Home made SPOT welder

This must be one of the quickest, easiest and (so far) most successful things I've ever made. I removed the secondary winding from a brand new arc welding transformer and replaced it with 4 turns of thick copper welding cable:



First tests produced so-so results but at least it didn't blow the 240V 13A fuse. Then I realised that I'd left the transformer shunt on the minimum setting, like for the original 45A arc welding output. Once I pulled out the shunt, it was then on the 100A arc weld setting, effectively. This is the transfomer from a low cost, quite small arc welder. But my new windings produce only 2.4V so the spot welding current is probably now nearer 2000A.

I succeeded in fusing together two sheets of car body panel steel (thickness about 0.8mm x 2) with one spot weld. I tried to pull the two sheets apart with pliers but I can't, so it seems pretty strong. Once I get a joggler I hope to use spot welds to form the initial 'tack' joints, followed by a continuous seam (arc) weld in order to satisfy any MOT requirements. I'll also see if I can use spot welds to do a butt joint.

There is negligible new metal above the surface. The weld seems neat and tidy compared to some conventional welds. Many of the marks on the metal are from earlier experiments on the same pieces of scrap metal.

Here's one side:



And here's the other side:

aero said:

Just hope you don't get a fault with the insulation on that transformer. It was in a case for a good reason

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WARNING - yes, there are all sorts of risks with this project. I hope I can manage those risks but I admit I'm no expert as far as high power electrical devices are concerned. I'm not suggesting that anyone else should try this out. Danger of death, etc. I take all sorts of precautions (not fully described here) but I can't say they're sufficient.

Hi George
Is that the thermal trip on the pink wire? if so should it be in contact with the transformer ?

Weld penetration looks good

jugs said:

Hi George
Is that the thermal trip on the pink wire? if so should it be in contact with the transformer? Weld penetration looks good

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Well spotted. Yes, that's the thermal trip. With the modified transformer arrangement, there's no longer any space for the thermal trip. I needed to use the hole where it was previously located, to feed in the new secondary wire. I will, however, try and find somewhere to put the thermal trip before I place the modified transformer back in the metal case.

I expected the modified transformer to get quite hot within seconds, especially on the maximum output of what was 100 amps and is now approx 2000 amps. But it doesn't even get luke warm. This must be good news. For spot welding, the duty cycle looks like it could be close to 100%.

I'm not sure you can really tell penetration from the outward appearance but I'll try to chisel the weld apart later today to see if it pulls away the base sheet on one side. I read on another thread that that's a good test of a spot weld.

I'm intrigued George, what are you using to squeeze the spot up with, apart from the electrical side of a resistance weld, the mechanical compression and preparation of electrode tips is the key to a sound weld,
The recommended test for spot welds is a peel test like this. it should tear a hole in one of the sheets and leave the spot intact to pass.

TIG Paul said:

I'm intrigued George, what are you using to squeeze the spot up with, apart from the electrical side of a resistance weld, the mechanical compression and preparation of electrode tips is the key to a sound weld. The recommended test for spot welds is a peel test like this. it should tear a hole in one of the sheets and leave the spot intact to pass.

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Paul - many thanks for the diagrams, which help me understand the destructive test I need to undertake. If this, my first ever spot weld is sound, it will be amazing. But, even if it's not, I trust I'll be able to improve matters when I build a proper 'clamping' array of some sort, to be able to apply a lot more pressure.

The above tests were carried out by sticking a copper slate nail into the end of the welding cable. The nails were my two electrodes, one on each side of the two sheets. I say ' were' because both nail heads vapourised! You can see the feint copper colour on the spot weld images. Now I know that my home made spot welder works in principle, I will buy some Sealey spot welder electrodes and hope they are designed not to melt!

The only pressure was by hand, pushing one nail against the two steel plates, whilst the other nail was semi-held in place by two bricks. But that hand pressure was negligible, compared to what is normally used for spot welding. The next stage is to fit proper electrodes and build a (portable) clamping array.

Just hope you don't get a fault with the insulation on that transformer. It was in a case for a good reason

aero said:

Just hope you don't get a fault with the insulation on that transformer. It was in a case for a good reason

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Thanks for the warning. It reminded me that I should have put a warning at the start of this thread, as I've now done.

I tore apart my first weld with pliers as suggested by Paul. It wasn't easy, so the joint was quite strong. Ultimately, however, it wasn't strong enough because it failed the test in that neither of the base panels was left with a hole. I'm still pleased with the result, however, given that I applied very little pressure and this was a very early attempt.

GeorgeB said:

........ whilst the other nail was semi-held in place by two bricks. .......

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Love it

I was wondering if the electrodes, suitably insulated, could be held in a drill press, thus giving a convenient means of applying pressure. Obviously not suitable for working on a vehicle!

rtbcomp said:

Love it I was wondering if the electrodes, suitably insulated, could be held in a drill press, thus giving a convenient means of applying pressure. Obviously not suitable for working on a vehicle!

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I was going to dig out a drill press. Then I came across an orange juice squeezer thing. I may use that, just for a simple test. I've decided that the next 'test firing' will be with a fixed position device such as the orange juice squeezer, just to test the effect of pressure. If that works and the joint passes Paul's spot weld test, I'll start to think about making it portable. First I need to buy the Sealey electrodes.

I knocked up a press using an old orange juice squeezer. Test results at our arc weld meeting were a bit weak because of too much resistance in my wiring joints, I believe. Back home, I rearranged things and tried again. Here is the spot weld on 2 x 0.8mm car body steel from one side:



And here's the other side:



You can see the greater identation, due to the greater pressure compared to my first attempt at a spot weld in post #1 above. The spot weld was stronger on this second attempt but still didn't leave a hole in one side, when I pulled it apart. I wonder whether a Clarke or Sealey entry-level spot welder would do much better.

I will try again in some months time with a larger transformer, like from a 160 amp arc welder. My present transformer is from a 100 amp arc welder. I will also build a clamp array which can exert greater pressure than my orange press. Those two changes should produce even better results.

I used MIG welding tips as my electrodes.

Good on you for experimenting george,just mind those open wiring bits an pieces!

Bigjoe said:

Good on you for experimenting george,just mind those open wiring bits an pieces!

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Thanks! The exposed main terminals were insulated by Aero on Sunday and he's right - it is a risk but I say it's a managed risk. I look carefuly at how everything is positioned and whether it seems safe, before I apply power for just a few seconds. It's not like leaving a toaster or something on a kitchen worktop 24/7 with exposed wiring and lots of people coming and going.

Have fun playing george,and your a grown man so you can blow yourself up if you want too!

George - possible dumb question here - are you familiar with projection welding methods, sometimes also called spot welds?

It's where the site of the weld is 'marked' on one of the two pieces to be joined by a little dimple or projection. Normally done in a fixture in a machine, when the two pieces are clamped together, there's obviously a gap bewteen them as they will only contact at the site of the projection.
When current is passed the heating process begins at the contact point and spreads outwards as the machine/fixture continues to press the pieces together - I wonder whether this sort of technique would help the home-made process?

bill_gj said:

George - possible dumb question here - are you familiar with projection welding methods, sometimes also called spot welds?

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No, I've never heard of projection welding but I'm no expert, of course! I will check this out and incorporate some of the ideas into any future attempt at DIY spot welding with a homemade unit. Thank you for the pointer. I did wonder if it's OK to pre-clamp and apply pressure before applying current, and then turn off current before releasing the work piece. That way there would not be an arc or spark (minimal already) and it would be easier to work with. I'm not sure. It's not what I did here.

A few minutes ago, I dismantled the entire apparatus. I unwound the transformer and threw away the orange press. It worked but I know I could do better with a larger transformer. From what I saw, a microwave oven transformer would be far too small for spot welding car body panels.

GeorgeB said:

... From what I saw, a microwave oven transformer would be far too small for spot welding car body panels.

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If you do mess around with microwave transformers bear in mind the secondary delivers about 2000 volts, that at 800 watts is lethal. Also there is a large capacitor in there which can hold a lethal charge even after the unit has been switched off.

I'd be happier if you hooked up a spot welding attachment to your wolf George!

Have you seen these plans?

http://www.esnips.com/doc/d33c6748-...ding-How-To-Machine-Tools-388208-pdf)-(TEC@NZ)

The one thing I noticed on your orginal set up (apart from all the open wiring) was that you had the core of the transformer wound all the way out. It needs to be all the way in if the transformer is going to make any power. Don't worry about the duty cycle, it simply won't be transfering enough magnetism to the output coils if you don't have the core in.

You really want to case up your wiring in an earthed box and have off switch that won't kill you if you grab it in a panic

Aero - I'm grateful for your concern and interest. As mentioned before, I'm really not suggesting that anyone else does this. It IS DANGEROUS.

If proprietary spot welders use low voltage and high current, I wouldn't expect the Cosmo or Wolf to work as power supplies. Perhaps the Wolf inverter is even 'smart' enough to shut down upon a short circuit, like for the 'anti stick' feature. I had the core wound out because I thought that produced maximum current, like the range on my Cosmo welder:

- core in is for about 45 amps
- core wound out is for a claimed 140 amps

Could I be mistaken? Is it the other way around? Too late now! It's been dismantled.

no maybe i was wrong, I'm not 100% up on how this type work. i was thinking the more iron in the transformer the less the magnetic losses.

aero said:

no maybe i was wrong, I'm not 100% up on how this type work. i was thinking the more iron in the transformer the less the magnetic losses.

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The ones I have seen, the core is a magnetic shunt. With the core out, the magnetic field passes to the secondary. With it in, it provides a shorter path for the field so it never makes it to the secondary.