Trouble welding thin sheetmetal Millermatic 180 mig

[dbsoccer51]

I'm restoring a '63 Impala and have had to replace the floor pans (18g). I using 0.024 wire and 75/25 shield gas. I'm running the voltage 2-2.5 and 30 on the wire feed. 18 CFH for gas flow. To start with new metal against partially thinned out metal is a challenge as I'm always burning through on the old metal and forced to repair holes. Aside from that I am getting a lot of popping and times when the wire doesn't arc. I feel I have a good ground and I brush the metal shiny with a drill-mounted wire brush. I am thinking maybe the voltage is too low but then I already have burn-through issues...... Any tips would be appreciated.

[MMW]

1) Are you trying to run beads or just multiple tacks? Multiple tacks is usually the best. I like to tack and as soon as the red goes away then another tack. Do this while jumping around to minimize warpage. 2) Are you trying to butt weld or over lapping panels? I like to over lap about 1/8", especially areas like floor pans that don't need to be ground flush. 3) Wire doesn't arc (stubbing) can sometimes be remedied by snipping end of wire before each start to give a clean end. Also a new tip may help. 4) Can you clamp/wedge a copper backer bar under the joint? This will help with burn through.

[manac]

Your basic setup sounds good. Are you trying to do a but joint or lap joint? A lap will be a bit easier. Start with spot welds, find a time that won’t burn a hole and do a bunch of spot/stitches and let things cool down. Once you have a cool spot bead you’ll have a little more material to make it easier. You may end up doing a lot of stitches to keep from burning holes. I have a spot timer on my welder but never use it, I always manually control the time.

[manac]

Sorry, while I was pecking at the keyboard MMW beat me to the post.

[dbsoccer51]

I've been butt welding the joints. I know it's harder but...... And I'm stitching. I do a spot and move and do another spot. The starting and stopping is where I get all the popping. I try to be more consistent about cleaning the tip of the wire each time. I should have added this first but I seem to get a lot more black soot on the metal than I would have expected. I figured a little but not sure what is burning to leave soot. I like the copper metal strip tip but as you can imagine, this would be pretty tricky to do on a floor pan. The stitching, I find, leaves a ton of little holes. It's harder to create a nice uniform bead when stitching and then coming back and adding more between the stitches and then another time. I am learning about letting things cool down. by watching the red glow go away. Some times the old metal is so thin that it will hardly take any time at all, almost immediately, before I burn through. And then I spend a bunch of time building things up again around the hold. Fortunately for my various grinders and the fact all will be eventually covered with carpeting and under coating.

Thanks for your input. It sounds like I'm basically doing the right things. I need to be more patient. After the grinding it is looking pretty good but the steps to get there have been frustrating at times. Thanks again.

[MMW]

You mentioned soot. Are you cleaning the backside of the old metal and new metal? If not then you are probably drawing in contaminents to your weld, especially a butt weld.
As far as multiple tack welds once you get the first one down, then each subsequent one goes half on the previous one and half on the new part of joint.
If the old metal is so thin as you say then maybe that should be replaced also.

[Weld dr]

You should also clean the underside if you are doing a but weld the paint down under is most likely the problem.

[manac]

The repair needs to go out to good clean solid metal. If the replacement pan isn’t big enough you may have to add a transition patch.
I always use that gray putty like seam sealer with an acid brush. Seals and hides sins with one quick step.

[dbsoccer51]

Some great tips!! Thanks. I have been taking pains to clean the welded side but not the back side. I'll do that moving forward. And I need to keep in mind that the floor pans will be covered with layers on the passenger side (carpet pad, carpet, etc) and undercoating on the bottom side so pin holes here and there probably won't matter much. But I'm doing my best to get a complete weld. It is certainly a different process and technique than straight welding. The seam sealer sounds like a good idea. What specifically do you mean, manac, by grey putty?

[manac]

Evercoat Brushable Highly Adhesive Seam Sealer for Seams and Joints - 32 Fl Oz
Your local auto body/paint shop will have this or something similar

[MAC702]

1) Are you trying to run beads or just multiple tacks? Multiple tacks is usually the best. I like to tack and as soon as the red goes away then another tack. Do this while jumping around to minimize warpage. 2) Are you trying to butt weld or over lapping panels? I like to over lap about 1/8", especially areas like floor pans that don't need to be ground flush. 3) Wire doesn't arc (stubbing) can sometimes be remedied by snipping end of wire before each start to give a clean end. Also a new tip may help. 4) Can you clamp/wedge a copper backer bar under the joint? This will help with burn through.

This first answer is worth quoting for repetition. All 4 of these steps make a huge difference.

[ryanjones2150]

No much more to add, all great advice. I’d only reiterate that if you’re burning through the old floor pan because it’s so thinned out, then it’s probably not good metal to be welding to in the first place and you may consider excising that cancer while you’re there and replacing it with good metal.

[dbsoccer51]

Sorry about not responding and expressing thanks for the continued input and advice. Thanks! So I am continuing with my welding and things are a bit better. I feel I do have some thin spots from old metal but, for the most part, I the old metal is pretty solid. I cut the bad out and feel I went well beyond the sections that were corroded. I may have missed a couple of small sections but I don't feel that is the primary cause of my burn-through problem. My primary problem is self-inflicted - meaning it can be corrected. And, just maybe when this project is done, I will feel that I can actually weld thin sheet metal (can hope I guess).

One issue that is causing burn-through is thin metal but the thinness comes from my over-grinding. For example, following the multi-tack method, I place several tacks along a section and then come back and fill in the gaps. Based on input in this thread, I watch to make sure the last bead cools down a bit before I lay and another quick tack. I move around to help avoid warping. After I get things pretty well filled-in I've been grinding down the welds to make them even with the sheet metal. Working with 18g sheet metal there is not a lot of room for error so I have realized I have thinned out sections via over- grinding. And when I get done grinding I see that I missed some spots. It's when I go back to fill in these holes that I get the burn through. And this leads to some similar to chasing your tail. Because the area is thinned out the small hole quickly becomes a much bigger hole and ...... you get the picture. Not pretty. So I need to back off on the grinding. I need to review the grit of my grinding wheels/tools and my method/technique. And since this is a floor pan, where the bottom will be covered with undercoating and the top side with carpeting, who should care if the weld joints are not flat? In my effort to make the final product show room quality I've created messes that I've had to clean-up.

feel if I do grind them, I'll flatten off the high points of the bead but leave the bulk of the bead a bit proud of the surrounding metal. I'll then lay on some seam sealer and call it done.

Thoughts? I could send some video or pictures but I don't want to cause illness for anyone.

[MAC702]

Yes, you've already solved your issue. Don't try to make it flat again.

[dbsoccer51]

Thanks. "He can be taught"