Here is a quick and simple spot welder, suitable for light metals used in building a model turbine. From a microwave oven remove power transformer, relay, and door micro switch. Remove secondary winding from transformer. For the secondary winding ,I used 2 strands of 8 gauge wire long enough for 4 turns, and soldered in a 4 gauge terminal to each end. The 8 gauge was laying around my shop, also having thinner insulation is easier to wind on the core than 4 gauge jumper cable...
I had 2 3/8 copper rods 8 inches long in my junk, along with some 1/4 inch copper machine screws.. The machine screws were turned to a 3/32 point and threaded into one end of the copper rods, and secured with a brass lock nut..
The relay needed 18 volts to operate, so I put together some batteries, laid the micro switch on the floor, fastened to a scrap of wood, wired the transformer to the relay..and you are in business.
I clamped one copper rod to the work bench, held the other by hand , and tapped the micro-switch with my foot.. With a little experimenting, you can get very good spot welds...If the points are not precisely over each other, you can blow holes in the metal...
The best thing about this welder, I put it together in about half a day,, and it cost me nothing....
This author assumes no liability what so ever.. You build and operate at your own risk!!!!!!
Welder Pictures 
Spot Welder
Spot Welder Points
Foot Switch
Schematic
