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Planishing Mig welds

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  • Planishing Mig welds

    I had a request to do a tutorial that would help someone understand the process of planishing as it relates to Mig "dot" welding. I'll preface this by saying that this is a "test piece" to assist in actually seeing how the panel will deform when shrinking occurs, and how planishing will help to negate those effects, more so than establishing a hard and fast measured amount of planishing. It is not intended to be the end all-be all of how much, but it should get you in the ballpark of a measured amount to keep the panel in relatively good shape with minimal warping effects during the initial planishing efforts.

    For your planishing test subject, you need two sheet metal strips about 1-1/2" wide by about 15" long. These will be tacked together on the long edge, and works best if you can cut it with a shear, as you can cut a piece 3" x 15 and then shear it through the middle for a perfect seam. For you to see the effects of the shrinking and then the planishing, it needs to be a perfect cut through the middle for best results. So if you have one or can find someone with a shear to help you out, it will help tremendously.

    As long thin strips are more susceptible to bending/curving as the shrink occurs, it helps the "student" to see what is taking place. We could use two 24" square pieces, but it wouldn't be as readily apparent what is changing, so wouldn't serve as well as a learning tool.

    Next, the process and specifically amount of planishing needed is going to be directly related to weld dot size and/or wire type/softness, etc.

    Now that you have your fresh cut test pieces, take your two pieces and align together TIGHTLY along the long sheared cuts and tack the seam at about one inch in from one end.



    DO NOT PLANISH at this point. Go another 3/4" and add another tack. Is there any change in panels positioning? Go another 3/4" and add another tack. What we are trying to do with this process is to monitor how much shrink is occurring and what is does to the panel. The first tack should "anchor" the two panels together at the end. With the panels tight together, each subsequent tack, and the shrinking effects realized once cooling takes place, will start to pull the panels even closer together and shrink along the long axis where the loose ends try to overlap each other. So If you haven't seen this happen, keep welding dots at 3/4" spacing until it does happen.

    Now that you see these panels overlapping, the next phase is to see how much planishing it takes to "undo" the overlap. Start at your anchor tack, and hammer and dolly once.



    Go to each subsequent dot and apply the same hammer and dolly in the same approximate force. The flat should be similar in size, but I'd gauge your effort more on hammer force than size of the flats. When you get to the end, check the overlap to see if it still interferes with adjacent panel fitment at the un-tacked end. If so, start at the beginning, repeating one dot at a time, monitoring overlap. When your panel overlap issue has been resolved, your weld dot planishing effort will be the number of hammer strikes as it took per weld dot to resolve the overlap, using approximately same striking force. This assumes your weld dots don't mysteriously grow in size to add the need for more planishing, so again the importance of OCD consistency. This planishing effort will not be the end of the metal bumping to your panel ie: once you get welding in your patch panel. It approximates the minimum needed to relieve the shrinking effects so the differing forces will relax a bit. So when you get to this process on the "real" panel, the planishing will proceed at the number of XX hammer strikes to to each weld dot, then move to the next weld dot. What we're doing here in this test is striking one dot once and then the next, etc, only for test purposes to identify the number of strikes you need per dot. On your actual repair, after this initial planishing, any remaining planishing needed will be based on what the panel looks and feels like, high spots, low spots, etc after welding, initial planishing, weld dot grinding, and panel reading is completed.

    Now that you have completed this, just for the heck of it, go to the first anchor dot, and start planishing it and it alone. Keep repeating until you see the adjacent ends start to separate as the weld dot is being stretched. Look at how wide the gap is. This approximates the amount of extra effort needed at each weld dot to overcome the shrinking and panel movement that happens when you leave a gap that size in the panel. It also demonstrates the differing planishing efforts that will be needed for inaccurate and inconsistent gaps. Keeping track of what, where, and how much is the tricky part, and again stresses the importance of consistency in all the processes, starting with tight gaps at fit up. Everything that you can do to keep consistency throughout all the processes only makes the planishing efforts more consistent throughout, lessening the need to keep track of the errant what, where, and how much.
    Robert



    Instagram @ mccartney_paint_and_custom


    MP&C Youtube Channel

  • #2
    Great info, thanks!

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    • #3
      Interesting.

      Would this process differ much if you were using a torch?
      I'm still learning

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      • #4
        Bob, with Tig or O/A torch, you have more control over the heat, so welding in a full weld pass is a better option.

        As a comparison, Using the Mig, we are just about forced to weld one dot at a time in order to have enough heat for a full penetration weld, where continuing in a full weld pass would certainly blow holes. The downside to using a Mig in this one-dot fashion is that as the weld dot and HAZ cools, it shrinks circumferentially, pulling the surrounding panel inward from all directions. At each and every dot. This is why it is more effective to planish these welds one dot at a time, then grind them flush to get them out of the way for planishing the next sets.

        By contrast, Full penetration Tig or O/A weld beads are typically less "proud" than their Mig counterpart, for less cleanup. Welding a continuous bead (as on a panel that has been tacked together) will have less shrinking and thus distortion issues than Mig. This is because the continuous weld bead heats up and cools down progressively across the panel, so the shrinking effects are comparatively less than that of the shrink that occurs around each weld dot in Mig welding, and on either the Tig or O/A you can easily wait until the full weld is complete to planish out the panel.

        Where the Mig is more of a "point and shoot" device, it does take less manual dexterity as compared to holding a torch, adding filler rod, and in the case of Tig, operating a foot pedal. Most home hobbyists will have a Mig in their shop over the Tig or O/A. Despite the added shrinking issues with the Mig, it is still a versatile device that can yield good results. Using Ez grind or ER70S-7 wire for softer weld beads helps out in the planishing process. Consistency throughout all the processes, starting with panel fit-up using consistently tight joint, does as well.
        Last edited by MP&C; February 4, 2014, 05:16 PM.
        Robert



        Instagram @ mccartney_paint_and_custom


        MP&C Youtube Channel

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        • #5
          What a great and thorough reply. Thank you.

          The MIG is typically a "harder" weld? And by harder, I don't mean degree of difficulty, I'm talking about the hardness of the material.
          Last edited by Bob Holmes; February 4, 2014, 05:32 PM.
          I'm still learning

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          • #6
            what is meant by "PLANISHING" ? keeping the material flush? i'm a welder, but had never heard that term before.

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            • #7
              Bob, Your typical Mig welder is going to come with a roll of ER70S-6, which yes, does show to be harder after cooling than it's Tig or O/A counterpart. The EZ grind wire is softer and helps out in this regard, but I have heard rumor it is no longer being produced, what is out there is what is available. I have recently switched to ER70S-7, which in addition to being softer, also has slightly more manganese than -6 for better wetting qualities to the parent metal, which means it has a flatter weld, less proud. Where the -7 is more expensive than the -6, having less proud means you will use slightly less than the -6 which just gets ground off anyway, equating to easier planish, less grinding, and more importantly, less abrasive consumables expense. Kind of a wash, given less labor in cleanup.

              So again, where Tig or O/A is a preferred method, especially in coachbuilding, there are options such as the -7 wire that will make the extra work of the Mig much less than it has been..

              Joe, basically "restretching" the shrink effects using a hammer and dolly when it applies to welding sheet metal. When forming sheet metal, using a powered hammer device to stretch and add shape to a panel.
              Last edited by MP&C; February 4, 2014, 06:12 PM.
              Robert



              Instagram @ mccartney_paint_and_custom


              MP&C Youtube Channel

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              • #8
                Sure wish you'd posted this before my last rust repair session! Great info! A companion video would be awesome.
                Thank you,
                BKB
                www.FBthrottlebodies.com
                Bruce K Bridges

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                • #9
                  Thanks again for sharing your knowledge with us. Pertaining to TIG on sheetmetal work - is it possible to do this sort of repair without using filler rod - ending up with a "valley" where the two parent metals are melted together? I remember in class we had to do a method like this on test pieces to prove our ability to control the pedal. It would surely take some practice to be able to do this again, but I'm not sure if this would end up with more clean up or a giant valley at the seam which is no good either.
                  There's always something new to learn.

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                  • #10
                    That would be fusion welding, no filler added, and requires an absolutely tight fitting joint. Here's a sample of that I've done...
                    Separate halves tacked together:



                    Note the lack of/minimal amount of HAZ around the tacks. This was accomplished by holding the electrode as close as you can without touching and using a quick zap. If held farther away from the panel, you see more blue HAZ surrounding the tack.

                    Here is the fusion weld, no filler added.





                    Back side.....



                    Then, using this anvil to planish out the weld and HAZ:



                    Results in this:

                    Front



                    Back



                    Nice and flat, no grinding required (in this case)





                    The undercutting you see on the bottom side is from too much heat. In using this method, the heat is normally controlled by the speed of travel with the torch, as the amps on the machine are set at approx. twice what one would normally use..
                    Robert



                    Instagram @ mccartney_paint_and_custom


                    MP&C Youtube Channel

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                    • #11
                      Wow, thanks for the great info. I was reading your thread http://www.bangshift.com/forum/forum...wagon-progress and because of that I had the basic knowledge to do a test piece for a museum exhibit I am building. The 'furniture' aspect of this piece has a 16ga sheet metal cap with a 4" skirt and 4" radius corners. I used your dot, planish, grind, repeat method when I started wondering about planishing and what it's doing. I was going to shoot you a message with a question about planishing when it occurred to me to try Google and boom, found this thread.

                      Thanks for helping us motorheads in the hobby and professional realms alike. Now I won't be as apprehensive to work on my '64 Chevy 2 wagon!

                      Here are some pics of my quick and dirty mock up for prototyping. Next time I will slow down, I had little cooling time between the weld steps. Maybe less heat, as you can see some under cut, overall I feel it's ok for a first try. Machine is Lincoln 140c with .030 ER70S-6 wire at 3 on wire speed and G for voltage.








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