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There's hundreds of welding Instructables around, many of them quite good. I'm writing this one here to share a technique I've developed that works quite well, even on copper tabs (for which it was designed).
The theory is simple- I've got a handful of ultra-capacitors (2.5 Volts, 2600 Farads) wired in series, then discharged through the material. The devil is, as always, in the details.
Anyone who's tried will tell you copper is a pain to weld- for many of the same reasons you probably *want* to be using it in your project. It's got about five times the thermal conductivity of iron, which basically means you need to pump in a lot more heat to overcome heatsinking. When it comes to resistance welding (which is how spot welding works, unlike other electric welders, which use arcs), you've got even more pain coming- the conductivity is another 5 or 6 times higher. So your welder needs to output 5 times more current just to create the same amount of heat.
Put all this together, and it means you probably need a welder that's over 20 times more powerful than a typical steel spot welder.
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I'll hardly claim that I've got the best setup here- but it does work, so this is probably a good place to start. To replicate my setup, you'll need:
* 4 ultra capacitors.
-I used Maxwell "Boostcap"'s, rated for 2.5V at 2600F. I believe mine are surplus from electric buses regenerative braking systems. I got mine for about $10 each from the Electronics Goldmine (http://www.goldmine-elec.com/), but it seems they're sold out at the moment. You can find them on eBay, too. Make sure they're rated to have low series resistance (in other words, they should be able to provide a lot of current- mine should do 600 amps).
* 3 lengths of heavy gauge wire, with suitable ring terminals on each end. I used scrap I had around- I think it's about 6 gauge, roughly 2 feet long. Each is a different color in my setup.
* thick copper tubing, about 3/8" ID, 1/2" OD. You need two pieces, each about 3 inches long.
* a graphite block(s), enough to yield two pieces about 0.5"x0.5"x3.0"
* two big hunks of copper. I used a 1/2" plate roughly 5"x5", and a cube about 3" to a side. It's not critical.
* Some sort of DC power supply- I used a standard benchtop variable DC supply. It's limited to 3 amps, so charging takes a long time. You could do much better.
* Some means of clamping your electrodes: I used some G-10 fiberglass stock (not the best choice, but fairly temperature resistant and non-conductive), with holes drilled for the electrodes, and a rudimentary hinge. Again, you could do better than my quick-and-dirty solution.
* 4 ultra capacitors.
-I used Maxwell "Boostcap"'s, rated for 2.5V at 2600F. I believe mine are surplus from electric buses regenerative braking systems. I got mine for about $10 each from the Electronics Goldmine (http://www.goldmine-elec.com/), but it seems they're sold out at the moment. You can find them on eBay, too. Make sure they're rated to have low series resistance (in other words, they should be able to provide a lot of current- mine should do 600 amps).
* 3 lengths of heavy gauge wire, with suitable ring terminals on each end. I used scrap I had around- I think it's about 6 gauge, roughly 2 feet long. Each is a different color in my setup.
* thick copper tubing, about 3/8" ID, 1/2" OD. You need two pieces, each about 3 inches long.
* a graphite block(s), enough to yield two pieces about 0.5"x0.5"x3.0"
* two big hunks of copper. I used a 1/2" plate roughly 5"x5", and a cube about 3" to a side. It's not critical.
* Some sort of DC power supply- I used a standard benchtop variable DC supply. It's limited to 3 amps, so charging takes a long time. You could do much better.
* Some means of clamping your electrodes: I used some G-10 fiberglass stock (not the best choice, but fairly temperature resistant and non-conductive), with holes drilled for the electrodes, and a rudimentary hinge. Again, you could do better than my quick-and-dirty solution.
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Just some added thoughts for this idea.
I am planning on building one myself soon. Beem trying to fing a resonable priced capaciator for the project as the 1f caps are NOT cheep. Had thought of trying some older computer caps, but rather use 1 cap.
This is a great idea as i could use one for some of the repair work i get.
The tips are another problem finding, Have seen some use thick copper ground to a point. Still working on some other items to use instead.
Keep up the great work and keep us posted on your progress!!
I'd suggest rereading the hyperphysics and wikipedia articles on capacitors:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capac.html http://en.wikipedia.org/wiki/Capacitors
"Capacitors block DC" is a general rule of thumb that fails in this instance. When your physics book says this, it means that they'll block DC over a suitably long timescale- enough for the cap to charge or discharge (i.e. timescales greater than R*C). see, for instance, http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capchg.html#c1
However, saying that this makes a AA potentially lethal is akin to claiming that a hunk of steel is a deadly weapon because you might be able to turn it into a gun. This is strictly true, yet absurd. In both cases you must actively take several complicated steps, impossible to perform ignorantly or by accident, in order to create this hazard. My system physically cannot pose more of an electrocution threat than a standard 9V battery.
Now, there's a large risk of accidental heating (e.g. you could badly burn yourself by touching a metal watch band or ring across two terminals), but that's not what you're talking about.
I was simply going to solder wires to replacement cells for a cordless drill but the battery vendor scolded me, saying the heat would hurt the cell chemistry. He said they'd rebuild my pack using spot welds. I learned later that this repair would set me back twice what the whole drill kit was worth.
If so, the negative end shouldn't be too tough but the positive will take a bit of thinking.
http://www.youtube.com/watch?v=GGTGIlT6JvM&feature=related
Based on this and the other links I surveyed, your machine adaptation is less hard than I thought it might be.
I'd do it the same way they do- my same two electrodes, mounted so they are nearly parallel, i.e. with the tips very close to each-other. . Mount the whole thing to a spring-loaded arm that swings down, pushes the tab against the battery, and switch 'er on. Ideally with some precisely timed pulse triggered via foot pedal.
My only thought is that you need a higher voltage than 2.5 V for some reason...
However, that would suggest that HarnessedDevilry is claiming to be able to spot-weld 2 *1-inch-thick pieces of a metal that "has about five times the thermal conductivity of iron" AND "5 or 6 times" the electrical conductivity!
My limited (ie. practically non-existent) knowledge suggests this is not likely, so I plugged '25 {mils}' into Waste Of Space's first formula, above, and got a thickness of 0.635 millimeter(s)...
you might have to copy/paste, or just google gouging rod uses
First, power supplies are generally more capable of handling volts than amps- it's easier and cheaper to source 20V @ 2A than 2V @ 20A.
Second, the switching mechanism is ugly due to all the amps you've got. No normal knife switch would handle this sort of thing- you'd weld the contacts. My impression of spark-gaps is that they work for high voltages, not high currents, so I'm not sure how much use they'd be here.
To make a switched version you'll probably need to make your own switches. You want the contacts to have a large surface area, and grease them with a good, conductive grease to help prevent them from trying to weld themselves together.
You could, of course, simply modify your current connection bars to be easily removable for the discharge half of the switch. The charge half could be standard 20A switches since I'm sure you can keep your charging current under that.
There is another project running around building a spot welder out of a microwave transformer. If you have one of those lying around you might be able to tune it to output the voltage you want, and I'm sure it could probably handle the charging current.