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  1. #1
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    Bending chain stay 25 degrees

    Need more help on this. Previously I posted a thread asking for ideas for bending small diameter tube. I am on my last piece of scrap chain stay and need to come up with a solution
    Radius of bend is about a half of an inch. Tube is squished to a dimension of 26 x 7 mm. The bend is to turn the tube correctly towards the rear dropout.

    Shoving rods in the tube and I can't remove them as they get stuck at the bend. Nothing in the tube and it crimps.

    Thought of using some mild steel rods of very small diameter in hopes of making them easier to remove, but this is last shot at it with "free" tube and want some more ideas.

    Ideas, anyone?

  2. #2
    Decrepit Member Scooper's Avatar
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    That sounds like a very tight radius for thin walled tubing.

    One trick we use in building experimental category airplanes is densely packing clean, dry sand inside the tube and capping both open ends of the tube with wooden plugs before bending. This greatly improves the odds of bending without flattening or buckling. This EAA article by the late Tony Bingelis explains the process on the last page of the article under "The Use Of Filler Materials."

    How to Bend Tubing Successfully (1.14 Megabyte pdf file).
    - Stan

  3. #3
    Randomhead
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    freeze soapy water in it. Use something like cerrobend. Bend first, squash later. Those are my three ideas.

  4. #4
    Decrepit Member Scooper's Avatar
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    Quote Originally Posted by unterhausen View Post
    freeze soapy water in it. Use something like cerrobend. Bend first, squash later. Those are my three ideas.
    Bingelis suggests Cerrobend in his article and describes it as a low melting temperature bending alloy. I'd never heard of it, but it sounds like it might work better than packed sand.

    CS Alloys Bend 158 (formerly CERROBEND)
    - Stan

  5. #5
    coprolite fietsbob's Avatar
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    Pitch and tar mix tightly packed , and a shoe bending jig offering the outside shape and the inside
    which can be in hardwood.. will perhaps help.

    the pitch - tar blend will burn out at low temperatures, to remove the fill ..

  6. #6
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    I was aware of the frozen liquid method, but not sure how to cap the ends and fear splitting the tube during the freezing process as I have seen it happen with aluminum tubing.

    I think the sand method is what I shall give a go. One end of the tube is already sealed with a brazed in piece so plugging the other end with a wood plug should be easy enough.

    Thanks to all who contributed!

  7. #7
    Nigel nfmisso's Avatar
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    Use something like this:
    http://www.amazon.com/Low-Melting-Po...ing+temp+alloy
    seal one end of the tube; melt the alloy in a double boiler over boiling water; pour in the tube; wait ½ hour; bend; heat up to melt the alloy out.
    Nigel
    Mechanical Design Engineer

  8. #8
    tuz
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    1/2 inch bending radius? This is impossibly tight IMO. Is there a reason for that?

    I've had success bending c-stays by packing them with sand and using my fork bender. It has a 6'' radius. One half is flat and has no groove for the blade, if there is one it'll leave an indent.

    I could not get the sand packed tighly enough and it resulted in a small indent at the bend. Incidentally that was beneficial. That was for a 3-4 degree bend (enough for a 26x1.85 tire @ 440 c-stays); not sure it will work with a 25 deg bend unless you shape the mandrel to the oval dimensions.

    I think I showed the results in your previous thread.
    homebuilt commuter, mixte, road and track (+ Ryffranck road)
    bla bla blog

  9. #9
    Decrepit Member Scooper's Avatar
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    I'm beginning to think filling the chainstay with molten low melting temperature filler alloy like Cerrobend, bending it, then heating the stay to melt the filler alloy and drain it from the stay is the best way to get such a tight radius bend.
    - Stan

  10. #10
    Randomhead
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    it is actually half inch radius? I don't think that will work either, not even with quarter inch diameter tubing. The angle of the chainstay to the dropout is only about seven degrees, I think I need to see a sketch

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