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Cracks around dimpled holes

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  • Cracks around dimpled holes

    I noticed a small crack on my lower cowl around a dimpled hole for a #8 screw where it attaches to the firewall. On further inspection, many of them have tiny cracks, both top and bottom cowls. Not the sort of thing I like to discover and a bit of a heart sinking moment. I had deburred all of the holes. These are the largest dimples I’ve done, and I hadn’t done any #8’s before starting the cowling.

    I’ve discussed it with my mentor today, and spent some time on various forums trying to discover where I’ve gone wrong. So far I haven’t found the same issue on any smaller holes.

    6114E949-5DA7-4651-A934-7062522DD534.png

    It seems the cause can be not enough deburring, too much deburring, metal riding up the drill bit, hole too small, low quality dimple dies etc etc.

    There appear to be a number of solutions including using a larger drill bit, smooth the hole very carefully, and using WD40 to lubricate (which apparently gets very good results).

    My main concern is what to do now. Most of the cracks are tiny and difficult to see, but I’ve kind of resigned myself to having to remake the cowlings. I figure that the lower cowling in particular will have a cantilever force on it, as well as air loads, so there’s probably quite a load on the dimpled holes. I couldn’t find any cracks on the firewall dimples interestingly, and the loads on the firewall are being taken by the nutplates.

    I did some test holes in .032 2024 T3 aluminium to see where I had gone wrong, yet none of them cracked. (even when I didn’t deburr the holes which doesn’t make sense to me)

    A review of the Vans forum on the topic shows that it’s not an unusual occurrence. One engineer said he had never seen a crack propagate out of a dimple and it appears some people just accept it. What to do ?

    Any advice is appreciated.
    Last edited by Nev; 04-22-2021, 08:42 PM.
    Nev Bailey
    Christchurch, NZ

    BearhawkBlog.com - Safety & Maintenance Notes
    YouTube - Build and flying channel
    Builders Log - We build planes

  • #2
    What is the angle of the back side of the screw you are using? Dimple dies typically are 100 degs. Verses screw head angles being 82 degs or 100degs depending on what you received.

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    • #3
      In my experience, the stuff you think will crack never does. The stuff you swear will never crack does. Make your mistakes on this cowl, fly it till it cracks out, patch it, fly it some more, then make a better one. By then you'll have a list of changes you'll want to make that you don't know about yet.
      Gerry
      Patrol #30

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      • Sir Newton
        Sir Newton commented
        Editing a comment
        100% wisdom.

    • #4
      The US Navy panics over cracks and grounds airplanes.. The US Air Force monitors them. For the exact same airframe.

      I have no definitive answer. Like you, this is my first build, even though both of us have built other stuff. One of the things that shocked me, was asking technique questions in sheet metal working, and getting 10 different answers. Despite the world of knowledge at my fingertips, (internet), I still got mostly opinion and technique.

      My personal technique, for now, is to monitor imperfections, even though I want to achieve perfection. I think if I only accept perfection, I won't ever get out of bed.
      I wish I had a better answer. But that is the best I can come up with.

      I guess I would say I am a manager of risk, and imperfection.

      Comment


      • #5
        I’m my experience those cracks are cause by the hole being to small. The nipple on the dimple die goes into the hole, stretches the metal when you squeeze and causes the cracks. I can’t remember the sizes exactly but in my case I was drilling the holes with a 1/8” drill bit rather than a #30 bit or something. Once I started using the right bit there were no more cracks.

        I can’t offer much a of recommendation. I didn’t paint my cowling so I would be willing to run that cowl and monitor the cracks. Making a new one wouldn’t be a big deal. If I were going to paint the cowl I’m pretty sure I’d be remaking it so I didn’t have to paint twice.
        Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.

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        • Nev
          Nev commented
          Editing a comment
          I think that’s what I’ve done too, used a drill that is too small. Thanks.

      • #6
        How did you support the back side of the dimple when you were forming it? I don't have any answers either but am curious.

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        • #7
          These panels are non-structural and, with the screw in place, are not going to be subject to any significant load or flexing. The cracks are therefore unlikely to propagate. I agree with Gerry ....

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          • #8
            Originally posted by quadra View Post
            How did you support the back side of the dimple when you were forming it? I don't have any answers either but am curious.
            Just the standard male/female dimple die - if that’s what you meant.
            Nev Bailey
            Christchurch, NZ

            BearhawkBlog.com - Safety & Maintenance Notes
            YouTube - Build and flying channel
            Builders Log - We build planes

            Comment


            • #9
              Paul beat me to it - the cowl isn't structural, so I wouldn't be too worried about it. It is also the one part of the plane that is most likely to need some modification after a bit of flying (cooling issues, etc.) I recommend moving forward and keeping an eye on it. It's fairly simple to use the old cowl as a pattern for a new one so maybe lay that out on paper now and set it aside for later (if it's still a flat piece of metal at this point)

              I don't have an idea why you got cracks, but I like the idea to lube with WD-40, I hadn't heard that before.
              Rollie VanDorn
              Findlay, OH
              Patrol Quick Build

              Comment


              • #10
                I'm with the 'keep an eye on it but don't worry about it' crowd. I drilled the holes a bit undersized, used a reamer to get a clean hole, deburred carefully but not too much and used a bit of WD40 before dimpling. Not sure if it was all necessary but the results were good.

                Comment


                • #11
                  Agree with others. If it was on the boot cowl I'd probably fight with it given the effort to pull the boot cowl after it's done, but given that this is on the cowling, I'd totally fly it until it's a problem as it's not going to fail all at once.

                  Comment


                  • #12
                    Remove the cracked material, otherwise the cracks will almost certainly grow. No need to discard the whole cowl as others have said, but you will want to remove the cracks first.

                    Comment


                    • #13
                      Thanks for all the advice here, just what I was after. My mentor and retired aircraft engineer took a look today and agrees with advice above to use the cowls as they are. He thinks perhaps the cracks possibly may not propagate because of how they were formed and their location. Lesson learned for me when forming the larger dimples.
                      Nev Bailey
                      Christchurch, NZ

                      BearhawkBlog.com - Safety & Maintenance Notes
                      YouTube - Build and flying channel
                      Builders Log - We build planes

                      Comment

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