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Old 09-12-2007, 06:30 PM   #1
jpgrego
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Default Mig welding thin sheet metal

I'm in the middle of fixing up an old motorcycle I got for my wife to learn to ride on. One corner of the tank has some pretty severe rust damage and I've decided there's no way to really patch all the tiny holes so I'd like to just cut out the whole section and replace with new steel. However it's very thin steel (even moreso thanks to the rust) and I'm worried I'll blow through it if I just start blasting away. Anyone got some pointers before I get started on this or is it even worth trying?

Thanks,
Patrick
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Old 09-12-2007, 07:03 PM   #2
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Default Re: Mig welding thin sheet metal

keep the metal cool. tack weld, dotn try to run a bead, just use tacks until you have it welded all around, but make sure the metal doesn't get hot in between or you will blow through. use the smallest dia. wire you can find. .023 for a mig with c25 shielding gas. its just gonna take some time, dont rush it. it is doable.
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Old 09-12-2007, 07:09 PM   #3
jimvannoy
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Default Re: Mig welding thin sheet metal

When I weld thin/rusty metal I hold the tip at a steep angle and try to let the wire hit the edge of the metal and not straight down on the top. Use a low setting and take your time.
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Old 09-12-2007, 07:32 PM   #4
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Default Re: Mig welding thin sheet metal

There is a MIG wire out called EasyGrind. It makes for a softer weld bead which will help you when grinding it down, especially on paper thin material. One note of caution though, be absolutely friggin' certain there are no fumes present in that tank. Best bet is to have it purged (boiled out) before doing any repairs on it. I believe this is the eigth commandment of welding.

"Thou shall not weld on an unpurged tank for the noise will be very loud and thy friends will console thy widow in ways generally unacceptable unto thee."
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Old 09-12-2007, 08:25 PM   #5
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Default Re: Mig welding thin sheet metal

Alright, thanks for the tips... I'll give that a shot and see how it comes out. Appreciate the advice

Patrick
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Old 09-12-2007, 08:59 PM   #6
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Default Re: Mig welding thin sheet metal

I'd keep the portions not being welded on wrapped in damp burlap or canvas. As stated keeping things cool is big.
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Old 09-12-2007, 09:37 PM   #7
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Default Re: Mig welding thin sheet metal

I would take the extra step to line the tank with liquid tank liner when you are done. MiG has a nasty habit of cold starts. One cold start can let gas slowly seep and ruin you paint work. A cold start weld can get past a air bubble test. Tank liner is easy to install. Plug all the holes but one, pour it in the unplugged hole and then plug, slosh it all around and pour it back out. In fact some tank liners work so well they will fill and seal daylight pinholes.
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Old 09-12-2007, 09:55 PM   #8
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Default Re: Mig welding thin sheet metal

Good advice so far. Especially about the tank liner.

Use .023" or 0.25" wire, C25, and GO SLOW!

Oh, and if you're using a cheap welder, or one with just a couple of voltage settings, go ahead and just buy another tank now. Proper machine setup is crucial.

Tack it every inch. Then tack halfway between those, then halfway between those. REALLY thin sheetmetal won't let you go very far in a continuous bead.
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Old 09-13-2007, 07:09 PM   #9
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Default Re: Mig welding thin sheet metal

Quote:
Originally Posted by 5wndwcpe View Post
"Thou shall not weld on an unpurged tank for the noise will be very loud and thy friends will console thy widow in ways generally unacceptable unto thee."
I have nothing useful to add to this thread but had to say that was the funniest thing I've heard/read today. thanks
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Old 09-13-2007, 07:19 PM   #10
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Default Re: Mig welding thin sheet metal

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I have nothing useful to add to this thread but had to say that was the funniest thing I've heard/read today. thanks
Don't mention it.
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Old 09-14-2007, 03:32 PM   #11
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Default Re: Mig welding thin sheet metal

Before you hit the tank, get some similar gauge metal and practice. Helps get all your settings before you blow a hole in the tank.
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Old 09-14-2007, 05:17 PM   #12
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Default Re: Mig welding thin sheet metal

if doing the little tacks all around, how do you avoid little pin holes, the start of each tack is a little cold sometimes. although if no breaks are taken the wire and the piece, should be warm enough, grind it down and test for leaks I suppose.

oh and if you position the piece somewhat more vertically it's less likely to blow through because the puddle won't tend to fall down.
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Old 09-14-2007, 09:30 PM   #13
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Default Re: Mig welding thin sheet metal

Heat setting and wire speed are going to determine the rate at which your tacks will either build up on the surface or blow through the paper thin metal. When I was welding the floor in my '33 Plymouth, I had much the same problem. What I did was lay down tack welds every inch or so at as cool a setting as I could get. Go back and stitch them together but don't obsess about it if you miss a few, the object is to keep the heat down and yes, it will look bad. Next, grind down the beads and start the process over filling in the pinholes, increasing the heat/wire speed as you go. Rinse, lather, repeat. The idea here is that as you add the tack welds, you're adding mass to the thin metal, allowing it to absorb more heat, which will in turn, give you better penetration.
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Old 09-19-2007, 09:00 AM   #14
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Default Re: Mig welding thin sheet metal

Quote:
Originally Posted by PoorOwner View Post
if doing the little tacks all around, how do you avoid little pin holes.

That is the main reason behind the tank liner stuff.

You can avoid pin holes by starting on the metal and tacking vertically over the gap.
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Old 09-30-2007, 09:28 PM   #15
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Default Re: Mig welding thin sheet metal

So how did it work out? Have you started?

I do a lot of thin sheet welding with MIG and all the advice you got was spot on! I think the best overall advice is to get some scrap sheet of the same gauge or thinner and practice practice practice.

Just remember, the grinder is your friend.

Also, if (when?) you blow through at some point, fight the urge to try and fill that hole up right away by piling in loads of wire!! Put the torch down and take a break!
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Old 05-03-2011, 10:22 AM   #16
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Default Re: Mig welding thin sheet metal

im gonna post here instead of starting a new thread...

Im in the same boat as the OP. im mig welding with my hobart 140 on 20gauge mild steel.
as i spoke with nastyzen he also recommend putting the piece on copper or aluminum to disperse the heat.

im welding a fairly big sheet metal chair and spot welding every inch then half that then half that will take a longg time. is there another more efficient way to do this? can you weld wet metal? i was thinking about welding and hosing it off or dipping it then repeat... will i have to dry it every time?

this might sound noobish but does anyone use jbweld? i was thinking maybe spot welding it every inch, then in the middle of those, then fill the rest with jb weld? is that dumb? i really cant afford to have the pieces warp as it throws the dimensions off...
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Old 05-03-2011, 11:51 AM   #17
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Default Re: Mig welding thin sheet metal

20 ga seems awful light for a metal chair, is that what came on it? Tack welding and filling in between with JB weld will give you a repair that is very susceptible to future damage from separation and/or cracking. (not recommended)

The following was in response to someone asking about patch panels on a car, but much of the same logic follows no matter what the sheet metal object is. Perhaps it will help...

Any welding process, be it O/A gas welding, arc, mig, or tig, is going to cause shrinking. Sheet metal, due to the limited thickness, is especially prone to this shrinking and the side effect of warpage.....no matter which process you used, gas, mig, or otherwise. With any sheet metal repair involving a long weld, you will be better served to first of all, insure a full penetration weld, and secondly, promote consistency in your methods. A joint fitment that is tight in spots and wide in others will require more filler (which means more heat) in the wider gaps, yielding more shrinking side effects for more warpage in those wider gap areas. You may not think panel fitment as a welding issue, but when it comes to how much filler is required, it most certainly is.

Welding in certain areas of sheet metal, specifically high crown vs. low crown, will tend to have different results in the way the panel reacts to the heat. Welding through a low crown panel will tend to shrink and further flatten out the panel, for an immediate loss of the shape. Welding in a high crown area (such as radius at the top of the quarter) still shrinks the area, but has a less noticeable effect on the shape of the panel. So in addition to consistency and improving skills and methods, if more than one option is available, you need to look at the area being welded and choose the weld location that will yield the least amount of adverse effects based on panel location and how the panel will react to the heat.

I'd suggest to eliminate some of this learning curve on scraps (same gauge thickness as what you're working on), and once you feel comfortable with consistent results, move on to the actual repair.

Fixing the shrinking side effects (warpage) from welding involves stretching, normally using a hammer and dolly. So that last bit to look at when planning where to put your weld should address access from the back side for hammer /dolly work. Not all circumstances will provide this access, but you can minimize some of the problem side effects by planning ahead. Any attempts at cooling an area with water will only serve to temper the weld, making it more difficult to planish, and risks cracking.

As far as a choice in the process, Gas or Tig is definitely the cleaner choice in welding in that they leave no splatter or slag all over the place. The amount of warpage is relative to the amount of heat you put into the panel via how long you sit there, so typically, MIG vs. TIG (when applying filler rod with the TIG), the MIG would have less of a HAZ. However, if you can trim your joints to zero gaps such that a no-filler weld can be performed (fusion welding) using the TIG or O/A, then you should have about as small a HAZ as possible, and as consistent in width as possible, for less distortion. To explain this further, various starting and stopping in your weld will cause inconsistent width of the HAZ, along with the shrinking effects that come with it. This may lead to a "wavy" distortion. Consistent heat, consistent speed, without stopping, in performing this weld will help to keep the HAZ, and all the other conditional reactions consistent, for less distortion. Having said all that, those conditions are hard to maintain when welding sheet metal with the MIG. Most enthusiasts will be using a MIG welder (as that's what they have) which is not very compatible with a full pass weld when dealing with sheetmetal. But you can still apply the same principles of consistency in using a MIG and "dot" welding. Single dots, skip around, use same overlap, same size dots (elapsed time of trigger pull) etc. This won't make the weld as nice as the fusion weld described above, but it will help with consistency if the MIG is what you're using. The biggest thing on consistency with the MIG is to practice on some scrap pieces the same guage as you'll be working with to insure your welder is set up correctly for a full penetration weld. It's hard to be consistent if you're putting in the patches and still fiddling with welder settings, (practicing on the good stuff).

Next, if you aren't very good at trimming the panels to fit tightly together, the O/A or TIG is going to be more likely to blow holes (depending on your proficiency to keep up in adding filler rod where needed); this is an area where the MIG is more forgiving as it is automatically feeding filler. The O/A and TIG processes prefer a tight joint for less chance of blowout. Lastly, TIG can sometimes be awkward to use in some remote locations (i.e.: under a car, any hard to reach location where using a foot pedal is cumbersome) O/A will be a bit easier as no foot pedal, and the MIG is more of a point and shoot type deal. The downside to the MIG over the other two processes is the additional grinding and consumables required to get rid of the excess weld bead. So I would consider all of the above in making your choice.
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