How to build a spot welder without burning your fingers off
Now With Pictures!
I wanted a spot welder. I nearly NEEDED one. While wandering around online I ran across the wholly
wonderful website of Kurt Bjorn who, besides making spot welders, also makes
rotary aircraft engines and miniature turbojet engines. (yow.) Anyway, he had a discussion of how HE made a spot
welder. Go there, read that first. Then read my version.
My needs were somewhat different than his so my construction follows a different path. I will try and explain how
I built what I built, while discussing what he did.
I have had no problems welding mild and stainless steel, and welding aluminum to steel. I have not yet
managed to weld aluminum to itself, and can't weld silver at all. But steel to steel, I can weld all day long,
and get beautiful welds very nearly as strong as the wire I'm welding in.
- You need an energy source. Both Kurt and I supplied our own homebuilt energy sources. He used a
transformer with an external secondary winding, that he removed. I used the guts of an old arc welder with a double
bobbin winding so removing the secondary was easy. In both cases, this adds a lot of time to the project
(compared to the total time required) but it has some advantages. You can buy a MIG welder and use it as the
energy source, a much much faster way to get this done, and many MIG welders advertise their suitability as spot
welder energy sources. However, MIGs and the prior incarnation of my arc welder run the secondary at about 25 volts.
If we make the assumption that they're pulling about 1800 watts at the plug, that means they will have about 70 amps
at the electrodes. Both Kurt and I wound to get 4V at the electrodes which means we'll have more like 450 amps.
(Making the probably invalid assumption that we're getting the same efficiency with so few windings.) The voltage
doesn't matter anywhere nearly as much as the amperage, hence my decision to go this route. It was also not that
difficult to get 4 or 5 loops of #2 stranded wire around the transformer core. Getting the power source ready to
run took maybe an hour, and over half that was the time it took to unwind the old secondary because I wanted the
old 6 ga copper wire as intact as possible for another project. But like I said, that's a significant portion of
the entire time it took to build the system.
One marked advantage that I have, starting with an old arc welder, is that it has a welding amperage control on it.
This is actually pretty nice, as I found when I tried welding a 26 ga type K thermocouple. It welded but it also
blew the end of the wire off, until I turned the amperage down. You can actually have TOO MUCH AMPERAGE. Who'da
- The electronics is where Kurt and I differ the most. He has a beautiful design with a solid-state relay and a
one-shot for timing. I made an orc-electronics setup where I use a $4 mechanical relay from jameco that'll
switch 220v@30A with a five volt signal. I supply the signal with a 9v battery and a pushbutton. Electronics
total cost about $8. I also have the massive, massive advantage that this is easily adaptable to computer control
with an optoisolator and a FET. I like this because it means I can write a little program where I enter the wire
specifications and it'll look up the details of how I ran it last time.
Because consistent timing is critical for good welds, use of a one-shot or of computer control to determine the
power-on time, is fairly essential unless you're just playing around with it. While I like experimenting with my
pushbutton version, every weld is different, and many are lousy.
- Mechanical construction. Kurt's design is absolutely gorgeous. Mine looks like me n my cousin Bubba threw
it together in the basement. Which is essentially true only no cousins involved. Anyway. His uses a DeStaco cam
clamp to put pressure on the jaws. Mine uses a hinged 2"x2" construction with a handle that supplies pressure.
The major functional difference is the application of pressure on the workpiece. Kurt's is designed to weld thin
sheetmetal. Mine can do that, but in addition the jaws can move during the course of a weld, and when I'm
lap-welding two pieces of 0.058" wire, they compress to just over half their original thickness during the course
of the weld, so being able to move the weldhead during the weld is essential. I will be adding something to do
profiled compression, either with (initially) just weights or (eventually) a steppermotor and screw.
Now, the place where Kurt's design is completely, utterly better than mine is in the electrode holder itself. He
uses holes bored through the arms, with screws pinching the electrodes in place, with the arms as the conduction
paths. I have a pair of aluminum plates that clamp the electrodes, but the current is supplied to the electrodes
by soldered-in #2 wire. Replacement of his electrodes takes a few seconds; replacement of mine takes half an hour.
However, my design has, by virtue of the aluminum plates bolted to the wooden frame, the advantage of fairly good
electrode positioning flexibility. Of course if I'd done a precise job of construction, the electrodes would meet
without having to do a lot of fussing about with adjustments.
Note the button at the top, that activates the relay, and the 9v battery beside it. Note also the cheap wood construction. The big silver thing to the right of the jaws is an aluminum weight on a threaded rod, allowing me to change the jaw pressure.
- And finally, the electrodes themselves. We both use round electrodes that taper to smaller diameter faces.
I use electrolytic copper (that I forged out of a solid bar) and he seems to use brass. In both cases, we turned the
electrodes on a lathe, with 45 degree angles down to small contact faces. Mine is 5/8" OD tapering to 3/8" face.
Copper is too sticky to use for a production electrode -- it is not mechanically strong enough to resist being
blasted in use. Brass has higher resistance and lower thermal conductivity and is prone to welding to the material
being welded. The ideal material to use is electrode alloy, which is copper with some hardening agents in it.
Between the jaws, there is a flat steel washer propping them in the open position. The lower electrode has tape on it to prevent problems with power flowing from the top electrode to points other than the bottom electrode.
I found something interesting: if you want to reface your electrodes nicely after the faces have gotten chewed up from lots of welding, a really good way of doing it is to take a piece of 220 grit sandpaper and fold it so that it has the sandy faces on the outside, put this between the electrodes, close them gently, and pull the paper out. This has the nice advantage of keeping the electrode faces parallel to one another.
This page created 3/15/02, last modified 4/23/04
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