spot welder did preform as it should have. It was able to weld smaller pieces of metal together and did a decent job of it. The pieces I used to test with were very thin and even with very little metal to hold them together, they were stuck together pretty well.

Step 6: Things to Take Away

Watch out for needless breaks in wire. Figure out which wires actually need to be broke to not waste time breaking and then having to re-solder wires.
Make sure that if you're in high school or in a public work space where many people will be traversing and working, don't leave the MOT out where others can get to it. I did so by mistake and when I returned to it, my initial multi-wire ground wire had been cut. This left me to adapt and work with what we had in the shop, which was a solid-wire. Luckily it was the ground wire and not the hot wire, for a solid-wire soldered to a multi-wire will work only for grounding. It's not aesthetically pleasing, but it works.
For a long term MOT spot welder, use a stronger, nicer looking wood or a metal as a carrying case and base. They will survive longer under the heat and just look better as a final product.
Cover all of the transformer and the more fragile wires in the case.
Using 2 or more MOT's together would definitely boost the power output, meaning thicker metals could be welded.
Instead of using zinc coated steel contact points, use copper contact points. I had issues with the metal sticking to the contact points as the two metals were both coated with zinc. Another option is to use copper acorn nuts on the zinc coated contacts points (If you use screws as contact points that is).