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I'm looking at purchasing a 78 Chevrolet truck which has some modest rust which would most likely need to be taken care of via replacement sheetmetal and welding. I'm a decent welder (a little better than a hack). I understand how to strike the arc, create a puddle, and generally get two pieces of metal to permanently stick together.

One of the issues I have when welding thin sheet metal like you find on vehicles is, the metal is too thin and I end up blowing through it (burning holes in the metal where I'm welding). This makes a huge mess because if I really want to get it done right, I then have to cut it all back out and basically start from scratch. To this point, most of the welding I've done on this type of sheet metal is only just trying to get metal to stay put and to heck with the aesthetics. Well, if I get this truck and want to do the sheet metal work on it, I'll need to do a better job and get it right on the first go.

My questions are:

  • How do you avoid blowing through the sheet metal on vehicles?
  • What techniques are best suited to providing the best overall finish?

For reference, I'm using a light duty mig welder. I have four power settings and a wire speed range from 0 to 10 (arbitrary, I know). It is a 240vac welder @60Hz. I use flux core wire which is .030" thickness. I'm do have .035" thick wire which I will go to when the .30" runs out (pretty soon).

  • I would recommend plug welding (thin holes, 20-30 mm apart) with a thin wire to minimize blow-through. Perhaps you can place a large copper block as a heatsink under the sheet metal – Martin Feb 10 '17 at 17:33
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A useful technique with MIG on thin metal is to create the weld by a series of overlapping tacks rather than a continuous run ie strike the arc, create a puddle then release the trigger and repeat, starting the next tack on the edge of the previous one.

Here you want to use a bit more current than the minimum to ensure decent penetration, the pause between each individual weld allows the surrounding metal to cool down a bit.

It can also help to put a moderately thick plate of aluminium or copper behind the weld if possible as this acts as a heat sink but won't stick to the steel weld bead.

As mentioned in another answer using a lap fit helps a lot too, if this is not possible try to get the fit as tight as possible with minimal gaps.

Another common issue with automotive welding is rust, paint or sealant on the back side of the metal which contaminates the weld so sometimes it is easier in the long run to cut out a large section to give enough access to properly clean both sides.

Also using thinner wire and an argon shielding gas with low C02 content works best for thin metal. Flux cored wire really does not work well on thin metal as it uses the reverse polarity to gas shielded MIG which puts substantially more heat into the base metal. It is not impossible to weld thin metal with flux core but it is significantly more difficult to get good results regardless of your skill level.

  • Great answer. Brings to light a couple of things I hadn't thought about. +1 for sure. – Pᴀᴜʟsᴛᴇʀ2 Feb 10 '17 at 18:55
  • To add to this. It's good to move your tack welds around so as not to over heat one area of the metal and warp it. – cory Feb 10 '17 at 19:39
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practise, practise, practise... Also, don't attempt butt joints - get a tool (usually air driven) to swage the edge about 1/2 inch to allow an overlap which helps prevent blow-through. Good luck...

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