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    Crossbow Lock Sideplates

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    benjaminh123
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    Crossbow Lock Sideplates

    Post by benjaminh123 on Sat Mar 28, 2015 3:00 pm

    Hi everyone,
    I'm about to begin building my first crossbow; a big thank-you to all you great guys here that have helped so much.

    One design question though; If I decide to make a drop in lock (like this one, just from aluminum) how thick should the side plates holding the sear and trigger pivots be?
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    Re: Crossbow Lock Sideplates

    Post by mac on Sat Mar 28, 2015 5:15 pm

    Ultimately, it's all about restoring the strength of the tiller which is lost by removing wood to make room for the nut and lock.  As such, it comes down to a question of "how strong must the tiller be?"  That is largely a function of the spanning method and the draw weight. 

    A bow which is spanned by putting your foot in a stirrup and drawing the string with your hands or a belt hook does not need much strength through the nut well. 


    By contrast a bow which is spanned be a mechanism that attaches on the "user side" of the nut, will put the tiller into a bending load, and the sides of the nut well have to be up to resisting that load.  So bows that use a goat's foot, a craniquin, a windlas, or a hook and pully need strength here in proportion to the strength of the prod.

    A bow that spans with a wippe falls somewhere in between because although the wippe pushes against a ring on the "prod side" of the nut, the tiller is used to steady the bow against the effort applied to the handle.




    Mac

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    Re: Crossbow Lock Sideplates

    Post by benjaminh123 on Sat Mar 28, 2015 5:39 pm

    I'm not sure if you really are understanding my question; lock plates that I'm speaking of (look at the link in the first topic) are just holding the pivots for the trigger and the sear in this mechanism that I plan to use:

    The nut will have its own separate side plates; what I'm speaking of will just have the trigger and sear putting weight on it; would 1/8" aluminium work?

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    Re: Crossbow Lock Sideplates

    Post by jaeger22 on Sat Mar 28, 2015 6:47 pm

    Yes 1/8" should work fine for bows up to 150 lb. or so. The two factors that it depends on on are the type of aluminum and the pull weight of the bow. If you use a 1/4" axle the area resisting  the force is 1/4"  times 1/8" times two (two sides) or .0625 inches squared. So even if you use soft aluminum with a yield strength down around 5000 PSI per inch, it still should not fail until you hit 312.5 pounds of pull. So a safety factor of 2 for 150 lb pull. Most aluminum will be stronger. Pure aluminum is only 4000 psi but hard to find any pure.
    However if you want to build a 250 lb pull xbow, not so much!

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    Re: Crossbow Lock Sideplates

    Post by benjaminh123 on Sat Mar 28, 2015 6:59 pm

    Thanks Jaeger22,
    That was just what the information I was looking for; how much space around a 1/4" pivot would be  "safe"? How close to the edge of the aluminium could the pivot be? Please bear in mind that I am only speaking of the trigger and sear, NOT the nut. You could refer to the drawing above, in my last post.

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    Re: Crossbow Lock Sideplates

    Post by jaeger22 on Sun Mar 29, 2015 5:14 am

    It looks like there will be only a little less stress on the shear so I would leave at least 1/4" of material around it and the trigger is lightly loaded so if you needed to for your design you could go as low as 1/8" . More is always better but those are what I would as minimums.
    Good luck and please post your progress.
    That is a very interesting trigger design. I may have to build one myself!

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    Re: Crossbow Lock Sideplates

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