The welder in my garage cost me about $400 and can only weld mild steel. Hmm... so I saw this on the Instructibles site and it's kinda interesting.
It goes over several ways to build your own welder using just spare parts. The design this guy prefers is a stick welder and he shows how to do it with either AC or DC. And he claims it's easy to do either mild steel or stainless steel welding this way, but that he's not gotten aluminum to work.
Hurm. All those times I've accidentally crossed a tool over the wiring and pseudowelded tools to connectors and vaporised tools, it didn't occur to me to try and do that on purpose.
Welding is an interesting and useful capability. One of the most expensive parts of a true welding machine is the part that allows the ability to adjust the current, low for thin metal, and higher for thicker metal.
Sooo, if you are always welding angle-iron, such as free steel trash bed-frames, it is easy and cheap to make a welder that is not adjustable, but is appropriate for the job at hand.
Permanent Magnet Motors (PMM's) and Permanent Magnet Alternators (PMA's) have permanent magnets on the spinning part, which spins closely next to stationary copper wire coils. Either drawing power in the case of a PMM or outputting power in the case of a PMA.
A car alternator has electro-magnets made of copper wire coils in the spinning armature, so, the power coming out of the field coils can be varied by how strong the armature magnets are adjusted to. a Voltage regulator varies the current to the armature so as to vary the armature magnetic power, so as to vary the output of the field coils.
By bypassing the voltage regulator, you can create a feed-back loop where increasing field output also increases the armature power in an upward spiral. By adjusting the RPM's of a car engine, you can then get such an alternator to put out 120 Volts when it was only designed to put out a regulated 12 volts
By regulated I mean high amps when the headlights, radio, and A/C are on, and low amps when they are off. Off-road Jeep websites have many "instructables" showing how to alter your system so that you can flip a switch, put the engine in neutral, and rev the motor so you can use the alternator output to make emergency welding repairs to help you limp home.
Also Google "induction motor conversion" and there will be info on taking an old washing machine motor and a lawn mower engine to make cheap DIY welder.
You can also weld using 2-3 12V car batteries in series. Protect your eyes, the arc is nothing to play with!!
I almost built one of these, but then I found a great Lincoln welder at a yard sale for $150 (Old mechanic died)