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  • Need Advice

    I need to attach a pair of 3/8" aluminum plates (about 3" x 6" ) to some 16ga steel. The plates have a decorative ball mill on them and need to line up in a very close alignment with each other. I was thinking of trying to drill them on the edge and insert a roll pin for the alignment, but I'm open to other ideas. On the back side of the aluminum I need to attach some machine screws (studs) so I can put nuts on them after they pass through the steel. Not sure how to attach anything to the aluminum as it's too thin to drill and tap and if I could find any aluminum screws, welding them might distort the ball mill design on the front and would certainly F up the threads. I was thinking of possibly using some JB weld compound to attach some machine screws to the aluminum.
    Any ideas? Thanks

    EDIT: The AL is only 3/16" not 3/8"......
    Last edited by nocheepgas; 05-25-2009, 09:44 PM.

  • #2
    Well, 3/8" you've got alot of meat there to tap. though bonding it to a mild steel plate without an insulator makes for immediate corriosion between the 2 disimular metals.

    Comment


    • #3
      As cruizer said 3/8” is a decent thickness to work with. I think what I would do is get a couple of #7 drills and grind one off flat. I would drill carefully to a depth of 5/16”, then drill again with the ground off bit to the same depth, giving a flat bottomed hole. Tap this with a ¼-20 bottom tap which should make at least ¼” of threads. McMaster and probably others sell ¼” aluminum studs (example part# 93225A869). I would epoxy these studs in and these would make the mounting bolts.
      You can also get plastic or metal dowel type alignment pins from the same source. (Plastic example part# 98703A740).
      There is still the dissimilar metal problem.

      Comment


      • #4
        Not too worried about the interaction of dissimilar metals, The aluminum will be clear anodized and the steel painted. But reading your responses made me realize my mistake, material thickness is 3/16".

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        • #5
          Originally posted by nocheepgas View Post
          Not sure how to attach anything to the aluminum as it's too thin to drill and tap
          I did some investigating in Machinery's Handbook (28th Ed.) and found the following on page 1443:

          ...the length of engagement of mating threads should be sufficient to carry the full load necessary to break the screw without the threads stripping.

          If mating internal and external threads are manufactured of material having equal tensile strengths, then to prevent stripping of the external thread, the length of engagement should be no less than that given by Formula (1):

          Le= 2xAt/[3.1416 Knmax[1/2 + .57735n (Esmin-Knmax)] ]...
          So I made a spread sheat and entered all the dimensions of common class 2A threads, fine and coarse, and found the minimum length of engagement Le for all of them. Any screw with a Le less than .1875" (3/16") can be used without stripping (provided it's a similar grade of aluminum). Here's the results:

          screw___thread___Min Engagement__AT_________Kn max _____Es min
          size_____TPI_____inches__________sq. in.______inches______inches
          4_______40_____0.087561624_____0.00604_____0.0939_ ____0.0925
          4_______48_____0.094257374_____0.00661_____0.0968_ ____0.0954
          6_______32_____0.101150431_____0.00909_____0.1140_ ____0.1141
          6_______40_____0.111695325_____0.01015_____0.1190_ ____0.1184
          8_______32_____0.124112652_____0.01400_____0.1390_ ____0.1399
          8_______36_____0.130004293_____0.01474_____0.1420_ ____0.1424
          10______24_____0.133231699_____0.01750_____0.1560_ ____0.1586
          10______32_____0.145589954_____0.02000_____0.1640_ ____0.1658
          12______24_____0.155182487_____0.02420_____0.1810_ ____0.1845
          12______28_____0.162915746_____0.02580_____0.1860_ ____0.1886
          1/4_____20_____0.172846497_____0.03180_____0.2070___ __0.2127
          1/4_____28_____0.194909137_____0.03640_____0.2200___ __0.2225
          5/16____18_____0.223025271_____0.05240_____0.2650___ __0.2712
          5/16____24_____0.242414386_____0.05800_____0.2770___ __0.2806
          So anything up to a 1/4x20 UNC can be used. One thing that suprised me is that I thought for sure a 1/4x28 UNF needed less engagement than a coarse screw. According to the equation, I was wrong.

          Anyway, if you can carefully drill it without breaking through (say with a drill press and a 2 flute end mill, you could tap it to, say, 10x24. And keep in mind that the Le calculated gives the depth required for the full breaking strength of the screw. Depending on how many screws you use, you may not need anywhere close to the breaking strength. So you might even drill 75% depth and tap to 60% depth.
          Last edited by Bodybagger; 05-26-2009, 04:01 AM.

          Comment


          • #6
            Originally posted by nocheepgas View Post
            Not too worried about the interaction of dissimilar metals
            If you find your self in a situation where you are, swab on a layer of this to each side to be bonded. Clamp them together according to time limit.
            Attached Files

            Comment


            • #7
              Besides you could always use rivets, or aircraft riv nuts

              Comment


              • #8
                Originally posted by Sonora Iron View Post
                If you find your self in a situation where you are, swab on a layer of this to each side to be bonded. Clamp them together according to time limit.
                Originally posted by cruizer View Post
                Besides you could always use rivets, or aircraft riv nuts
                The assembly needs to be removeable, and I didn't want anything showing on the face of the AL otherwise I'd just use a machine screw through both pieces of metal.

                Bodybagger: Thanks for the info. the problem I see is the hole depth would be 1/8", and that does not seem to be enough room to get a tap started. Thinking of seeing if I can find a T-slot mill that's only 1/16" and sliding screws in from the ends???

                Comment

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