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    Samostrel-Crossbow

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    Drunken Sailor
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    Samostrel-Crossbow

    Post by Drunken Sailor on Sun Sep 29, 2013 2:39 pm

    Hello guys.

    I am making a crossbow.Here are some pictures of my progress,and a design.I plan to make it over next couple of months.

    Design process:

    http://postimg.org/image/6oi5m39s1/

    http://postimg.org/image/3n57aux2d/

    http://postimg.org/image/9krv0ise9/

    I would upload a CAD file but i don't know how.

    Manufacturing process:

    http://postimg.org/image/6x7u73gf9/

    http://postimg.org/image/w8r8jydcn/

    And I have a question: Does any one know what glue is good for gluing aluminum-aluminum,and aluminium-stell? I was thing about using a 2 component glue on acrylic base.





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    Re: Samostrel-Crossbow

    Post by Seventeen76 on Sun Sep 29, 2013 4:38 pm

    Welcome to the forum Sailor.

    You cannot use glue to connect metal parts that will be subject to vibration or stress, if you must try it then roughen the surface, remove any oxides with sanding or acid, clean with MEK and then apply your 2 part epoxy and clamp. Your parts will be "stuck" together but the connection is just for show and cannot be trusted under strain.

    Al to Al connections can be welded, brazed or soldered but Al to steel cannot be welded and can only be brazed or soldered with special materials and processes. I suggest you alter your design to allow for mechanical fasteners between the Al and steel and mechanical fasteners or brazing between aluminum parts.
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    Re: Samostrel-Crossbow

    Post by Gnome on Sun Sep 29, 2013 4:42 pm

    Hello, Sailor!
    Welcome to the forum. Good to see you documenting your process, it's a great tool to improve your skills, design and fabrication. Regarding gluing alumium or steel, I'll quote Geezer from another thread: "Stop, go no further... This one is a non-starter.... go back.... go back.... go..... back....!"
    The appropriate way to join them depends on what parts they are and what sort of stress they'll be under. Could you provide some more details?
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    Re: Samostrel-Crossbow

    Post by ferdinand on Mon Sep 30, 2013 1:02 am

    Or u call the people at the Lotus car factory!! They glue the aluminium chassis for years!
    But if they give away the secret?
    glue is only suitable for big surface to surface connections. 
    So for u that means weld, solder, bolt or rivet!
    Good luck, keep it safe!
    (u dont want to connect to many different metals together by the way, u will create a galvanic corrosion due to the difference in electric potential. )
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    Re: Samostrel-Crossbow

    Post by Drunken Sailor on Mon Sep 30, 2013 2:44 am

    I was thinking about gluing a part of mechanism that holds the string and the pulleys.
    That 2 component glue has strength of 160 kg/cm^2. I will firs try it on some scrap aluminium to see if that works. If not i will solder them.

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    Re: Samostrel-Crossbow

    Post by tubis on Mon Sep 30, 2013 4:34 am

    I would use screws to mount alu parts on other metals.

    The surface between the trigger and the latch needs hard hardened metal not aluminium. It looks like you made the trigger out of aluminium.

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    Re: Samostrel-Crossbow

    Post by Seventeen76 on Mon Sep 30, 2013 5:17 am

    By all means try it and see what happens, if the glue is holding secure your aluminum will break or tear before the joint fails.

    Soft solder (lead/tin) is a reliable bond but it's only as strong as the solder material itself so for small or high stress joints I use hard solder or braze. Of course lead/tin is easier to apply and costs a small fraction of what brazing rods or hard (silver) solder cost so for some applications lead/tin is ideal.
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    Re: Samostrel-Crossbow

    Post by Drunken Sailor on Mon Sep 30, 2013 5:57 am

    Yes i made a trigger out of 6mm thick aluminium. Ok, then i will made the part that hooks the string out of steel. I was thinking of that,but i don't have the tools for that. I will go at local shipyard and they will fabricate that part for me.

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    Re: Samostrel-Crossbow

    Post by tubis on Mon Sep 30, 2013 9:27 am

    Drunken Sailor wrote:Yes i made a trigger out of 6mm thick aluminium. Ok, then i will made the part that hooks the string out of steel. I was thinking of that,but i don't have the tools for that. I will go at local shipyard and they will fabricate that part for me.
    Well, the trigger part that grinds on your next part should be made of steel too or it will grind down in no time. Alu is soft as butter.
    Either the whole trigger out of steel or put at bit of hardened steel on the  surfaces that grinds.....
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    Re: Samostrel-Crossbow

    Post by kenh on Mon Sep 30, 2013 12:18 pm

    From everything I've seen, you're better off avoiding aluminum altogether.
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    Re: Samostrel-Crossbow

    Post by dutchbow on Tue Oct 01, 2013 12:13 am

    If you want to use aluminium you can thinking on hard anodizing the parts. this will give a very hard and wear resistant layer.
    After anodizing the color of the part will be black.

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    Re: Samostrel-Crossbow

    Post by PierreC on Tue Oct 01, 2013 1:01 am

    Sending the trigger part out to be hard anodized will be more time consuming and expensive than buying a grade-8 bolt at the auto-parts place, drilling and tapping a hole in the trigger...  Personally, I find that grade 8 at about 40 Rockwell is still a bit soft for a wear surface, so I'd heat it with the torch to cherry red and quench in oil.

    Most grade 8 bolts will be 4140 Cromolly, and should come out of the oil glass hard.  You might want to temper to light straw or so, but either way, screw the bolt into your aluminum with a dab of red locktite, and shape the head with your grinder to get a nice sear.
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    Re: Samostrel-Crossbow

    Post by Drunken Sailor on Tue Oct 01, 2013 5:42 am

    Ok,I will then make the whole mechanism out of steel.

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    Re: Samostrel-Crossbow

    Post by tubis on Wed Oct 02, 2013 11:52 am

    http://en.wikipedia.org/wiki/Case_hardening

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