Methods for welding sheet metal for autobody restoration

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Need some advice from the crowd.I'm helping a neighbor with some restoration work he's doing on a late 60's muscle car.  He's making the rear fender wells 3" wider to allow space for some slicks.What's the best method for welding the 3" sheet metal spacer into the well?He brought the first fender to my shop this morning.  He'd tack welded the 3" spacer every 1" using a 110V MIG welder.  The root opening ranged from 1/16" to 3/16" and the tacks were proud of the surface by about 1/8".  He'd ground down a section of the tacks, but also feathered the edges of the sheet metal in the process.  This made finishing the joints very hard as the feathered edges were real easy to burn away.He brought the fender well to me because he was having major issues with burn through while trying to weld with his MIG welder.  I tried to TIG weld, but the heat input was too much and the sheet metal was starting to distort because of the heat.  Welding on the ground areas was easier to get started, but also super easy to burn the thinned edges away.I'm using my Miller Maxstar 150, 1/16" Ce-Tungsten, 1/16" filler ER70-3, about 30 amps max on the dial but with a footpedal to control heat.My questions are:1 - Is distortion unavoidable and my neighbor needs to accept that there will be lots of work with a hammer and dollie once the welding is done?2 - I suspect that butting the joints tight and welding is better than welding this thin sheet metal with any kind of root opening.  True or False?3 - TIG can produce a flat bead with good penetration, but a lot of heat buildup and distortion.  Would a heat sink, like a copper block, held to the back side of the joint help prevent the distortion?  Or is MIG welding a series of tacks, moving around alot to avoid heat buildup, the best way to go?  4 - Is he better off figuring out how to put a flange on the joint and welding the spacing strip in with lap joints on either side?5 - What's the best overall procedure to use on this kind of project?  MIG or TIG?  When is the right time to grind on the tacks and/or finished welds?  How much distortion must be dealt with?He has parts for the second fender well waiting, and if there's anything we can do to make this easier, we need to know before he starts working with the second fender well.Thanks for any help you all can give...Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:He needs to work on his fit up. With root gaps that large it's gonna end up like a pretzel regardless. Lap joints... can't metal finish it, will invite future corrosion, a quick and dirty way to patch a car up in IMOTIG would be my preference...  http://weldingweb.com/vbb/showthread.php?t=23036 Overlapping tacks with a MIG gives less distortion but there's shed loads of grinding to finish. A chill bar does help a bit but it needs to be a tight fit- more hassle than they're worth most of the time IMO
Reply:Thanks, very helpful. I missed the link you posted when I searched the archives earlier.  So, no gap at all, short welds, evenly spaced, and TIG weld...I'm not sure what my neighbor's going to do with the one he has tacked up so far....But I'll pass your advice along to him when it comes to the second tub.Regards,DaveBenson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:I'm guessing it's 18 gauge wheel tubs you are working with.You can fill a 3/16" gap, but with that much gap to fill there's no way to fill it with weld and have it come out looking "pretty" or professional. The guy who tacked the panel together f***ed it up, it's not your fault. His poor prep is going to show in *your* final product. I don't know what your reputation's worth to you, but when I see poor quality work I generally avoid doing business with whoever did it. In the welding business word of mouth/reputation brings more business than any other kind of advertising. You might be losing money on the job unless you have an opportunity to make it right.If you want to TIG it, I think you are better off cutting out the botched attempt and starting from scratch with a new filler panel for the tubs. Zero gap fit with about 50 amps would be my starting point. Amps on thin stuff depends a lot on your particular style so YMMV. Spend as much time as you need to make the new filler panel fit right. Prep is 90% of the job. If you only spend 10 minutes on fit-up with a cutoff wheel and a claw hammer it's going to show.If you elect to MIG it use .023 wire and C25 gas. Flux core or Straight CO2 is too hot for thin work. For filling holes and large gaps I use a backer bar. If you need one, you can make a copper "spoon" by flattening a piece of 3/4 or 1" copper water pipe and bending the flat to a shape that suits you. If you know any electricians, ask one for a piece of scrap Buss Bar. Some people use aluminum tape .Skip your welds around to different sections to prevent excess heat buildup. If the heat starts getting out of hand, walk away for a few minutes and let the whole piece cool off. If the panel's a little taco'd or warped, don't stress out - you can finesse it after the welding's done.-Matt------------------------------------------
Reply:some of the cheaper 120 volt migs have such a narrow sweet spot they can be very difficult to use. I would use a quality mig. If it's a good one 120 volt is fine for this job. Do the grinding with a flap wheel, it will heat less.Tough as nails and damn near as smart
Reply:Not to worry about my reputation.  My neighbor is a friend of mine, and used to be a welder many moons ago.  This is my first real go at sheet metal, and he knows this.  Since this didn't got smooth and by the numbers, I told him to pay for the consumables, and we'd chalk this up to a learning experience for the both of us.  I think the finished product will be good, and I've no worries about him speaking poorly about my skills.  It's  weekend project, so it's not like I'm under the gun or anything.I hear you in regards to filling a 3/16" gap.  I can push a lot of metal into that, and bridge the opening.  But it sure ain't pretty and the heat really builds up quick.  Still looked better than a MIG weld over that large a gap, but it ain't a stack of dimes either.  I agree with you about the idea of cutting out what we did today and starting over.  I'll discuss this with him.  It might be faster to start over than if he tries to flatten out the sections we worked on this morning.  he did the fit and tack job on this first piece, so I'll see if we can work together to get the next trial perfectly fit up.  Maybe 4 hands are better than 2...30 amps and the 1/16" filler seemed to work well for me, but will keep in mind that I may need more juice with a zero gap butt joint.Thanks for the tip about the copper 'spoon' as a heat sink.  I think I can manage this if we need it.Regards,-DaveBenson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:Roger that, I'm probably not going to MIG this.  I have to take it to work, and I'm not sure they'd approve.  I'll stick with TIG for now.I'm already a fan of flap wheels and I gave some to my neighbor so he could use them.  He's a convert now and was running out to buy some for himself this afternoon. Originally Posted by tresisome of the cheaper 120 volt migs have such a narrow sweet spot they can be very difficult to use. I would use a quality mig. If it's a good one 120 volt is fine for this job. Do the grinding with a flap wheel, it will heat less.
Reply:You know what may be a good option for your neighbor (if their is a budget) would be for either you or him to buy a resistance welder for spots, specially if you can see in the future doing more projects,  with little expierence, then their is nothing to grind, Miller sells theirs for around $500, but you can get offshore or Matco spot welders that are the same thing, in the U.S.A., you could get this off ebay for $150 with shipping.He has already sealed his fate when he ground the tub edges down with a grinder.And if you need to put some shrinks in their Harbour Freight has a nice shrinker/stretcher $85.00. If you need to shrink elsewhere take a o/a with a 0 tip and make dime sized red spots, tap down the high spots and quench with wet rag, you will realize how much control you can put into this by shrinking.Careful not to work harden the steel by using to much heat melting the steel.As for warping, .230 and alotta breaks if you mig it for spot welds, remember to put as dolly behing the spots when still hot to normalize and strike with body hammer until the spot welds are slightly proud or flush, then you will not have this warpage, take your time.
Reply:I agree that cutting out what's been done and starting over is the way to go- probably faster, definately give better results if you make sure he fits it well. It sometimes works out better to cut out a larger area and make a larger panel with welds placed near to/at areas with more shape.It obviously depends on the complexity of the fender well but if it's a relatively simple shape then to add a 3" section i may for example cut out a strip say 2" wide and replace with a 5" section- the welds are further apart which helps and closer to the edges of the wheel well where there's more shape to resist distortion. Not necessary but rolling a bead into the replacement section to stiffen it works well although not always asthetically pleasingPersonally i feel that 1/16 filler is too thick for this sort of work- takes more heat to melt than the base metal. 18swg i would probably reach for 1mm (0.040"), 20swg and i straighten 0.8mm MIG wire. I only reach for 1/16" when i've got gaps to bridge
Reply:hotrodder, thanks for the advice.  Will see about getting some smaller diameter filler metal before we try again.  1/16" was the smallest I had in the shop, and I was trying to bridge some large gaps in the joint.  When we re-fit the pieces, I'll make sure the joint is butted up tight, and use some thinner filler metal.Benson's Mobile Welding - Dayton, OH metro area - AWS Certified Welding Inspector
Reply:refit.  you should be able to keep you gap to a minimum of .045.  when i have done this i use a metabo and it takes just as long to fit as it does to weld  even long if it is a complicated fit.  i would tig it with a mig wire filler.  you will notice if the gap is tight then the filler only is used to level the weld.i can't give you the specifics on the tungsten or the amps as i haven't actually tigged sheet metal.  i have used a jeweler's a/o torch.  the wire was .023 er70-6.  i replaced the back metal in a 68-9 camaro and also did some work on patching a 65 gmc truck.  the warpage was minimal.