Examples, if you use thicker wire and less of it you will get higher current but lower voltage, if you use thick wire (not as thick) and a few more turns of it you will get high current and slightly higher voltage. or if you keep the original windings you will get one to two thousand volts at one to two amps.
Now to jump right in here's what you will need!
1) A microwave oven transformer
2) An angle grinder or if you don't one and your very keen.. a hacksaw.
3) 1-5 Gauge insulated copper wire.
4) A grounded 3 pin wall plug
Onto the next step!
Step 1: Removing the secondary windings
Step 1) Take you microwave oven transformer and secure it tightly into a vice or onto a solid bench with clamps. Then plug in your angle grinder or pick up your hacksaw (with proper safety gear of course)
Step 2) Cut away the secondary windings, the large amount of thinner wire. and use whatever means necessary to just remove all of the secondary windings.
Step 3) there will be a small length of usually red wire in the middle below where the primary was this is the filament windings, remove these as well. then there is the magnetic shunts, you can also remove these, just tapping them out with a flat head screw driver will work. once all this is removed leaving only the primary left,continue to the next page!
also pictured below is what you should have, but in the pictures I have not yet removed the shunts. (the primary connections are on the opposite side.)
Step 2: Adding in new secondary and the Final product. (VIDEO)
Step 1) Now that you have removed the secondary windings, shunts and filament windings and its as tidy as possible, you can take your new secondary wire (whatever gauge your using) and wind as many turns as possible on your transformer as you can fit, the more turns the more volts. and the thicker the wire the more current you will get. to wind the new secondary, just wrap your new wire round the core of the transformer where the old secondary was.
Step 2) Now you can attach whatever you want to your new secondary's two output leads, whether it be some big heavy duty alligator clamps or connecting it up for use as a spot welder or stick welder
Step 3) Get your 3 pin grounded wall plug and connect the ground to the base of your transformer, and the hot wire and the neutral wire to your transformers primary inputs.
Step 4) Plug it in, turn it on and you have yourself a power supply that theoretically should be able to supply up too a kilowatt of raw output power!
And of course a video. 2-3 turns approx 3v AC and around 500 amps of steel melting power...
Heres a snapshot of one rewound MOT i put a bit more effort into. I wound it into a power supply that has 5 voltage taps which once rectified the voltage on each is 0v (GND), 12v, 24v, 30v and 50v DC! all of the voltages besides the 50v can put out 30 amps+ and the 50v limited to around 20 amps.