Spot welding

[Andrewinpopayan]

I have been into workshops with spot welders in use. Most appear to be a clamp with copper electrodes and I was wondering if it was possible to use a 140 amp stick welder as the power supply for a home made spotter?

 

[kf2qd]

A stick welder is not the proper power source for spot welding. Spot welding uses lower voltage and turns the current on after the electrodes are clamping the part, and turns off the current before unclamping the part.

 

[Mcgyver]

like kf said, spot welding is resistance welding and needs low voltage, 1000's of amps. Read this : http://www.millerwelds.com/pdf/Resistance.pdf At low voltage the steel passing that many amps gets hot where as the copper, a better conductor does not. The tough part is finding a transformer that you can rewind that will deliver say 1.5 V on the secondary and 3000+ amps. I've got a complete one built, rewound a 1KVA transformer, getting 1500 amps through it, not enough.

here's some pics of mine, timer, toggle, foot operated arms and electrical switch work to perfection but only on very thin stuff

the front of these bell cranks go to a large lever type foot switch (build now, not at the time of this pic) that also starts the cycle with a micro switch, all on a low voltage control circuit that fires a pair of hockey puck scr's.

 

[Andrewinpopayan]

Hmmm, thousands of Amps is a lot more current than I expected. I was only going to use it to weld some custom wire mesh joints, the mesh to be made of lengths of 10g mild steel wire. MIG-ing it would be repetitive and a shop bought job out of the question. The actual weld area where the 10g wires cross would be very small and even a small amount of pressure from the tongs would result in considerable clamping force on the joint.

How about a separate overwind secondary on the stick welder transformer? 1.5 volts would only be 3-5 turns of welding cable, that is if there is space inside the bobbin.

 

[Bluechip]

Hi Andrew

Turns ratio is not the end of it unfortunately. If you want a lot of VA out you need a BIG core, otherwise saturation occurs, and you're limited by this.

Many smaller transformers are deliberately designed thus to stop a s/cct burning out the secondary more or less instantaneously.

One of lifes many annoyances .. unless you like destroying transformers.

Dave

 

[Mcgyver]

if its just wire, that's a different story and needs less amps. guys have built these, for wire, i believe rewinding transformers as small as a microwaves. Mine won't do, at least until i a bigger transformer(s), 24 gage sheet steel and its putting out around 1500 or 1600 amps but there should be much less resistance with wire. oh yeah, mine looks like lots of turns, its not, its just 1.5 turns x 4 in parallel

 

[Stan]

Yes indeed, resistance welders run a lot of current. Many years ago, I repaired a seam welder used to weld the side seam in ordinary oil barrels. The seam was run between two driven wheels with 250,000 amps going through the wheels. Even the welders that weld 10 gauge reinforcing mesh run thousands of amps. Of coarse they are welding about 20 wires with each cycle.

 

[Loose nut]

A couple of years ago an electrician friend of mine and I decided to McGyver a spot welder, not that we needed one just wanted to see if we could, We rewound the transformer, the original windings were 1/8" square copper wire and after it was finished minus the current limiter on the primary side we gave it a try.

The timer was set to 1/2 sec. and on trying it, it drew 156 amps on the primary (220v 30 Amps circuit and it didn't blow) and the output was 3600 amps (calculated figure, didn't have equipment rated high enough to measure).

The first pieces about 30 GA. vaporised, the second were two sections of 16 GA. welded together but it left a crater on both sides about 5/8" in dia. it should weld at least 1/8" if not more.

Should give you an idea whats required.

 

[Mcgyver]

wow, what was the transformer like - need one of those! I mean that's like a 20-30kva transformer for it to not saturate - it would be massive. how did you measure the amps? .....also voltage would have been way to high for a spot welder, no?

 

[Loose nut]

Can't remember to much about it and my friend is laid up with an injury so I can't contact him at the moment but the transformer came out of a 3 phase industrial battery charger. Their are actually 3 of this type and 3 of a different type that came out of the charger (the kind of transformer you use to make a homemade spot welder is very important, some types won't work). It is a bit larger than what is usually recommended for this application. We had intended to make a current limiting devise on the input to control the power it puts out, like a shunt, but haven't got around to it since he has been injured.

 

[Kludge]

One fairly easy type of spot welder is the capacitive discharge welder. They use about a farad (which is a really huge massive much lot) of capacitance charged to around 25-35 volts then discharged through the short circuit formed by the joint to be welded. The capacitors available for auto audio systems will work with some modification but "computer grade" caps are better suited to the task. I could probably calculate the theoretical welding current but in the real world, there are way many much lots of variables and my brain's not up to it.

Anyway, it's an alternative approach if anyone's interested.

BEst regards,

Kludge

 

[Mcgyver]

Can't remember to much about it and my friend is laid up with an injury so I can't contact him at the moment but the transformer came out of a 3 phase industrial battery charger. Their are actually 3 of this type and 3 of a different type that came out of the charger (the kind of transformer you use to make a homemade spot welder is very important, some types won't work). It is a bit larger than what is usually recommended for this application. We had intended to make a current limiting devise on the input to control the power it puts out, like a shunt, but haven't got around to it since he has been injured.

sorry to hear about your friend. I'd like to learn more about the transformer, I've been going mental trying to find a a transformer that i can work with, had this thing built for 6 months with a bunch of things I'd like to use it for....all ready except the transformer. Next one i think I'll do a mitered copper bar secondary, even have the copper. lol. I guess you don't have any pics since your buddy's out? any leads on where it came from, as in might there be more?

thanks

 

[Loose nut]

I don't have the plans anymore, I think he has them but they can be found all around the INTERNET, variations of the same thing. Basically you take a tran. of the right size (check the plans, and it depends on what you want it weld) strip off the the outer coil, Rap heavy welding cable around the inner core as tight as possible, any gap reduces output, and thats basically a spot welder. Now for the extras, current control and a timer to control the spot weld time (again check the plans), handy for not melting the wiring in your shop and necessary housing and mechanical accessories to make it work.

And the most important thing, Please don't electrocute yourself.

I will try and locate something on this subject and let you know.

P.S. Kludge if you have anymore info on that capacitance discharge welder could you post it, looks like a interesting piece of equipment to make.

 

[Mcgyver]

hey Loose, i've got the mechanical and electrical design done, in fact got it built (as per pics above) all i need is a big enough transformer to rewind so any idea or leads you might have on where to get one are greatly appreciated, thanks

 

[Stan]

Any big spot welders I worked on used bare copper for the secondary winding. Usually square wire but I don't know why. Some used one turn of flat copper the full width of the core. Most had copper water pipes soldered to the secondary winding for cooling water.

The other big consideration is resistance. At several thousand amps .1 (one tenth) ohms will drop your entire secondary voltage and no current will flow at the electrodes.

 

[Kludge]

P.S. Kludge if you have anymore info on that capacitance discharge welder could you post it, looks like a interesting piece of equipment to make.

I'm looking for it. As soon as I posted I knew someone was going to ask.

The basics are pretty much the same - a clamp thingie to hold the pieces to be welded that include the electrodes, a power source and a trigger. The trigger even works the same but the weld current comes all at once and can't be stretched out over seconds. The caps take a while to recharge and the power supply used for charging has to have a resistor in series with the load to prevent it from being short circuited, either during discharge or during recharging.

The one I made wasn't used for anything major and there are probably some serious power limitations but nothing I welded with one ever came apart. Well, not at the weld and not due to the welding process. (Things that fall great distances that aren't intended to fall great distances often don't fare very well on landing.)

I know I have that site somewhere ... *grumble*

Best regards,

Kludge

 

[Loose nut]

hey Loose, i've got the mechanical and electrical design done, in fact got it built (as per pics above) all i need is a big enough transformer to rewind so any idea or leads you might have on where to get one are greatly appreciated, thanks

I know where there are two more just like the one we worked on [size=10pt]BUT[/size] they are sitting in a cupboard at work and I can't get them, we use to be able to get "scrap" for free (a reasonable amount) now days we can't have anything, period. No doubt they will end up in the scrap bucket some day. Sorry.

If anything changes I'll let you know.

 

[Kludge]

P.S. Kludge if you have anymore info on that capacitance discharge welder could you post it, looks like a interesting piece of equipment to make.

Here's the original version: http://www.philpem.me.uk/elec/welder/?elec/welder

I puffed mine up a bit (higher voltage & more capacitance) because I wanted to do more than battery tabs. It and the other baby welders are put away until the shop is significantly more accessable. It probably would be a lot less expensive to build a regular spot welder but the solid electrical *WHAM* of all that current all at once is just so cool.

BEst regards,

Kludge

 

[Mcgyver]

i didn't see where he addressed it, may have, i just quickly skimmed, but those caps may not last long unless they are of the quick discharge variety

Too bad Loose, seems a waste. I understand why, but its shame the abuse by a few crooks have made so many companies ban dumpster diving.

 

[Kludge]

i didn't see where he addressed it, may have, i just quickly skimmed, but those caps may not last long unless they are of the quick discharge variety

"Computer grade" caps can handle about anything you do to them including discharge via the expedient of a short circuit. The ones used for auto stereo systems are too new to have any sort of life expectancy guestimations but given they're a much newer technology, I wouldn't doubt they can handle quite a bit of abuse as well.

Gathering up enough photo discharge caps to make a serious weld would be a lot of fun; 250-300 volts and maybe half a farad (since we have voltage working for us as well) would make a really nice weld, I'd think.

My fatted up one had 3 farads at 48 volts worth of capacitor which I charged to 40 volts. When I can get to it, I want to rebuild it so I can select the charge voltage and capacitance using jumpers so I can customize the zap to the need a lot better.

BEst regards,

Kludge