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Homebrew Spotwelder

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LAN_403

Part Scavenger

Sat Jan 06 2007, 03:22AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Have any of you guys built a homemade spotwelder? I ca'nt get mine to work, and I can't figure out why. I am passing well over 100A, and the only thing that gets remotely warm is the metal I'm trying to weld.

I read you don't even need that many amps to successfully weld thin metal, I tried to find that link for you guys and I couldn't.

The first thing I tried was something similar to this:

I was using brass electrodes and the biggest copper wire I could get on the MOT and still have 5V open-circuit. IIRC, that turned out to be 6ga wire, wait, no 8GA. I first tried welding galvinized steel, but when that failed I tried ungalvanized. Yes, it is thin. Thinner than lunchbox metal, about the same as the shielding on the back of computer monitors. My meter only goes up to 100A, but I could tell I was doing well above that. Little iron filings on the table were jumping on and off the lead wires and the poor MOT that doesn't hum under normal circumstances was dancing all over. I forgot to check the wattage, but I had to be running somewhere over 1500W because the power-cord (rated 12A) was getting pretty warm. With the MOT, the wires would get hot after about 10 seconds of continous running, which to my understanding is way longer than you need. The metal to be welded would heat a similar amount, but you could still hold it in your hand. Nothing else in the system was getting hot that I could tell.

After that, I tried hooking my assembly to my dad's big arc welder, and eventually had it turned up to 250A, in which case the metal again didn't weld, but was the only thing that got hot.

Then, I made the top tip smaller down from about 3mm down to about 1mm. No significant change. The bottom tip is still about 3mm, but this doesn't seem to make a difference on other people's spotwelders.

So, my questions are:
1. 70-80 amps doesn't seem like enough to do much, but that was the only information I found on the amperage of spot welders. Am I right that they were wrong? If so how many amps do I shoot for?

2. Secondly, I have tried to keep everything as low resistance as possible: Short, large-as-possible diameter wires, etc. I saw several welders with brass electrodes that worked only their owners complained of sticking and heat. I have seen no heat as of yet. As long as I get enough amps, who cares right?

3. Why is it that when hooked to my dad's welder, which I've personally seen burn though 3/16" steel about halfway turned up, nothing happens? I mean pretty big shavings were jumping off the floor to stick to the wires and still nothing really even got hot?

Any suggestions?

...

Sat Jan 06 2007, 05:18AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

From the sound of it, your electrodes are flat? You need one of them to be pointy, as for a spot welder to work you need a high current density.

Think about it, if you have a .5mm point, at 50a that is the current/area as a 3mm point with 2k amps!

Steve Conner

Sat Jan 06 2007, 11:43AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Spot welding just needs a bit "More Of Everything". 100A might be OK for welding fine wires in vacuum equipment like Teralab's machine was meant for. But to do sheet metal you need a few thousand amps of current, and ball-ended electrodes with a few hundred pounds of contact pressure (enough to deform the metal so the two pieces only touch in a small spot) Regular arc welders are too high voltage and too low current by a factor of about 10.

The hand-held spot welders I've seen have a transformer about twice the size of an MOT with one secondary turn, made of copper bar, and a lever mechanism that really mashes the two pieces of metal together before the current goes on.

Electroholic

Sat Jan 06 2007, 03:26PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

will an impulse from say a capacitor work?
if it's just current density you are looking for.

Marko

Sat Jan 06 2007, 03:40PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

will an impulse from say a capacitor work?
if it's just current density you are looking for.

Those are called pulse welders and work on a bit different approach than spotwelders.

I don't have much experience there but, looking on TDU's and some similar designs, one MOT should be plenty for the job.

If you use some 3 volts you can get over 500amps out of it. (that's some1-2 turns of really thick busbar in place of secondary).

About electrodes, it seems that tungsten is preffered because of it remains solid and hard on very high temperatures and heavy pressure can be put on workpiece.

Copper looks like it would need water cooling (correct me here if i'm wrong).

Carbon_Rod

Sun Jan 07 2007, 03:17AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

a MOT with a few turns of thick copper does work. High current contacts and short leads are needed.

The 106 amp 4.6vAC spot welder for battery tabs I made works just fine. I used two broken 1/4" tungsten carbide drill stocks as the probes. Additionally a blow torch was used to solder the probe holders to the thick copper straps and terminal blocks. Brass contacts are nice as they don't stick as bad. I am still working on a controller/timer based on a 46 Amp triac. Also considering a transmission oil radiator cooling system to extend operating periods. However, time grows short these days.

So far it handles two 1/16" sheets of iron with no problems (weld contact area is about 1/8"). Again controlling it so it does not melt through is important.

Good luck.

Tesladownunder

Sun Jan 07 2007, 11:48PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

For my spotwelder, I use a transformer rated at 90A continuous at 6 volts or about 812A at short circuit. Even this is a bit wimpy.
I dont think that 1 MOT really does it well. Maybe 2 in parallel with a few turns each of rectangular wire or many paralleled windings of a few turns with smaller guage wire with very heavy bolt terminations.
I use tungsten rod contacts on a heavy swing arm so you can put a lot of pressure on it.

Alternatively I have seen a commercial setup using 600,000 uF caps at 5 or 6 volts. I did a temporary lashup but it didn't seem as effective.

Peter

1168213719 10 FT19540 Spotflashsculpture

Part Scavenger

Tue Jan 09 2007, 07:50PM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Thanks guys! Big help, I'll have to go try that out.

Dr. Shark

Sun Jan 28 2007, 06:41PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I also want a spotwelder! However I want to go down the capacitive route (pulse welder), since I dont have any big iron around. My problem is that I can't find reliable information regading the voltage / capacitance required for welding battery taps. Most people seem to use 60mF at 16V or so, which is twice of what I have right now. 30mF probably wont do the job.
There seems to be a third way though: Discharging a high voltage (ca. 300V) capacitor through a pulse transformer to match the impedance down. I would like to go this route since I have a 6mF 400V electrolytic and suitable switchgear, and I am sure I can figure out a way to make a pulse transformer.
The thing is just that I don't have the slightest clue about pulse transformer maths. I imagine that the huge currents involved would drive any "normal" transformer into saturation, so I want something with a huge airgap or maybe even just a "stick" core like in a car ignition coil. I want to store about 100J in the magnetic field, which is not going to happen by magnetizing some iron :)

Any rules of thumb I could use to make that kind of pulse transformer?

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