Technique for MIG welding sheet metal

[Doug_B_928]

Hi folks,

I'm a new member. I didn't know there was a forum just for MIG welding; very cool. I'm new to welding. I'm planning to restore an old car myself (I'm new to this too). I purchased a Lincoln 180 Dual before Christmas. I've been doing some practicing (using 75/25 gas). I understand that for thin sheet metal the technique is to tack the pieces together, then do a series of spot welds. I was under the impression that, to fill in the seems once tacked, one should keep the individual spot welds sporadic to avoid heat build up. I watched the tutorial video on this site and it shows doing a series of spot welds in a 10mm row. Would limiting it to 10-20mm be enough not to heat the panel such that it warps? If the preferred method is to jump around with the spot welds, then what is the technique for putting a spot next to a cold spot. Do you start on the seam and then move back to the spot, or vice-versa, or ?? Wouldn't a really cold spot need some extra heat to get penetration compared to the sheet seam next to it?

Also, I read about some .023 wire that is easy to grind (don't recall the exact name). The local welding shop told me that .025 (Lincoln made in Canada) is just as good for sheet as .023 (I had been practicing with .030). Is the easy grind chemically different, or is it just because it's .023 that it's easy to grind.

My first project has been to refurbish an old muffler. I did it with .030 and poor technique. The metal was extra thin (33 year old muffler) and the new metal was 22G, so lots of fun with blow through and chicken feces.

 

[keith19]

Hi Doug, welcome to the forum. When you say spot welds, seems to me like you are meaning tack welds? Like you first tack your metal along the seam to get it aligned correctly, then gradually fill in between the original tacks with more tacks?
If that's so, then personally I hit the seam first when putting a tack in close to another one, then push it back into the previous tack. The thinking behind this when working with thin metal is that it can get hot and burn through pretty quick as you move along, even though you are maybe only making the tack one quarter inch long. So by the time the weld is hottest it hits the previous cool weld, which should prevent blowing through.
In effect you are working around the job in say, a clockwise direction while putting in the spaced tacks, but as you put in each tack close to a cool one, you are pushing the weld pool anti-clockwise back into the cool tack. (If you're right-handed).

Lots of practice and experience soon show you how long you can make the tack before blowing through on different metals, and how far apart you need to keep the initial sets of tacks when setting out the job. Maybe every couple inches - 50mm - is a good start on thin metal, all around the job, then go around again, one tack backwards onto each original and now cool one. Then around again and again, etc, etc.

Get in plenty practice on a variety of metal thicknesses before doing anything important on your restoration. Old mufflers are about one of the hardest things to practice on, so you've done well to stick at that and make something of it. Try to get good on bench work first, then it won't come quite so difficult when you have to start over again under a vehicle.

Best of luck with the resto', and try to put in plenty of regular practice to help learn more quickly.

 

[the snooper]

exhausts are terrible to weld anyway, i think i would have made a complete new silencer would have been easier i think and would last longer

good luck with the rest of it

 

[Doug_B_928]

Hi Doug, welcome to the forum. When you say spot welds, seems to me like you are meaning tack welds? Like you first tack your metal along the seam to get it aligned correctly, then gradually fill in between the original tacks with more tacks?
If that's so, then personally I hit the seam first when putting a tack in close to another one, then push it back into the previous tack. The thinking behind this when working with thin metal is that it can get hot and burn through pretty quick as you move along, even though you are maybe only making the tack one quarter inch long. So by the time the weld is hottest it hits the previous cool weld, which should prevent blowing through.
In effect you are working around the job in say, a clockwise direction while putting in the spaced tacks, but as you put in each tack close to a cool one, you are pushing the weld pool anti-clockwise back into the cool tack. (If you're right-handed).

Lots of practice and experience soon show you how long you can make the tack before blowing through on different metals, and how far apart you need to keep the initial sets of tacks when setting out the job. Maybe every couple inches - 50mm - is a good start on thin metal, all around the job, then go around again, one tack backwards onto each original and now cool one. Then around again and again, etc, etc.

Get in plenty practice on a variety of metal thicknesses before doing anything important on your restoration. Old mufflers are about one of the hardest things to practice on, so you've done well to stick at that and make something of it. Try to get good on bench work first, then it won't come quite so difficult when you have to start over again under a vehicle.

Best of luck with the resto', and try to put in plenty of regular practice to help learn more quickly.

Excellent, helpful, advice Keith. Thanks as well for your encouragement!

 

[Doug_B_928]

exhausts are terrible to weld anyway, i think i would have made a complete new silencer would have been easier i think and would last longer

good luck with the rest of it

Yeah, but in this case I'm trying to maintain the originality of the car. No new ones are available (MY 1979). I purchased a used non-rusty one so welded-up the original to have as a back-up and for practice.

 

[Daz1968]

I am only self taught but I use small clamps to hold material together and put a few tacks approx 50mm apart then i find it easier to pull the torch rather than push when welding thin material and I release the trigger before it blows through, let it Cool for a few seconds then strike the arc where I left off. I do this to get short runs of about 20mm and move around the panel until they are all joined together. But best to practice and see what works for you, as the main thing is you need to practice to get it right. Try getting a short run on some really thin stuff you cut out and when you can do this the new stuff will feel easy.
I then grind the weld flush and look for any small holes, weld these then grind it again
Forgot to add I use a 5% mix gas as I think 25% is for thicker stuff..