Can you spot weld with a normal Mig?

[TroubledSoul]

I'me still very new to welding. I' ve done a basic course but have very little experience otherwise. I have to replace some bits on one of the cars and they are meant to be spot welded.

Is that the same as essentially tacking something or is there something more to it? Can I use my regular (gas) mig welder?

[finlo]

You could do a plug weld to achieve more or less the same result

[phillpot]

drill or punch holes in one or other panel, clamp panels together and "plug weld" each hole.

Make sense?

[colin_p]

You even buy (or used to be able to) a stepped nozzle that holds torch the correct distance off the work piece.

[Crafty_]

TroubledSoul said:

Is that the same as essentially tacking something or is there something more to it? Can I use my regular (gas) mig welder?

A tack isn't the same as a spot weld.

A tack is designed to temporarily hold something together so one can check alignment/fit etc. You don't care about penetration, in fact penetration is undesirable, because if you need to pull it apart a decent solid weld will make it difficult.

A spot weld (or plug weld) is a permanent fixture, you want proper penetration.

Tack weld, note the lack of discolouration of the metal, there is likely little or no penetration here, thats just enough to keep the two bits attached to each other. This is quite messy actually, the white powder/dust is because the gas is up too high. This will not hold if you attempted to pull the two pieces apart, which is exactly what you want from a tack - if everything is good you can then turn up the power and run a weld.



Look at this page for plug welds - note the penetration on the back side of the piece in the 4th picture.
http://www.mig-welding.co.uk/plug-weld.htm

Even with only one weld you wouldn't pull that weld apart, ever. It'd need to be drilled out.

So the difference between the two is penetration. Tack for alignment and fitting up, spot will permanently join the two pieces.

[GreenV8S]

I get the impression a mig welder has the wrong resistance characteristics to work as a spot (contact) welder, but you can get things like this which claim to do more or less the same job:

http://www.frost.co.uk/mig-spot-welding-kit-mig-we...

[TroubledSoul]

Cheers all, very helpful. I've since done some more research myself too and see that there's a Sealey spot weld nozzle for my welder. That Eastwood spot weld kit says something about Tweco torches but I'm sure my Sealey Supermig has a euro torch.

[lufbramatt]

I didn't know spot welding nozzles existed.

What you want to do is a plug weld. Drill an 8mm hole in the top sheet, make sure the two sheets are tightly clamped together, aim the nozzle at the middle of the hole and when the hole is nearly filled with weld spiral out to the edge of the hole. This makes sure the bottom sheet gets good penetration.

Its easier than it sounds, practice on some scrap first.

[colin_p]

lufbramatt said:

I didn't know spot welding nozzles existed.

What you want to do is a plug weld. Drill an 8mm hole in the top sheet, make sure the two sheets are tightly clamped together, aim the nozzle at the middle of the hole and when the hole is nearly filled with weld spiral out to the edge of the hole. This makes sure the bottom sheet gets good penetration.

Its easier than it sounds, practice on some scrap first.

Yes, that is how I now do it.

But when I started and got my own mig (over 20 years ago now) I bought a spot nozzle for it. All it does is hold things the correct distance from the work piece. With practice it soon got left in the toolbox as doing spots the way you have described is quicker and easier.

The only downside is, if you want anything near a factory looking spot is you have to get the grinder out afterwards.