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    Crossbow nut questions

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    Seventeen76
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    Post by Seventeen76 on Sat Sep 14, 2013 11:57 am

    How do you decide on a material? Tradition aside is there a specific draw weight that nylon is better because it's lighter or is steel always preferable?

    Also why are the nuts round rather than half or 2/3 round? Looking at them it seems to me that raising the nut higher and cutting off the top of it would improve the trigger pull. I'm picturing the bowstring catch say 1/2" above the nut axis while the sear is say 1" below it.

    Also is there some way of using a simple nut lock to tie into a trigger or is there too much force involved?
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    Post by mac on Sat Sep 14, 2013 12:58 pm

    Seventeen76 wrote:
    Also why are the nuts round rather than half or 2/3 round? Looking at them it seems to me that raising the nut higher and cutting off the top of it would improve the trigger pull. I'm picturing the bowstring catch say 1/2" above the nut axis while the sear is say 1" below it.

    1776,

    Nuts are round because they typically bear on their cylindrical surface, rather than on an axle.  The socket in which the nut rotates is inlaid with a heavy piece of bone or antler, and it is this which bears the "weight" of the spanned bow.

    Does that make sense, or should I try explaining it differently?

    If you make it to bear upon its axle, you can make the nut  be any shape you please, but the axle, and the places on the tiller that support that axle, must be strong enough to do that job.

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    Post by Hermit on Sat Sep 14, 2013 4:58 pm

    1776,
    Are you building a traditional,or a modern type crossbow?
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    Post by Hotspur on Sun Sep 15, 2013 1:00 am

    For the medieval roller nut many use Delrin rod with a steel insert where the sear contacts it.  I have an ABS nut on a 250 lbs with no ill effects yet.  Delrin is suppose to be superior to ABS in strength.  Not sure what the maximum prod weight would be for Delrin?  Steel and brass nuts will be inefficient for all but the heaviest bows, you will lose FPS pushing the nut forward.  But if you are building that 400 or 500 lb spear thrower steel may be the choice. 
     
    The medieval roller nut is relatively easy to make without a lot of special tools but the modern trigger designs are more efficient.
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    Post by Seventeen76 on Sun Sep 15, 2013 9:58 am

    Thanks that makes sense, I'm looking at making a simple roller nut crossbow just to try it. Usually I make moving parts out of hardened steel so the idea of using bone or plastic is a new one.
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    Post by Todd the archer on Sun Sep 15, 2013 10:16 am

    Don't over look the obvious material "wood", I have made quite a few nuts from walnut for crossbows up to and including 150 pounds. I glue in (J-B Weld) a steel plate for the tickler/trigger to bear against. To keep the odds of success in my favor I make them 1 1/2" wide and orientate the grain vertical when in the cocked position. 

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    Post by Hotspur on Sun Sep 15, 2013 10:36 am

    Make sure the geometry of the nut and tickler is right, particularly on a heavy bow.  When pulling the trigger you don't want the tickler face to be pushing the nut against the weight of the string.

    There is a basic nut geometry plan on Alchem's web site http://www.alcheminc.com/plan2.jpg 

    The whole bow plan is there to give you a starting point.

    Caution ordering parts from Alchem as he is notoriously slow in delivery (often 6 to 8 months).

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