Building a spot welder

Over christmas break, me and my family attempted to turn a cheap, off the shelf stick welder into a spot welder for home use.


The original welder in question was a 70 Amp Vaper Arc Welder, purchased for ~80$ at Pep Boys on a whim. It’s terrible at its intended function, so there was no harm in trying to get it to perform another. After popping the lid, it was fairly easy to rip out the existing secondary coil. We rewound it with 3 turns of some spare 4 gauge cable.


We measured the dead-short current to be 995 Amps. The meter at right shows the welder’s input current to be a solid ~20 Amps. Plenty happy with that figure, and after some promising trials at welding thin sheet metal, we decided to build our own electrode jaw assembly.


Trying to keep it fairly small and portable, we opted to build off to the side of welder (it’s plenty heavy enough to serve as a base). The arms are made of 1/2″ aluminum U-channel, with a simple plastic cam mechanism to open and close the top arm. While the aluminum is electrified, we decided to add runs of 4 gauge wire to carry the bulk of the current.


The tips we used are from a MIG welder, as these turned out to be markedly cheaper than bonefied spot welding tips. They are attached to the arms and the 4 gauge with copper grounding lugs.

While we had some small success with welding thin scraps of sheet steel before, this did not translate into our final setup. We saw a dramatic decrease in amperage and welding ability, presumably due to the extra losses incurred by the extra copper/fittings/cable now in the mix. Either the original machine was too small to begin with, or we need one or two more turns of 4 gauge on the secondary coil (or both).

Either way, still a lot of high voltage fun.


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