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Old 03-05-2013, 09:46 PM
foamcar foamcar is offline
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Default Stretching Mig butt welds

My first question on this great forum. Some of you may recognize me from the metalmeet forum, as I have been working on this car for a good 10 years now,mostly in the winter. I am doing a long(4 ft.) butt weld on a 356 Porsche fender repair panel. Trying to use the technique described by the "55 Chevy fender" thread wherein consecutive tack welds are made after slapping the series of tacks. You can see pictures on this 356 Porsche forum
http://www.thesamba.com/vw/forum/vie...5fde47769a90f2

My question, which I don't think needs a picture(at least yet) is: Do the bottome of the tack welds under the fender need to be ground smooth? I am using a cheap dolly that I ground to fit the high crown top to bottom and low crown front to back and it is getting full of pits from the protruding welds when I slap the HAZ. I have been getting the HAZ pretty smooth except for the area really close to the weld. Using one of my good Martin dollys does not help raise this area. Maybe I am expecting too much, as a little filler will fix this, but I have seen some nice smooth work on this site.

Thanks - Phil
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Old 03-05-2013, 09:55 PM
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Phil, I will typically grind down the weld dot both front and back.
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Old 03-05-2013, 09:55 PM
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The simple and quick answer, all welds have to be as close to original metal thickness as possible to properly planish a weld. Otherwise, there is too much mass and inherently too much stretch in the area you are planishing.

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Old 03-05-2013, 10:04 PM
foamcar foamcar is offline
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Marty and Robert

Thanks for the quick replys. I was just going to edit my post, as I did not indicate the type of weld, which may not matter. It is MIG. My mentor, who restores 356s for a living is very anti MIG welding outer panels. I am hoping to convince him it is doable to a pretty good quality level. So, tomorrow I get to crawl under the fender and try to grind just the weld without nicking the fender.
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Old 03-06-2013, 11:42 AM
weldtoride weldtoride is offline
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Hi Foamcar, and welcome. Ask 3 guys your question and you will likely get 4 answers, so here's my 2 bits. I understand you want to work with what you have, so I'll stay on topic with Mig. When you butt weld with Mig, do you use a gap?

Admittedly with that question, I have just opened up another area of discussion that is subject to differing opinions. That said, my experience with Mig on thin sheet has been that with no gap, there is less deposit on the backside.
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Old 03-06-2013, 01:31 PM
foamcar foamcar is offline
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Mark

Fair question. I have a very slight gap in most areas, although it has closed up in a few spots. I sliced through both layers with the Dremel Thin cutoff wheel. I am guessing 1/16" at most. As I will be under the fender shortly grinding down the weld, I will see how much less there is in the zero gap zones and post. I will also measure the thickness of the cutoff wheel.
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Old 03-06-2013, 01:42 PM
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I prefer zero gap, high heat settings (set the welder for 3/16" or 1/4") and VERY little trigger time. Much less grinding once you get the hang of it, and 100% penetration each time. Much smaller HAZ and weld buildup which means.... less distortion

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Old 03-06-2013, 02:00 PM
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Quote:
Originally Posted by foamcar View Post

I have a very slight gap in most areas, although it has closed up in a few spots.
Phil, that is a good reason for using no gap. If you do leave a gap, when shrinking occurs, it pulls the sheets together. If you HAD the panels aligned prior, now the shrinking, compounded by the gap that allows more pulling, has drawn the panels together that much more, likely losing some of the crown and creating a loose panel.
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Old 03-06-2013, 06:49 PM
foamcar foamcar is offline
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Thanks for the additional info. I will try a 1/4" setting and see what happens. I measured my cutoff wheel and it is a little under 1/32", so the gap was probably at 1/32 or so when I started. I ground down a lot of the weld underneath. Not a lot there, but now most of it is pretty smooth. One section will need to ground with the Dremel, as it is too tight to get the big air grinder in there.

I suppose there is a trade off with my method of getting a good gap alignment by overlapping the new and old and slicing through both. This eliminates any final fitting to get an even gap, or none, but does leave a gap the thickness of the cutoff wheel. That is why is use the thin Dremel wheel. Took six of them to cut the full length.

Anyway, had a little time to do some more weld slapping where I ground underneath and it definitely is helping. I will shoot some pics tomorrow when I go back at it. Had to repair a leaky toilet and clean up an old gas stove we are going to donate to a needy family today, so not too much time on the fender.
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Old 03-06-2013, 09:53 PM
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When ever I run onto somthing that has been mig welded I grind both sides also but I will work the haz hot about 2 or 3 inches at a time. Very dull red not bright red, it always seems like a miged seam is a little harder to get it to move. Others here know way more about metalurgy than I do but think about how fast that mig weld cools and how much slower a tig or OA weld cools I think it doesn't stay hot long enough. Every time you turn something red hot it will shrink to some degree but also think of the shenanigens happening in that puddle you have elements moveing around trying to make the weld puddle relative to the materal so I think a little heat relaxes and helps make the weld relative to the surrounding surface allowing you to planish it smooth. I have seen very very good panels migged together I know that it is possible but thats not for me and I fear change

Pat
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