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5 posters

    Nut bearing blocks

    mac
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    Nut bearing blocks - Page 2 Empty Nut bearing blocks

    Post by mac Mon Apr 25, 2011 2:00 pm

    First topic message reminder :

    I've been looking at some of the projects over on http://armborst.forum24.se/ and was struck by a clever method to keep the front and back bearing blocks aligned while in-letting them into the tiller. In these pics, the two bearings have been machined out of a big piece of antler and left with a very substantial attachment at the bottom.

    Nut bearing blocks - Page 2 Img_2410

    Nut bearing blocks - Page 2 Armbor10

    Nut bearing blocks - Page 2 Armbor11

    I have always made the front and back bearing separate, but getting everything to be coaxial and properly let into the tiller can be a lot of trouble.

    last night I imagined this method....
    --The two bearings are machined up using whatever method works.
    --The nut is wrapped in a strip of paper to provide the proper clearance.
    --The two bearings are glued temporarily to a wooden jig, with the nut captured between them.
    --The whole assemble can be let into the tiller, using soot, or whatever other marking method you chose.
    --When everything fits properly, the nut is removed, and the bearings are glued in place, leaving the jig to assist in retaining the orientation.
    --When the glue is set, the jig is removed.

    Mac
    [img]Nut bearing blocks - Page 2 Bearin10[/img]


    Last edited by Ivo on Mon Nov 14, 2011 5:37 pm; edited 1 time in total (Reason for editing : Inserting images)
    juancheco
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    Post by juancheco Tue Apr 26, 2011 5:38 pm

    Thanks fellas!!!!
    I remember to read Todd´s example of the tree and the robe!!!
    Todd, I can´t still imagine how you make the sit of the nut directly on the stock, without make an extra piece....

    Well, done the mathematics, result in a nut of diameter 35mm (1 3/8") and 30mm length (1.18")
    Geezer, can you explain a little mor about that Teething problems....i don´t figure it out......
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    Post by Todd the archer Tue Apr 26, 2011 7:50 pm

    Hi Juancheco, hope these pictures show better what I am talking about.
    Nut bearing blocks - Page 2 Easter2011011
    Nut bearing blocks - Page 2 Easter2011009
    I try to find the simplest and most effective way to do things. Can not say how this would work on heavy crossbows (over 200 pounds).

    Todd
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    Post by juancheco Tue Apr 26, 2011 8:36 pm

    Todd!!!!! Tanks man!!!!!!!!
    I´m sorry, but i think that i forget to tell all of you, that i want to do no side carving on the stock!!!!! Nut bearing blocks - Page 2 602584
    Sorry again my friend for bother you......but anyway this pictures of a wooden nut are useful to me, for i can see the metallic insert on the bottom, and the way that the nut maintain holded by the socket...!!!! I can said now that if there is no axe or cord holding the nut, this is not going anywhere.....
    Again, thanks a lot Todd!!!!!!!!!!
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    Post by Geezer Tue Apr 26, 2011 10:29 pm

    For Juancheco, concerning teething problems, from Geezer
    My first crossbow used an old Whamo Powermaster prod. I mounted the semi-symmetric whamo prod too low in the stock, which meant I had to pitch it forward to keep string drag to a minimum. If you pitch too much, you get string-hop on release. That's problem no. 1
    Problem no. 2: Payne-Gallwey recommends the 1.5 in dia. roller be sunk only about 3/16 inch below center... so about 3/5 below the top and 2/5 above. That means the bowstring contacts the lugs of the nut fairly near the center. This means the string gets less leverage, particularly with a very heavy brass nut (mine was 1.5 in diameter and 1.25 in. wide.) The great inertia of the nut, coupled with the minimal leverage on the lugs, and substantial pitch on the prod made the bow misfire, particularly when shot with the bolt-clip in place. Without the bolt-clip performance was pretty good, but with clip in place, the clip stripped the bolt off the string as the roller nut turned s l o w l y around. Sometimes the hop was bad enough that it actually bent the brass bolt-clip straight up. That was my clue. Eventually I drilled a bunch of holes through the nut to lighten it, and cut deep grooves in the back of the lugs, to delay string-hop. That helped. The final solution took several bows to discover... sink the roller nut by 2/3 rather than 3/5. So a 1.5 in dia. nut should have its center 3/8 in. below the top of the socket, rather than 3/16. A 1 & 3/8 dia. nut should have its center 5/16 in. below the top of the socket, etc.
    If you bury the nut 3/4 of its diameter, you won't be able to remove the nut from the top of the socket... it will have to go in from the side.
    Does this help? Geezer.
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    Post by Todd the archer Wed Apr 27, 2011 4:26 am

    Hi Geezer,

    As I mentioned above I use a nut that is 1 1/2" diameter. I set the center 3/8" from the top. This works to be 3/8" of the nut is above the deck and 1 1/8" below the deck. With all respect does this not mean that the nut is sunk 3/4 into the tiller? Also with this layout I have no problem taking the nut in and out from the top.

    Juancheco, if you are not cutting in from the side then the earlier methods discussed would be the way to go and probably
    more traditional as well.

    Todd
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    Post by juancheco Wed Apr 27, 2011 8:35 am

    Good morning, and thanks a lot for the tips, you must be teachers or something, because i get learned instantly!!!!!!!!!

    So, Todd, Geezer, I´ll start with a soft wooden mock-up before make the real ones, following wath you just teach me...!!!!!!!

    Thank you very, very much!!!!!
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    Post by Geezer Wed Apr 27, 2011 10:23 am

    "With all respect does this not mean that the nut is sunk 3/4 into the
    tiller? Also with this layout I have no problem taking the nut in and
    out from the top."
    Todd: You are right of course, it comes out to almost precisely 3/4 of a circle when cut with the forstner-bitt, assuming it doesn't drift any. But by the time I have done all the sanding and finishing, I expect to lose at least 1/16 of an inch, maybe a bit more, so I end up with about a 2/3 round socket, which generally works very well. Geezer.
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    Post by juancheco Thu Apr 28, 2011 9:16 am

    Guys, sorry, but do the section of the string have any relation with the inner diameter of the nut´s tooth?? I mean, same diameter for both?? Because i´ve seen, in every draw or detail, that diameter doesn´t change at all!!!
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    Post by mac Thu Apr 28, 2011 9:53 am

    Juan,

    It is my impression that the shape of the nut is that same for all sizes of crossbow. Historically, the draw weight of the bow determines the thickness of the string, and that determines the diameter of the nut.

    Modernly, we are making relatively light bows, and frequently stringing them in modern materials. All this leads to thinner strings. This should lead to smaller nuts, but that would not look good. So, instead, we end up using full sized nuts with skinny strings.

    In my experience, this is not a functional problem. Undersized strings will work perfectly well in a nut that is really designed for something thicker.

    If you were to make the notch for the string smaller, without changing the size of the nut, it would end up farther away from the nut's center of rotation. This would make the nut tend to lift the string off the surface of the tiller as it rotated.

    It is best, I think, to simply make the nut the same shape and proportions as historical examples, and not worry that the string looks too thin.

    Mac
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    Post by juancheco Thu Apr 28, 2011 10:45 am

    Ok Mac, anyway i realise that i can do some thickening on that zone of the string to correct that behavior while making the first shots....
    Thanks Mac!!!!!! (I used to said that to my old computer..... jocolor)
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    Post by mac Thu Apr 28, 2011 7:40 pm

    8fingers wrote: I know a gunsmith who uses cheap lipstick for his inletting markers. It is fast and easy to see on dark woods.

    I have heard of using lipstick, but never tried it. It's probably a lot less messy than the grease based inletting compounds I usually use.

    Here's a link to a discussion of inletting compounds on a machinest's forum. http://www.practicalmachinist.com/vb/gunsmithing/qt-home-made-inletting-black-197205/
    ...and another from a gun forum...
    http://forums.gunbroker.com/topic.asp?TOPIC_ID=373268

    Some commercial products.
    http://www.midwayusa.com/viewproduct/?productnumber=123915
    http://www.trackofthewolf.com/categories/partList.aspx?catID=5&subID=63&styleID=210&AspxAutoDetectCookieSupport=1&as=1

    Mac

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