bushing inserts into aluminum plate

Discussion in 'Metal Boat Building' started by Charly, May 14, 2014.

  1. Charly
    Joined: Dec 2009
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    Charly Senior Member

    Hey guys, my plans call for bushing inserts in the chainplates and the rigging plate on the bow beam of my cat build.
    more about it here:

    I have very little experience working with metals, but I figured I might be able to do this at home, on site. Over at onlinemetals they have some 1/2" 316 ss round stock, and some seamless tubing that has an id about 005" larger, so I figure to use the round stock instead of a clevis pin, and cut the tubing to length for the bushing.
    I thought that I might simply freeze the pin inside the bushing overnight, in the refrigerator, and bore a hole in the aluminum piece, and then stick it inside before it warms back up., making a good tight fit. Though I 've never done this it seems straightforward enough.

    Some questions I still have, and would appreciate any answers:

    Is there anything special with this techniques I should look out for or be aware of?
    What about galvanics? best way to insulate aluminum from 316 ss?
    Is there a better choice of metal for the bushing and pin?
    I already cut the plate to shape with a jig saw (its 3/4 inch stock) how to best polish out the saw marks?
    Does it really matter to polish out the saw marks?
  2. upchurchmr
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    upchurchmr Senior Member

    Polishing saw marks out of aluminum matters a lot if you want long life.
    Aluminum tends to start a crack at such a feature then slowly progress over the life of the boat until it fails.
    A boat gets lots of load cycles in waves as the mast bounces around.

    Sand them out, then polish to a shine. Doesn't have to be a real good shine.
  3. waikikin
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    waikikin Senior Member

    Got a belt sander? work around the chainplates with tat along the cut edges.

  4. Charly
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    Charly Senior Member

    Thanks guys, that makes sense. I will try the belt sander this weekend.

    My guess is the best way to insulate the ss press in bushing from the aluminum (6016), would be to have the aluminum piece anodized first? Is there a better way? How much build up do you reckon anodizing would add to the inside of the hole? I had planned to paint the thing afterwards with some of Paul Oman's magic Aluthane as a primer.
  5. Charlyipad
    Joined: May 2014
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    Charlyipad Senior Member

    The sander works fine, now the aluminum plate pieces are shiny and slick

    So next I will bore the holes before I get the parts anodized. I am wondering what is the best fitting tolerance to minimize corrosion? I f the holes are a little big, or sloppy, I could put more insulation in between the ss bushings and the aluminum plate... And what would be best for that? Zinc chromate? 5200? Aluthane? OR... Would better practice be to get the tighest possible fit by heating the aluminum and putting in a cold bushing, and then trying to just keep the salt water off of it (more 5200?)

  6. Petros
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    Petros Senior Member

    corrosion will occur with dissimilar metals if they are incompatible, no matter how tight the fit. if you are using the recommended materials they should be okay to be in contact with each other presuming the designer choose metals that do not tend to react with each other. You can go from a "net fit" to interference of about .004". Usually just careful taping with a hammer and appropriate tool will set it, or you can carefully drive it in with a vise. You can also install the bushing "wet" with sealant to minimize moisture intrusion.

    even if press fit and swagged or staked into place, the bushing will tend to work out unless they are captured in place with the fastener head or washer large enough to hold it from drifting out. Hopefully the design of the installation accommodates that.

    Too many designs depend on the interference fit to hold parts into place, this is always a bad idea, the part should also be captured by the built up assembly to make sure it does not drift out under load or because of temperature cycling.
  7. Charly
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    Charly Senior Member

    Thanks Petros

    Seems like if I anodize it and then press in a bushing tightly, it will scrape the anodizing off the aluminum and defeat the purpose, so I guess I will leave some play in it and work in some 5200 as insulation. A few of these holes (smaller ones) will be out of sight, buried down inside the wooden beam. They will be what fastens the alum plate vertically to the wooden insides of the beam. There will be no chance of them working out, but there will also be no way to inspect the thing after it all goes together. So there can't be too much protection of the aluminum to suit me.

    Thanks for the advice.
  8. Charly
    Joined: Dec 2009
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    Location: st simons island ga

    Charly Senior Member

    more nitpicking

    Today I got some more metal pieces from online metals.

    The ss piece I chose for the bushing is 1" od 1/4" wall thickness and 1/2"id.
    After looking at it in person, I am wondering if It is too heavy for the application. It's not like the 1/2 inch pin will be rotating inside, and wearing on it,so maybe I don't need 1/4inch wall thickness, and could get by with 3/4od 1/2id, with 1/8" wall.

    And here's another question: won't the likelihood of galvanic corrosion be lessened if the mass of the more noble metal is reduced in ratio to the aluminum in proximity to it? IOW are my bushings too fat?:) Is it worth changing to a smaller bushing wall thickness for that reason alone?

    Finally I wonder what the size of the hole for the bushing insert should be in relation to the size of the aluminum from a structural standpoint. The last photo here shows the bushing material standing on end atop the chainplate, in about the position it will be installed in. Is there a ratio of strength calculation just like when cutting a lightening hole? Does a larger insert add strength or does it matter?

    Thanks for any comments

    Attached Files:

  9. Charly
    Joined: Dec 2009
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    Charly Senior Member

    I downsized the bushing to 3/4 inch od stainless, with 1/2 inch id.

    Today I drilled a test hole with a drill bit from ace hardware. It's fairly sloppy. You can kind of tell in the picture here, where I have just stuck the bushing stock in the hole and left it standing upright. Behind it is the spreader with the rigging plate etc. laid out ready to weld up. With no experience, I have no idea how much play is too much. I do know that 5200 is fairly strong stuff, but does it have enough compressive strength? Will it hold the bushing in place? The bushing cant really go anywhere because of the shackle or turnbuckle jaws.

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