Just looking for a few thoughts on Mig welding in a patch panel. I was piecing a rocker panel where there is no backside access...very careful with tacking, letting cool and then skipping around the weld slowly. Still have shrinkage, not bad but fill will be needed- just a light coat.
Is it worth trying to pull the weld bead back out to metal finish or is it even possible given the hardness of the weld? Should I just let it ride? What do the pros do in this situation?
I'm even wondering about hammering on the MIG bead - what about the potential for cracking?
Mig shrinkage will occur no matter how slow you go, no matter how you skip around. Those practices will HELP to curtail the issues, but will not eliminate them unless you have the welder's heat set so low that you have cold joints. Access for planishing is preferred, and also why areas such as the rocker will realize better success in replacing the entire thing. The other option is to open a hole to the back side for planishing and weld it closed afterward, where any defects are less noticeable. At this point hammering on the bead will not do much other than pounding the weld in further, requiring more filler.
Regardless of appearance, all body panels will have crown in at least one direction to help hold the shape of that panel. A flat sheet of metal has no support and will flap in the breeze, so ALL panels will have crown somewhere. If we were to look at the horizontal weld seam along the top of the quarter in a cross-section view (top-down) you would see that despite appearing flat, that panel actually has crown from front to back. It looks like a slight arc. Now, anytime you apply the heat from welding, you are going to get a shrink as that weld cools. When we weld one dot at a time (using the MIG), each and every dot is going to pull at the metal around it, from all directions, causing a shrink. Once you've added all those shrinks from all those weld dots together, along the entire weld seam, it adds up to a substantial amount of shrink such that what used to look like an arc is now more closely resembling a straight line. This is why the panel is pulling inward, the crown is shrinking. If this seam were in the middle of the panel, it would appear as a more pronounced valley. As to the gap question, I prefer no gap at all. The weld seam has a tendency to shrink, and the use of a gap between will permit more movement as this shrinking occurs. This risks losing more crown and will require more effort in planishing to restore the panel's crown. If you can properly set up the welder with more heat, you don't need any gap to get a full penetration weld.
As to how I address the weld dots, lets look at this again:
Quote
When we weld one dot at a time (using the MIG), each and every dot is going to pull at the metal around it, from all directions, causing a shrink.
I have found that due to the manner in which each weld dot shrink pulls from ALL directions, you will have better luck in planishing to remove said shrinking effects if you can planish the weld dots while they are singular, sitting all by their lonesome. This will more effectively STRETCH that weld dot back out in all directions. And by stretching as you go, you eliminate the panel being pulled into a valley as your picture shows. As far as tacking the panel, you would want to start your tacks at one end and work toward the other. I know many people will tell you to skip around to minimize heat buildup, and I have been one of those. But if you tack one end and then move to the opposite end, you run a greater risk that one panel may have more material than the other. Once things get all tacked up, this results in a panel bulge on one side of the weld. So tacking from one end and working progressively to the other will help to eliminate this by being able to align the panels together as you go. Now that the panel is tacked and weld dots are spaced about (2 or 3"), go back and planish each weld dot individually, to add a bit of stretch. At this point, I use a 3" cutoff wheel to grind down the dots to just above flush. This gets them out of the way for planishing the next sets of dots, and by leaving them just above flush, you can do the final cleanup with a roloc sander all at once. by trying to grind things down to perfectly smooth after each, you run a greater risk of inadvertent sanding of the metal to the sides of the welds, which may thin and weaken the panel. So I hold off on this until the end. For your grinding disc, I prefer to use cutoff wheels about 1/16 thick. This gives a much smaller contact area than most any other method, so you will have less heat buildup from the grinding process.
Once these initial welds are planished and ground down just above surface, then continue, adding a weld between each one until your welds are spaced about 1" apart. At this point (still planishing and grinding after each time) instead of hitting the center between for weld location, start overlapping by about 1/3 of the last welds. By overlapping, you will have less risk of missed spots or pin holes. Continue with the weld, planish, grind, repeat until the seam is done. I typically weld from start to finish using weld dots only, none of the longer passes at the end, in order to keep everything consistent throughout the process.
For the cutoff wheels, I spend the extra coin and get ones rated for stainless steel. This makes them last longer and put less of that brown haze in the air that you see from the cheap HF or swap meet specials. By the time you figure out the cost of how quickly the cheap ones wear away, you haven't saved a thing. With the bulk of the welds being removed by a cutoff wheel, we are only dressing what little remains of the weld and blending that into the parent metal. I use a 60 or 80 grit roloc, that should be as coarse as you need to go.
There are a few different considerations in locating weld seams on low crown panels, such as the quarter panel or a door skin. In most cases, as mentioned above, a seam horizontally through the middle of the panel is just asking for trouble as there is little shape (strength) in the panel to resist any movement/distortion from the shrinking, and why a weld here normally results in a severely caved in valley. (given no planishing to counteract the shrinking). For the most part one would put the weld up as high as possible, as most quarters have more shape toward the top where the quarter slopes inward to help resist movement and distortion. It also puts the seam up where most if not all is better accessible for planishing. Alternatively, a bodyline crease often serves the same benefit. That is the normal scenario.
In other cases, where the panel is blocked by an inner wheelwell or other structure that prevents/discourages planishing the weld. In this case, one can be creative in making a dolly on a stick, say a piece of steel flat bar that would fit in the void, welded to a pipe to allow better reach. I've also employed the assistance of my nephew in remote cases where his youth permitted more of a contortionist approach over what my body refuses to do anymore. This is also why it is important to planish those weld dots individually, and then grind them out of the way, front and back. This way two people can better work together on either side of a panel to planish out the welds, and find the correct weld dots in doing so. Next, you have the option of removing the outer wheelwell to better address an exterior panel that everyone will see, and then replace the wheelwell after you are satisfied with the metal bumping and finish work on the quarter.
Thanks, Robert; mapleleaf, I'm just following your thread as its timing is great...hope you don't mind the questions.
If I am reading this correctly, the first tack in our progressive movement from one side to the other side of the panel is extremely important. I do not know the term used, but it should hold/pinch/fuse/anchor the entire panel for the subsequent tacks-correct? My only analogy would be in roof framing & how you'd nail one end of a facial board to a rafter; then worm it up flush for nailing the next rafter, and so on. I'm thinking this method of tack-welding from one end, and working toward the other might help me a lot with fitment.
Also, using the "no-gap" method mentioned above, and assuming the panels suck up (or shrink?) into each other as is common, does "planishing" the weld tacks individually and stretching them out need to be almost immediate; while still warm? I didn't see a picture, but I understand this is a common question regarding the valley, and you'd probably answered it before.
The very hardest weld I have had to deal with was the inside bottom of an AD door. There is no structural support even close and it's about 3 feet across (guessing). Since you know this is going to be an accident waiting for a place to happen, really think about what is coming. You know it will shrink, you know if you are not real careful with the welding AND grinding, it will warp. No access whatsoever to the back side. I used butt weld clamps with enough gap to allow for expected shrinkage. I reasoned I would have about 1/16th of shrinkage, so allowed for 3/32. Tack in the middle once its clamped perfectly and frequently. Move to the edges by tacking every inch or so. Then no surprises at the end.
I learned the hard way and I listen carefully to Robert because he is a real professional. My first shot at doing that door job wasnt pretty. Now I can do any of this with confidence. Now if my painting skills were the same... There is just no substitute for practice.
I used butt clamps as well and had a good tight gap. The rocker has minimal crown where I pieced it - I probably could have planned it better.
I would have loved to been able to work it from the backside but little access. I could make some type of dolly and cut an access hole but is it too late now to planish now that the weld is complete and hardened? Reheat it with a jeweler's torch before dollying?? I'm thinking that could create even more work if done improperly...
No heat added, it was planished cold, and no cracks. Despite all the hoopla online about Mig welds cracking, I've never seen it first hand.
At this point, given no access for planishing, I would leave well enough alone, grind down the weld, and use some filler to repair the low.. If you can't get a dolly in there, you're done with the hammer.
If I am reading this correctly, the first tack in our progressive movement from one side to the other side of the panel is extremely important. I do not know the term used, but it should hold/pinch/fuse/anchor the entire panel for the subsequent tacks-correct? My only analogy would be in roof framing & how you'd nail one end of a facial board to a rafter; then worm it up flush for nailing the next rafter, and so on. I'm thinking this method of tack-welding from one end, and working toward the other might help me a lot with fitment.
Also, using the "no-gap" method mentioned above, and assuming the panels suck up (or shrink?) into each other as is common, does "planishing" the weld tacks individually and stretching them out need to be almost immediate; while still warm? I didn't see a picture, but I understand this is a common question regarding the valley, and you'd probably answered it before.
I do know how to make a valley happen.
Brad
See picture in link above, planished cold, no cracks. If you want to planish when something is warm, help yourself. Would it help? Perhaps. What I would suggest is it may cause you to rush in your process. If you start dropping things in a rush to grab a hammer and dolly, perhaps it's not much worth the effort. If you want to heat something back up with a torch to planish a weld, you are now introducing more heat, which causes shrinking. Counter productive IMO.
For Mig welding, I use a systematic process in welding patch panels that actually promotes some cooling. Let me use some pictorials to explain a bit better.
Let's say we are installing a lower door skin to repair some rust. The repair panel has been trimmed for zero gap, and the panel is tacked on, starting at one end, and working progressively toward the other. Don't skip around from one end to the other as you have a greater chance of misalignment in that you may have more metal on one side than the other, which will result in a buckle from the excess metal on the one side. Start at one end, tack, move a couple inches, align the panels together, make another tack, repeat. This insures the panels are correctly aligned as you work progressively from one end to the other:
Once done with the last tack, go back to the beginning and use a hammer and dolly to planish each weld dot, working in the same start to finish pattern. Now go back to the beginning and use the 3" cutoff wheel to grind the weld dots down to just above flush, both FRONT AND BACK, as this gets the weld prouds out of the way for planishing the next successive sets of weld dots, and also removes all the excess weld that is going to act as a heat sink and affect your weld penetration. I grind to just above panel height for a weld seam like this, and final cleanup with a 3" roloc sander will be used at the end to dress the seam to the parent metal. Here's a video that shows the grinding process, but as this is a plug weld it is dressed immediately following.
Now that those welds are down and out of the way, let's add the next set. Here I'll overlap the last set of welds by about 1/3 to 1/2. Whatever the distance of your overlap, keep it consistent throughout.
This method helps to eliminate any missed areas like what may occur if you kept skipping around. Again, start from the same spot you did before, overlap the first weld dot done, go the next, overlap, repeat, until you get to the end. Then go back and repeat the planish from start to finish, then grind weld dots from start to finish, both FRONT AND BACK, and then repeat the overlap process again. Keep repeating the process until the weld seam has been finished, then use a roloc sander to dress the little bit of remaining weld to flush with parent metal, both front and back side.
If this sounds like a slow, monotonous process, that is the intent. This promotes consistency in the welding process, from fit-up through to final welds dressed. At no point is it necessary to cool the welds because they are too hot, at no time do we need to heat the welds up to planish.
Planishing per Ron Covell is to flatten or straighten out a bumpy panel (if I remember that correctly). As it applies to metal shaping, one would use a blocking hammer and a shot bag to add shape to a panel, but once complete it would appear as if it had walnuts all across the surface of the metal. A planishing operation would smooth these out to a more consistent "flat" finish (where flat means the panel has some crown to it, but no more lumps). As it applies to welds, planishing is the use of a hammer and dolly (or other appropriate devices as the job dictates) to add some stretch back into the weld and HAZ for the purpose of negating any of the shrink that has occurred. I would add that seldom will you get it perfect the first time as you go through the weld process as described above. You will most always have a bit of planishing to complete after the welds have been dressed out, or even some shrinking if you got too carried away in your planishing efforts.
"Despite all the hoopla online" is becoming the new way to say, "well, here in the real world..." That's just a side comment, but I was curious about the cracking welds the same as mapleleaf . Specifically, I have heard of them as something to look out for, and that is why I asked about planishing beads while warm. I like your way.
A LOT to digest & study here- and a LOT makes more sense to me now. Great questions for all of us in the real world, mapleleaf- thanks!
I'm trying to learn...I've gone through the Key to Metal bumping a couple of times now and I'm learning theory...there is lot more to it than beating out the dents and bondoing it...I honestly never saw metal finishing around here..even in the old days. the old boys I saw were more interested in fixing the rust (even with pop rivets) and getting it back on the road. My first 55 Belair had a good part of a stop sign riveted on the quarter over the wheel well and the bondo sculpting was..ahem..artistic.
What I mostly saw was brazing in patches or welding them in with coat hanger wire. Not much concern with warping anything LOL.
The Z I am working on now has several old skool repairs from years ago, a couple of area ares are brazed and some are steel welded...but I have to say it is the prettiest steel welding I have ever seen..almost like a nice MIG weld..the puddles are even and concise..must have been a small torch by someone who knew what they were doing. Not at all like the "functional" ones I have seen here years ago.
Maybe off topic...Any tips for removing brazing beyond just grinding it off? It is in a couple of structural areas - not sure I trust the bronze- and I want to replace it with MIG. Concerns with MIG welding over a previously brazed area?
That's just a side comment, but I