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Old 01-05-2007, 11:08 AM
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back side of a welded seam

Hi Guys -

When butt welding together body panels (or whatever) say in 18 gage, what should the back of the seam look like?

On my practice pieces, (tight seam with very little gap) I tack along the seam, grind the tack down to just above the surrounding metal, make another tack about 1" away, rinse and repeat until the seam is done. The tack is about 3/16" diam and maybe .06 to .08 tall (guestimating). When everything is cleaned up, I have a nicely welded part with no sign of the seam.

The backside however, is a different story. The seam is visible in many spots, with just tiny little weld beads sticking thru the seam. Its not closed up at all.

So, do I need to increase my heat and actually have a bead of weld on the backside the same size as the front? It seems to me if that were the case I'd be very close to blowing holes.

Thanks for the help

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Old 01-05-2007, 01:41 PM
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back side of a welded seam

What are you welding with ? Im Kinda Old school >> << Means No real book learnin other than High School Metal Shop ? Then years of doin it . Mostly Fenders On Trailers of Semi truck's Mud flapp Brackets . I Use a lott of mig <<, Cause it's Quick and Easy . But I gas a lott too with oxyacetylene , Best too tack in with migg when fabricateing parts . I like too see Back side of my seams with a heat disfiguration bead <<<< Good penetration << But With Migg Gotta really watch Heat Warpage Etc Same with gas ? But Tig is the Way too go, when weldin in . Tack with mig when fabricateing then tigg in . Im not a Tigg Talent Just migg and gas with lotts of Brazeing <<, Old School . I can yellar a car up with some brazeing rod. But What are you weldin in with ? And what and Where are you weldin on the Car ? I use Migg On Baja Buggys , Grind Welds down and braze in . But there Just Buggys ? As For me Im Lookin real hard at Tigg Machine <<, Cant hardly stand it 2500 Buck;s . But Really need Too Build A Plasma Cutter first so i can cut flames in metal bracketry . But Maybe a pic of weld , and what your weldin with. along with what your weldin . try to tell you what your doin wrong or right <<,wich is what im really good at ? But it really depend on what im weldin as too what I weld with and how critical it is as too how im gonna do it . Baja Buggs <<< Migg In 1 Inch beads Skippin around So doesnt Warp Then grind and braze in . 10k 57 Chevy ???? Floor board ? Or Qaurter Panel ??? Just depends ? What it is as too how it should be done ? If Runnin a Migg I like .08 MM Wire , Argon gas at I think my Regs set at 18PSI << I get pretty flat bead Im On 220 Amp Lincoln. Works For Buggys pretty Good But Buggy panels Are spott welded in . So Lotts of Spott weldin in of complete replacement panels rather than butt welding . But I do Both ? Lookin too do more . 2 1/2 inch sqaure tubeing for motor mount towers with triangular sheet peices centerd bracketry Too The Frame . << I really wanted too Cut Flames intoo Triangular sheet metal brackets . <, Needin a Plasma cutter too do it > Cuttin torch and lotts of rotary Files ? But Motor mount Towers with Flames cutt intoo the bracketry commin off the side of the motor mounts ? What ive been workin on last week or so . Just need Plasma Cutter too do the flames ? Im thinkin I Can Build one for around 300 buck's ? << any one gott the scoop on buildin a plasma cutter ? Sean
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Old 01-06-2007, 11:32 AM
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I welded up 22 gauge panels in the engine compartment of my mustang with my miller mm251 and had similar results that you are describing. Imo I don't think you can have complete penetration on the entire back side due cold starts. I tried to crank up the heat and it helped a little but for the most part it just tends to blow through. I know it probably wasn't necassary but I decided to weld the backside up the same way I did the front.
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Old 01-08-2007, 11:23 AM
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You said "very little gap". That's the likely problem. You need a gap between the pieces, should be about the width of your wire thickness. You'll have a little "keyhole" as you weld -- a hole that fills in as you go along. With the proper gap you should have 100% penetration that will look just about like it was welded on both sides.

I used to teach basic welding, but it's been a few years! That was "stick" welding on 1/8" thick and thicker too, but the principle is the same. Taught gas welding and a little TIG, but no MIG. Have a MIG of my own but don't use it much. Takes me 30 minutes to an hour to get back in practice every time I use it.. once or twice a year now. Welding is more practice than anything!!

If it's not strutural having less than 100% penetration isn't going to matter. As you mentioned, you can turn up the heat and risk blowing through, or continue as you have been -- or get the gap right. If you get good "stitches" on the back side (the beads that go through aren't far apart, say no more than 3/8") you should be fine.

You'll want to throw some paint or undercoating behind the panel if possible to at least slow any rusting down. If it does start to rust, when it comes through it will be in those gaps between "stitches". Sounds like you have at least 50% penetration all along, and that will keep the seam from cracking. If unsure, take a couple pieces of similar metal and butt weld them. Break apart and see how it looks (if it will break -- if not, great!).
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