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    Epoxy brands and sorts

    FrenckBrambo
    FrenckBrambo
    Fresh Blood

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    Fresh Blood Doesn't meanI'm new to crossbows

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    Post by FrenckBrambo on Sun Jan 08, 2017 12:27 pm

    Hallo to you all, 

    I started building a RDT crossbow, and problem after problem arises. But I am determined to overcome them all, maybe with some help of fellow bowyers all over the world!

    Besides the cams being too small (I could not make them large enough form the 10 cm wide aluminium plate I bought) the main problem now is the following:
    For the limbs (do you call it prods?) for my RDT project I considered 3 options:
    1. Laminate; 2. Fiberglass rods molded in Expoxy; 3. Steel.
    I started to try the laminate. For the 40 cm (15 3/4 inch) long limbs I glued together (sail battens);
    1. 3 mm glassfiber (from sail-boat);
    2. 5 mm bamboo;
    3. 4 mm yew sapwood
    4  4 mm yew heartwoord
    5  1 mm bo - tuff.
    I still have to cut out a V-shape in the middle where the cams come inbetween, but the whole is incredibly stiff. When I stand on it, it hardly bends half an inch..
    For the epoxy I used an industrial epoxy (BISON Kombi PRO XL, strong repair). I know epoxy EA 40 is often recommended.

    Can the immense rigidity be caused by the wrong type of epoxy, or will the amount and thickness of the layers be the cause?


    Last edited by FrenckBrambo on Mon Jan 09, 2017 6:48 am; edited 1 time in total
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    kenh
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    Post by kenh on Sun Jan 08, 2017 4:14 pm

    You're not using the right kind of fiberglass laminates if you're using boat battens.  Battens on my boat are solid extruded fiberglass not glass strands in an epoxy matrix which is what "bow glass" is made from!  Bow glass and wood laminates is in the range of .03" to .1" not 3-5mm  thick!

    Using the wrong materials can get you in some trouble.  EA-40 as epoxy for wood/glass laminates has decades of proven results.

    And your stack thickness is what controls the strength of the final bow, not the stiffness of the epoxy. 

    Even if the prod only bends a bit when you stand on it, you're not going to be giving it a 6" or 8" brace height -- more like 3".  My loose-lam glass tension bar prod has a 3" brace and at 13" is drawing over 250#.
    FrenckBrambo
    FrenckBrambo
    Fresh Blood

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    I'm new to crossbows


    Fresh Blood Doesn't meanI'm new to crossbows

    Posts : 15
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    Post by FrenckBrambo on Mon Jan 09, 2017 2:54 am

    Thanks Kenh,
    the bamboo laminate is sold here at 5 mm thickness. Even though it is meant for longbows, I guess that should be ok.
    I don't fully grasp your comments on the brace height. I am making a reverse draw bow. An equivalent of 3"brace heigt would be fine. I only wonder if the stack is not too stiff..
    Is that the 250# bow in the picture? And how much do the limbs bend if you support them at the ends and stand on it (assuming you have a more or less normal weight)?
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    kenh
    Crossbow Junkie

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    Post by kenh on Mon Jan 09, 2017 4:56 am

    The bamboo laminate is fine I'm sure; it's the overall thickness of bamboo and fiberglass I question, as well as the kind of fiberglass.  All you can do is try, and pray that things don't break.

    Yes that's my 250# prod.  It is 28" long and when I stand in the middle my 215 lbs bends it only an inch or so.   When I suspend it from the ceiling and hang my weight on the string (like a bolt) to draw the string back from its 3" brace height to cock at 13", my weight only draws the string back about 8" rather than the 10" needed to cock.
    FrenckBrambo
    FrenckBrambo
    Fresh Blood

    Doesn't mean
    I'm new to crossbows


    Fresh Blood Doesn't meanI'm new to crossbows

    Posts : 15
    Join date : 2017-01-01
    Age : 58
    Location : Groningen, Netherlands

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    Post by FrenckBrambo on Mon Jan 09, 2017 5:40 am

    That inidicates my limbs might work. I am going to try (cut out the V-shape first) and let you know later (might take some weeks).
    Thanks for the answers!

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