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    Nut bearing blocks

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    mac
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    Nut bearing blocks

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

    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.







    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][/img]


    Last edited by Ivo on Mon Nov 14, 2011 5:37 pm; edited 1 time in total (Reason for editing : Inserting images)
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    Re: Nut bearing blocks

    Post by Todd the archer on Mon Apr 25, 2011 8:22 pm

    How about this, glue antler block/wood block/antler block together with a wood strip on the bottom, then bore for nut. Then drop assembly with paper shimmed nut into motised tiller.

    Todd
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    Re: Nut bearing blocks

    Post by mac on Tue Apr 26, 2011 6:49 am

    Tod,

    I'm having some trouble following you. Do you mean that there will be another wood block on the bottom of the assembly, which gets let into the tiller?

    Can you make up a sketch?

    Mac
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    Re: Nut bearing blocks

    Post by juancheco on Tue Apr 26, 2011 8:34 am

    How do you machine the interior concave support, where the nut seats?
    There is a need for a shaft or a string that acts as a hub for cases where the bearings are made ​​in one piece, which is introduced from above into the stock?
    The nut is not trapped inside the bearings? I see to dificult to make (by machine or by chiseling) the stepped bottom inside the in-letting hole on the stock....
    I know, to many questions, but i´m right now making a pile of sketches about how to do it on my crossbow..... pale
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    Re: Nut bearing blocks

    Post by mac on Tue Apr 26, 2011 9:01 am

    Juan,

    I usually make the concave surfaces of the bone bearings this way...
    --Make up a cylinder that is the size of the nut on a lathe.
    --Wrap the cylinder in abrasive paper, and use it (still on the lathe) to machine the bearing surfaces.

    I am making the bearings in two pieces because that is what was done historically. Making them in one piece is a clever idea, but it is modern.

    The nut is not trapped in the bearings. It can be easily removed when it is rotated to the proper place. This is typical of crossbows. Some have threads tied through the nut to keep it from falling out. Some have a rod or pin in place of the threads, but this typically does not bear any force. It's just there to keep the nut from getting lost. The inlet bone bearings carry all the loads.

    The mortise (inletting hole) is a pain in the ass to make. There is no denying that. But it is quite possible, and with practice, you get better and faster at it. It is important to work slowly and thoughtfully. If you use the soot of a candle to cover the bottom of the thing you are inletting, it will leave a black spot on the wood that you must remove. Gun makers do this when letting barrels and locks into stocks. We must borrow techniques and processes from other craftsmen.

    I am sorry if I have told you things that you already know!

    Mac

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    Re: Nut bearing blocks

    Post by juancheco on Tue Apr 26, 2011 9:31 am

    Mac, tanks for the answer, and don´t be sorry at all, you have been helpfully, now i´m realise the issue about the nut stuff.
    Anyway, when you make the bearings, first you make a unique piece following the shape of the sand paper wraped cylindrer, and then split it in two parts?? Or still work with two pieces at all???
    I´m thinking about make a brass nut, and i believe that a hard wood for the seat of the nut will be ok, because bone is hard of find out here.....
    Thank you again!!!!!!!! bounce
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    mac
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    Re: Nut bearing blocks

    Post by mac on Tue Apr 26, 2011 10:30 am

    Juan,

    I make the two bearings as separate parts right from the start. That way I get to maximize the use of my material, and chose for dense surfaces. If the bearings have porous surfaces, they are not as strong. I also worry that porous surfaces will retain dirt, and contribute to wearing out of the nut.

    A brass nut will work OK. They are heavy, and will rob a bit of your stored energy. Aluminum sounds like it would be a better choice, but Geezer tells me that they make ugly black marks on the "table" of the bow.

    The wood of the tiller is probably just fine for bearings. Lots of people do that, especially for light weight bows.

    My wimpy little crossbows don't really need bone bearings, but I have always put them in anyways. I hope that the experience will lead me to have a fuller understanding of the bearings by the time I start playing with heavier bows.

    Mac

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    Re: Nut bearing blocks

    Post by juancheco on Tue Apr 26, 2011 10:47 am

    I got it.....i´ve ben seeing on this pictures, i understand this way, but still think that you way is required of important handwork skills

    http://www.flickr.com/photos/swifthoundbows/sets/72157613494516821/

    Now i´m plan to do a hard wooden nut seat, and let the time do the rest. If that don´t works fine, i can allways change for other material.
    Even i was thinking about make a brass plate wrapping the nut sit exactly the shape of the nut....
    Again, thanks a lot for you support Mac!!!!!!!
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    Re: Nut bearing blocks

    Post by mac on Tue Apr 26, 2011 11:14 am

    De nada, Juan.

    Mac
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    Re: Nut bearing blocks

    Post by mac on Tue Apr 26, 2011 11:18 am

    Todd,

    Is this the sort of thing you are talking about?

    http://www.flickr.com/photos/swifthoundbows/3265117114/in/set-72157613494516821

    Mac

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    Re: Nut bearing blocks

    Post by 8fingers on Tue Apr 26, 2011 11:39 am

    I know a gunsmith who uses cheap lipstick for his inletting markers. It is fast and easy to see on dark woods.
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    Re: Nut bearing blocks

    Post by Geezer on Tue Apr 26, 2011 1:11 pm

    Well, I see somebody has posted a nice pic of a pre-cut bone-socket from Lightly's photo archive. That saves me the trouble.
    We make those by gluing two pieces of bone/antler into a block of wood, then use a forstner-bitt on the drill-press to cut a 2/3 round passage between the two. Then we cut the tickler-passage in the bottom/rear. Once all that's done, we cut the finished socket down to the size we want and inlet it into the top of the stock.
    If you don't want to do a fully mortised lock, just glue the blocks of bone/antler into the stock and cut the roller-passage between the two. That's how we do most of our reinforced sockets (anything over 125 lb. or alternatively, anything fitted with a bone/antler roller.
    (For non-English speakers, I use the term 'antler' to differentiate from 'horn' which in English can mean hard, bony stuff like moose and deer horn, or soft, flexible lamellar stuff like cow-horn. The term 'horn' can mean either of those. 'Antler' is specifically the hard, bony stuff. Even so, there are some antlers, like American Elk, which are nearly useless for roller-nuts. I have had good success with the base of Moose horn or Axis Stag. Geezer.
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    Re: Nut bearing blocks

    Post by juancheco on Tue Apr 26, 2011 2:12 pm

    How much is ussually the diameter of the roller nut that you guys make???
    I find only 35mm diameter forstner-bitt, that´s about 1.3 inches......it´s enough?? Remember, for a brass nut..... bounce
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    Re: Nut bearing blocks

    Post by Todd the archer on Tue Apr 26, 2011 2:37 pm

    I make my nut 1 1/2" in diameter from wood and use no bearing block other than the stock itself.

    That photo essay by Lightly was the one I was refering to.

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    Re: Nut bearing blocks

    Post by Geezer on Tue Apr 26, 2011 3:04 pm

    Nut diameter? Payne-Gallwey suggests 1.5 inches. These days, I use mostly 1 & 3/8 inches. You see a lot of bows with rollers about that size. The original Padre Island bow has a very small roller, about one and a sixteenth inches in diameter and 15/16 inches wide. That's about as small as I have seen. One and a quarter to one and a half inches seem most common.
    35 mm should work very well, though Mac is right, a Brass roller will be heavy and will eat some power. My very first medieval crossbow had a 1.5 inch by 1.25 inch brass roller. It experienced some odd teething problems that were made worse by the heavy nut, but eventually it worked well enough to encourage me to make more. Geezer.
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    Re: Nut bearing blocks

    Post by juancheco on 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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    Re: Nut bearing blocks

    Post by Todd the archer on Tue Apr 26, 2011 7:50 pm

    Hi Juancheco, hope these pictures show better what I am talking about.


    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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    Re: Nut bearing blocks

    Post by juancheco on 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!!!!!
    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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    Re: Nut bearing blocks

    Post by Geezer on 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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    Re: Nut bearing blocks

    Post by Todd the archer on 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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    Re: Nut bearing blocks

    Post by juancheco on 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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    Re: Nut bearing blocks

    Post by Geezer on 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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    Re: Nut bearing blocks

    Post by juancheco on 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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    Re: Nut bearing blocks

    Post by mac on 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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    Re: Nut bearing blocks

    Post by juancheco on 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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