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Gregs04.5
02-20-2016, 07:06 PM
Question on sheet metal welding. After tacking the patch in, do you "stitch weld" the entire seam, or is it ok to leave gaps?

Rick_L
02-20-2016, 07:53 PM
Greg, here's what I do, and I think most.

This is for mig welds mostly, though it applies to any welding equipment or procedure. Tack your panel in place. The tacks should be separated widely. Usually I tack one end, the center, and the other end first - for each weld seam/direction. Make sure that there is little or no gap, and as you make more tacks, that the gap doesn't change. If it does, grind the weld and break it, and do it again. If at all possible, avoid square corners. I.e., trim the new and old panels so that you have a radiused corner.

Then come back and tack again halfway between the original tacks. Keep doing this, tacking halfway. Once you have tack welds fairly closely spaced (say 1"), it's time to grind them flat and planish the tack welds to make sure that patch is level with the original metal. Then you can resume tack (stitch) welding until you have a solid joint. Then you can dress the welds with a grinder and planish the welds.

Robert (MP&C) has posted this procedure with photos. You may want to search his posts.

You don't want to leave a gap, ever.

MP&C
02-20-2016, 08:39 PM
Greg, what are you welding in? Can you post up a picture?

rockytopper R.I.P 5-13-2017
02-20-2016, 09:44 PM
I did not show this in my qtr pnl patch thread and not to hi jack but it has relavance. I tacked my panel in place about 2 inch apart. I then ground all down almost to grade each tackweld per Robert teaching. I recall he said he then starts on one end and welds grinds and planishes each weld going one one direction. I welded each spot along the patch and then stopped a ground each weld. Then over lapped per Robert direction and just repeated until done. So I welded say 5 or 6 spots each pass and and then ground them down not just one, grind, & planish as I believe Robert teaches hope I'm making sense. I was unable to planish as planned during welding because of limit access so I got lazy should have made custom tool as Robert teaches but didn't. Latter found I could squeese my hand behind and planished after I already welded the entire panel. I used these eastwood clamps to secure patch. I fit my patch pretty tight along the entire edge of patch but I cut small indents in edge to allow for clamp thickness .05 or so gap. I used magnets to hold clamps in place to attach and remove once tacked. I'm pretty sure these gaps created most of the distortion issues I experienced even thou I did not planish as much as should due to limit acess. It just seem when my repeat process was welding the gaps it went south quick. Any input would be appreciated before my next one about to start.

http://i282.photobucket.com/albums/kk243/rockytoppers1/Mobile%20Uploads/image_164.jpg

Gregs04.5
02-21-2016, 04:49 PM
Greg, what are you welding in? Can you post up a picture?

I have not started welding yet, but I will need to weld in a patch piece in the rear of the trunk, and will also replace floor panels.
I have a Miller 135 Mig, and plan to practice before touching the car this spring, so I'm looking for advice/ videos to help me out so I don't f**k it up.

chevynut
02-21-2016, 09:09 PM
Just jump in and start practicing. Once you get going you'll get the hang of it.

5557mad
04-25-2017, 08:39 PM
what dose it mean when you say planish the weld

chevynut
04-25-2017, 09:00 PM
what dose it mean when you say planish the weld

Welding always causes the metal in the HAZ to shrink. Planishing with a hammer and dolly, using a hammer ON dolly technique stretches the metal back out reducing or eliminating the warpage caused by the shrinking.

5557mad
04-25-2017, 09:03 PM
thank you i understand that now. what do you do if you cant get a dolly behind the weld. just learning to run my welder

chevynut
04-25-2017, 09:34 PM
what do you do if you cant get a dolly behind the weld.

You do what you can to minimize warpage. That is cutting patches and putting weld seams in high crown areas or close to creases, rounding the corners, and keeping the heat down to minimize the HAZ.

MP&C
04-26-2017, 07:46 PM
thank you i understand that now. what do you do if you cant get a dolly behind the weld. just learning to run my welder


As an example, I have seen some outer rocker repairs posted online that install less than a full rocker as the rust damage seen appeared to be confined to one end or an isolated area. The entirety of the rocker is held in with spot welds around the perimeter that in effect minimize the weld distortion if the replacement involves the entire rocker. Perform a vertical weld with the installation of a partial patch, absent any planishing because you can't reach the back side, and your result will be a guaranteed low area as the vertical weld pulls into a valley. Any effort you thought was saved by doing a piece meal job just gave you more work in final finish. I have also seen where many people suggest you install as little of the patch as needed to repair the rust. My preference is to repair what is needed to put the weld seam in the location that makes it most accessible for planishing, locates the seam where panel features best resist distortion, and do yourself a favor and leave the panel size as secondary to these considerations. You'll have a better repair in the long run.

Bitchin'57
04-27-2017, 07:19 AM
Since the heat from welding causes shrinkage, would it help to remove the heat from the HAZ quickly by cooling each tack with compressed air?

chevynut
04-27-2017, 07:29 AM
Personally I don't think cooling does much of anything. Its good to keep the panel cool so you can see where it's going to end up, but the HAZ is created during the welding, not during cool-down. Once the arc is stopped, the metal immediately starts to cool so I don't see how cooling it faster helps. I just jump around the weld seam and let it cool naturally. If you cool it too fast you could even harden the steel. At least that's my opinion. ;)

Bitchin'57
04-27-2017, 09:10 AM
Personally I don't think cooling does much of anything. Its good to keep the panel cool so you can see where it's going to end up, but the HAZ is created during the welding, not during cool-down. Once the arc is stopped, the metal immediately starts to cool so I don't see how cooling it faster helps. I just jump around the weld seam and let it cool naturally. If you cool it too fast you could even harden the steel. At least that's my opinion. ;)
Thanks. I saw a guy on youtube doing it while welding in a floor pan, and it made me think about it.

Rick_L
04-27-2017, 09:21 AM
The fact that the heat was applied is what causes shrinkage. How fast it cools doesn't matter with one exception. Cooling it faster lets you see the end result quicker.

This applies to both welding and applying heat to shrink a high spot. Also you can straighten (or intentionally bend) a round tube by applying heat to one side.

LEE T
04-27-2017, 11:11 AM
Sorry Rick, its not often that I disagree with you
We just recently had a conversation about mig welds being harder (more brittle) than a tig weld, and that is because of the mig does not heat the metal very much so the cold surrounding metal quenches the weld much quicker the tig weld. That is to say that when a weld is cooled too quickly, it becomes harder and more brittle.

In that same conversation there was mention of speed welding with a tig. Its done without filler rod and it produces almost no shrinkage. Additionally, if you think about the shrinking disc, you can increase the degree of shrinkage if you use a water soaked rag to cool the metal after using the disc.

If you could heat a complete panel to several hundred degrees evenly--as in an oven--and then let it cool slowly to room temperature, there would be no shrinkage. Paint and rust strippers do that all the time to burn the paint and undercoating off.

So I would recommend allowing the weld to cool naturally, even without the help of compressed air.

chevynut
04-27-2017, 12:22 PM
The shrinkage is caused by the "upsetting" off the metal meaning that it compresses where it's heated. When you heat the center of a sheet of metal, the surrounding metal keeps it from expanding and that's when it "upsets". If the entire sheet is heated and cooled, as you mentioned, it won't upset anywhere, the whole piece will just expand and contract with temperature.

You can shrink metal cold if you can upset it as you do with a shrinker/stretcher. The "upsetting" happens a lot easier at high temperatures when the metal is softer such as when it's heated during welding, shrinking disc, or a torch. Once it starts to cool, it doesn't matter how fast it cools because the "upsetting" has already happened. So no, using water with a shrinking disc shouldn't make the metal shrink any more than without it in my opinion.

5557mad
04-28-2017, 06:31 AM
this is all very interesting since i just got a new welder and am going to teach myself how to use it. thank you to all

Bitchin'57
04-28-2017, 10:48 AM
this is all very interesting since i just got a new welder and am going to teach myself how to use it. thank you to all
Buy scraps of steel in various thicknesses, and practice, practice, practice. Lots of videos on youtube to get tips from.

55 Rescue Dog
04-28-2017, 01:21 PM
I've been practicing a lot the last couple months. Downsized my MIG from a Miller 200 that weighed at least that much, and capable of welding all day long to a new Miller 211 inverter, dual voltage, and only weighs 38 pounds with plenty of power for most stuff. Welds differently than my old one, and have a new learning curve. So after getting bored welding scrap over and over, I thought practice would be much more fun if I started making stuff instead.
71597160

Rick_L
04-28-2017, 02:26 PM
Lee T, we'll just have to agree to disagree. It's one of those oft debated never solved issues. Doing it with cooling causes no harm, and like I said, you have a cool panel sooner so you can see what you did.