Crossbows - Everything about Building, Modding, and Using your Crossbow Gear

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    Nut dimensions

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    benjaminh123
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    Nut dimensions

    Post by benjaminh123 on Sat Mar 28, 2015 5:43 pm

    For a 1.25" round nut, how far up do you usually put the notch for the string from center?

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    Re: Nut dimensions

    Post by phuphuphnik on Sun Mar 29, 2015 7:04 am

    My rule of thumb is to have it 1.25-1.5 times the string diameter. My strings for the 150# are about 1/4" so the hook is an east 3/8"  I'm using dacron (D-50) Old horn nut hooks seem huge, but the strings were really thick as well.
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    Re: Nut dimensions

    Post by Geezer on Sun Mar 29, 2015 8:03 am

    My rule is to sink the roller nut 2/3 into the stock.  So 1/3 projects, and that's how high my lugs are.  If you sink the nut as much as 3/4 you won't be able to take it out of the top of its socket... will have to come out sideways.  Geezer.
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    mac
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    Re: Nut dimensions

    Post by mac on Sun Mar 29, 2015 11:45 am

    I think I am seeing something like a ratio of 7 to 3 in these X ray pics by Harmuth.  So, a 1 1/4" nut would end up with 7/8" buried, 3/8" exposed, and it's axis 1/4" below the surface of the tiller.

    This has been my "go to" image when planning a crossbow, and I have had good results by making my sections through the nut well look more or less like this.

    By Geezer's reckoning it's 7/10, which is bit deeper than his recommendation of 2/3, and just shy of his "don't go there" depth of 3/4. 



    Mac
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    Geezer
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    Re: Nut dimensions

    Post by Geezer on Sun Mar 29, 2015 5:51 pm

    If it works, it works, and certainly in most respects a deeper socket is better, so long as you have enough lug to hold the string and can get the nut in and out when you need to.  Geezer
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    Re: Nut dimensions

    Post by mac on Sun Mar 29, 2015 6:11 pm

    That is the sine qua non

    You have me wondering, though, exactly where that nut-removal limit is.  I will do up some two-dimensional models some day and see if I can determine it to an exactitude.  I have a sneaking suspicion that it may depend on the shape of the nut fingers, but I have not gotten beyond that yet.

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    Re: Nut dimensions

    Post by Geezer on Sun Mar 29, 2015 7:06 pm

    Got it in one, Mac. It's possible to finess the nut removal by changing shape of the lugs, deepening the platform behind it, and also whether you want a lip of the platform to actually show above the back of the socket (I prefer to do that, rather than having it fit flush for various reasons) If all else fails, you can always back-bevel the edges of the socket a bit to squeeze things in.  Geezer.
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    Re: Nut dimensions

    Post by mac on Sun Mar 29, 2015 8:45 pm

    Curiosity got the better of me, and I made up a series of drawings to investigate the nut-depth thing. I drew this out with a 1 1/4" (32mm) nut, and set it into the "tiller" at different depths based on 1/16" increments.  Since 1 1/4" is 20/16,  I described each depth in terms of fractions of 20, and then as a percents. The string notch is the most typical shape for historical nuts from the high middle ages on.


    The first column shows the nut in the locked position, with a 5/16" (8mm) diameter string against the fingers. 

    The second column shows the nut rotating through the middle of the release phase.

    The third column shows the nut passing through (or falling to pass through) the tightest place in the nut well during the removal process. 

    At 65%, the lock is good and the nut will come out very easily.  The only down side is how the string is carried up above the "deck" a little bit.  Making the notch a bit deeper would fix that if it turned out to be a problem.

    At 70% the string lifting thing has worked its self out.

    At 75% the nut is still readily removable.

    At 80% the engagement of the string with the fingers starts to look a bit dicey, and the nut may or may not come out.  At this level of modeling, I can not be sure.

    At 85% the fingers are treacherously short and the nut is definitely stuck in the nut well for good.



    All things considered, I feel good about the range from 65 to 75 percent.  I think that for medieval nuts, one would do well to shoot for 70% and accept an error of up to 5% either way.

    Mac
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    Re: Nut dimensions

    Post by Anatine Duo on Mon Mar 30, 2015 11:31 am

    That illustrates it very well, Thanks Mac

    benjaminh123
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    Re: Nut dimensions

    Post by benjaminh123 on Mon Mar 30, 2015 11:45 am

    Thanks everyone; this forum is great

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