I have seen a few projects which use an old microwave oven transformer re-wound with a low voltage secondary to make a hobby spot welder, and I decided to have a go at making one. I ended up designing my own using copper sheet to make the secondary, following an unsatisfactory experience attempting to use wire I made from about 100 strands of stripped down CAT5 cable, which at nearly 6mm diameter wasn't thick enough and melted its insulation, causing the winding to short circuit.
The copper strips were cut from an old domestic hot water tank, of the sort usually equipped with an immersion heater, though this one had a heat exchanger (which looks like it really needs to be made into a tesla coil primary... hmmmm....). The steel parts came mostly from an old rotary washing line which had lost it's line, and the electrodes were made from a brass arm from an old ball cock (still trying to think of something to make from the ball). The wooden parts came from an old pallet and a bit of old shelf. The brass electrodes tend to soften and deform and really need to be made of something tougher. I used the wire from my experimental winding, doubled over, to connect the secondary to the electrodes.
My original experimental winding (100 strands of CAT5) gave a cross sectional area of less than 28.3mm2. Standard wire of this thickness would be rated at only 104 amps in free air,
Using 5 x 0.5mm copper strips, 25mm wide gives a cross sectional area for the winding of 62.5mm2 , which if it were standard wire would be rated at about 185 amps. Retrospectively, I should have used 6 layers of copper, giving a rating of approximately 212 amps, however I had anticpated an extra turn of my winding and 6 layers would have taken up too much space.
Today it blew a 5 amp fuse, showing it to have been pulling at least 750 watts. Unfortunately I have no way to measure current in the primary as my meter only measures up to 1 amp, so I don't know what the actual power being delivered is.
Afterword - If I were building this thing from scratch again, I would position the transformer to the side of the jaws, so using shorter wires, and house the transformer to cover up the mains wiring. I'd connect the wires to the where the electrodes emerge from the jaws too, so the current has less electrode to go through.
Step 1: Tools and materials
For the welder
1 microwave oven transformer
0.5mm copper sheet for re-winding, taken from an old hot water cylinder
10cm (4") hinge
19mm steel tubing from old rotary washing line
Steel rod taken from old rotary washing line
Brass or copper rod
Nuts, bolts, screws
Copper tubing offcuts
3 core mains cable, connectors
Steel sheet offcuts
For the footswitch
Microswitch from microwave oven
A long bolt
Spring from liquid soap bottle
19mm drill or hole saw (same diameter as steel tube if different)
small wood drill bits
high speed drill bits
taps and dies