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    My crossbow project

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    ZigiMan
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    My crossbow project

    Post by ZigiMan on Sat Dec 15, 2012 12:46 pm

    Hi,

    I can say that this is the second crossbow I am building. (the first one was 20 years ago when I was a young boy.)
    This is beginning of my project. I want to share it with you all and also to get your opinions and advices.
    I'm building my crossbow from scratch using leaf spring as prod.
    There are some contemplations about how to start and what to do.
    In the next post I will send some pictures and details.


    ZigiMan


    Last edited by ZigiMan on Fri Dec 21, 2012 3:01 am; edited 4 times in total
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    Re: My crossbow project

    Post by ZigiMan on Sat Dec 15, 2012 1:44 pm

    I bought 4 used leaf springs with different measures:



    These are the measures (in mm):


    no.lenghthighthicknessplace of hole
    1875456465 (middle)
    2765456405 (middle)
    3820505380-520
    4980506.5450-620

    And in Inches, for those of you who use the United States system of measurs:



    no.lenghthighthickplace of hole
    134.441.770.2318.3 (middle)
    230.111.770.2315.94(middle)
    332.281.960.1914.96-20.47
    438.581.960.2617.71-24.40


    First, I wonder if all of them are suitable for use as prod...


    Last edited by ZigiMan on Sun Dec 16, 2012 4:34 am; edited 2 times in total
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    Re: My crossbow project

    Post by ZigiMan on Sat Dec 15, 2012 3:06 pm

    For the start, I want to choose the most expandable leaf spring since this will be the first time I use leaf spring as prod so if I'll make a mistake, it won't be a lose.

    The first question I have been asking my self is whether I have to tiller the spring?
    And second, how should I tiller the spring so that I can use the best of it? Which means, how to trim it so it won't be too week yet not too strong to span it by hands?


    These are the dimensions of the first leaf spring:



    These are the options I thought for the tillering (I took the first leaf spring as an example):



    The measures in the options are just suggestions. That is to say that it could be 6mm at the tips (as the wiki suggests), so the tips will be no less then the thickness of the spring. Or it could be 10mm (as the example) or 9 or 8... You got the idea...
    That's the same as for the middle of the spring or the high, or maybe a combination...

    I'll be glade to read the opinions and suggestions of you all.

    ZigiMan



    Last edited by ZigiMan on Sun Dec 16, 2012 7:25 am; edited 2 times in total
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    Re: My crossbow project

    Post by B.Cheers on Sun Dec 16, 2012 4:48 am

    Hey Zigiman
    This should help you out with your leaf spring prods
    http://crossbow.wikia.com/wiki/Bow_design
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    Re: My crossbow project

    Post by ZigiMan on Sun Dec 16, 2012 6:16 am

    I've read this article but since the measures of my leaf springs are different, I believe that the tillering should be different also: Let's say the middle part a little bit longer then the 40mm suggested in the wiki. Or the tips thicker then the thickness of the spring.
    I don't want to tiller the spring according to wiki measures only to find out that the prod will be week... (or too strong to span it by hand)
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    Re: My crossbow project

    Post by Gnome on Mon Dec 17, 2012 7:43 am

    Welcome to the forum, ZigiMan. I'm curious about your requirement that you are able to hand-span the bow. Do you mean without any mechanical aid at all, cocking rope or belt hook, etc? Seems like that would put a severe restriction on the potential power of a leaf-spring sourced prod, or your restricting your shooting enjoyment of shooting the bow. I can span my 200# steel prod bows by hand, if I do so more than a half dozen times in a short timespan my fingers are sore and my arms get shaky, and that's no fun. Please set me straight!

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    Re: My crossbow project

    Post by ZigiMan on Mon Dec 17, 2012 10:15 am

    Well, I have the impression that the bows are divided into two: those which are draw by hand and those which need a cocking device like a lever or a rope.
    I don't want to put restrictions over the power of the bow...
    If I understand you correctly, you can span your 200# (in that I believe you mean 200 pounds, right?) prod by hand but in order not to sore your fingers and arms you use cocking rope or belt hook?
    In that case, this is what I want from my prod.

    b.t.w I saw your works and I love them... Smile this is the directions I want to go with my crossbow, and I hope I'll get to the beauty of yours...
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    Re: My crossbow project

    Post by Gnome on Mon Dec 17, 2012 2:09 pm

    Thanks for the praise! I've been away from this forum for a while, caught up in other projects, and just the other day I got inspired to finally finish carving the grip texture on my favorite wooden stock. You're braver than I am for going the leaf spring route, that big hole in the middle of everything makes me too nervous. One of these days I'll try to make my own prod, but it's easy to be lazy when there are folks out there selling better steel prods than I could ever make, and reasonably priced.

    Yes, 200 pounds.

    I was a big fan of Northern Exposure when it was originally aired, haven't thought of it in years! The chef played by Adam Arkin was my favorite, and if I recall correctly there is an episode when he brandishes a crossbow.

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    Re: My crossbow project

    Post by ZigiMan on Wed Dec 19, 2012 2:48 am

    If anyone that has experience or knowledge with tillerring a leaf spring could advise me on how to trim the springs I have, I would appreciate it.

    ZigiMan
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    Re: My crossbow project

    Post by mac on Wed Dec 19, 2012 10:03 am

    Zig,

    The more material you leave around that hole, the better off you will be. You really don't want it flexing there at all if you can help it. To that end, option "C" is probably the best way to treat the center.

    Out at the tips, you will need to leave enough material for the shoulders of the nocks. If you skimp on these, the loops of the string will tend to slip over them as the bow returns to brace after the shot. That's a problem you want very much to avoid.

    What you want to strive for is to have every part of the prod equally stressed. There's the hard part. If the prod is the same thickness everywhere, and it starts out flat, when it is drawn, the limbs will fall on a circle. Starting out, as you are with a thing that is already curved, it is difficult to see when everything is equally stressed.

    A thing you can do is model the prod in a different material. Start with an aluminum yard (meter) stick or a wooden lathe, and put a hole in it like your leaf spring has. Your model should have the same length and width as your leaf spring, but it should be thinner, so it bends easily. By alternately flexing and reducing the height of the limbs, you will be able to see when you have a thing that bends gracefully and still has enough material to form the nocks. As soon as you have the model bending enough, put a string on it. This will make things easier. You may or may not be able to make your model flex far enough to simulate full draw, but as long as you do not make it take a permanent bend or blow up, you will be OK. When you are satisfied with your model, you can use it as a template for cutting your spring into a prod.

    Mac

    (edited for clarity....see below)


    Last edited by mac on Wed Dec 19, 2012 12:45 pm; edited 1 time in total
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    Re: My crossbow project

    Post by ZigiMan on Wed Dec 19, 2012 12:16 pm

    Mac,

    Thanks for the informative reply. I believe that this is the kind of answer I was looking for.
    Two clarifications: As I understand, the model should be in arc form like the leaf springs, and not flat, right?
    And second ( just to make sure we are talking on the same thing), by
    mac wrote: reducing the thickness of the limbs
    I understand that you mean the high of the spring? (what I showed in the first diagram as 45mm).

    I still wonder if there is some kind of calculation that can give me the answer. For example if the strength at a given draw distance is provided for a raw leaf string, including the other dimensions, can this calculation tells how to tiller the spring to get a prod with certain strength.

    ZigiMan
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    Re: My crossbow project

    Post by mac on Wed Dec 19, 2012 12:43 pm

    Zigi,

    First off...You are right. I meant to say "height" or "width" rather than "thickness". I will go back and change that in my post.


    Second. The model can be flat, rather than curved like your leaf spring. That's part of the beauty of the model. When you "tiller" a bow, it is the difference between the resting shape and flexed shape that you are trying to see. If the resting shape is "deflexed", (like your leaf spring), this can be difficult to see. Using a flat model eliminates that difficulty.


    I avoid math when I can; partly because I am not good at it, and partly because formulae are likely to simplify things to a point where it no longer applies to the problem at hand. For example, a formula might not be able to handle the extra material for the nocks, or the hole in the middle.

    I am sure the shape can be done mathematically, but it takes more math-savvy than I posses. Perhaps someone else can help you here. (Nerd Flintstone? Are you listening?)

    Mac
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    Re: My crossbow project

    Post by Zardoz on Wed Dec 19, 2012 1:01 pm

    I have to disagree with Mac. In option C, the longer 100mm straight section will be stiffer on each side of the center hole, but will then put more leverage on the steel above and below the hole, causing it to bend more there. I say go for option A or B where the taper starts sooner. Look at it this way- make a 45mm x 45mm square piece of thin wood with a hole in it and try to bend it. Then make a 100mm by 45mm piece with the same hole and try to bend that one. The rectangle will be easier to break by the hole than the square. It is the same thing when the rest of the limbs are putting tension on the center section. You can maybe even make the taper slightly concave to make the prod bend less at the center and more as you go toward the tips. More like a D shaped bend rather than circular. I am not an expert, but if you make a wood test prod, I think you will see what I am talking about.
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    Re: My crossbow project

    Post by mac on Wed Dec 19, 2012 1:32 pm

    Zardoz,

    You are absolutely right! I stand corrected and dejected......

    ....better to weaken the areas just outboard of the hole, rather than strengthening them. As you say, "C" just gives the limbs leverage over the hole.

    It would be better to bring the lower edges of the limbs together at a point below the hole. What you want is as much material above and below the center of hole as you can.

    Mac
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    Re: My crossbow project

    Post by ZigiMan on Fri Dec 21, 2012 2:59 am

    Regarding the hole in the center, I don't care about it too much since I'm gonna reinforce the center with steel plates...
    The dimensions I gave are just an example: i.e. the tapering can end in 6mm, 7mm or any given dimension. Same as the base which can be 100mm, 80mm or some other measure.
    About the model with flat aluminum - somehow I'm failing to understand it... I'm still trying to figure how a flat rod can simulate a curve one. But I'll give it another thought and will figure it eventually.

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    Re: My crossbow project

    Post by BrianlaZouche on Fri Dec 21, 2012 5:57 am

    from what little i know, i am led to beleive that metal prods were introduced to allow for greater draw weights, which by that very nature meant mechanical means of drawing the crossbow were also required so pretty much as ''GNOME '' mentioned why a heavy draw prod, if you wish to draw by hand ? HOWEVER i also know sometimes its nice to have something that looks how you want it,

    i know very little of crossbows, ( i'm primary a longbow man) but i have seen a 90lb draw longbow fail and when i say fail i mean ''expolde'', i think for safety reasons you should always look to the weakest point, and on the leaf spring it seems to me to be the hole, i have no idea how much spings cost, or ready made store brought metal prods, but i wouldnt have thought one made by a smith to any measurements you wanted would cost an awfull lot more,

    i have made a couple of longbows, both of which may not be up to a professional bowers standard, but the pleasure of having something i have made from scratch is fantastic, in time i hope to also make a crossbow, so i find this and other threads really really interesting Smile good luck with the project, look forward to seeing it as it progresses
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    Re: My crossbow project

    Post by Zardoz on Fri Dec 21, 2012 6:23 am

    Since the leaf spring is already tempered into a curve, you will not easily be able to see how the prod naturally wants to bend. When you make a thin flexible model, you will be able to see what shape makes a fair, even curve. When you then transfer that pattern to the spring, it should be almost perfect. Making the model will be kind of like making a pyramid style flat hand bow. I hope I am describing this properly.
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    Re: My crossbow project

    Post by mac on Fri Dec 21, 2012 9:14 am

    Zigi,

    Since I don't know what your current state of knowledge is, I will start with the basics. If any or all of this is "old hat" to you, please accept my apologies.

    The process making a bow stave bend in the desired way is called "tillering". The bowyer puts the bow stave on a "tiller" or "tiller board" and draws the string. The string can be drawn by hand or with a pulley system, according to how powerful the bow is.

    Most tillering is a subtractive process. The bowyer flexes the stave as much as he dares, and then judges how much material to remove, and where it is best to remove it. His goal is to make the stave bend evenly from one limb to the other. He must also keep the strains on the stave within acceptable limits at every point.

    If he starts with a stave that is straight, and of an even thickness, he will have a relatively easy time telling when it is bending correctly. In this case, the bow stave will fall on a circle when it is flexed.

    Unless it is very thin and light, a steel or wood bow stave for a crossbow can not start out straight. It must have some "deflex" Otherwise, the act of drawing it fully will cause unacceptably great strains in the limbs. That is to say, it will either break outright, or will take a permanent deformation. In the case of steel, there is another possibility; and that is that it will be so close to its elastic limits that it will fail in use. (This last failure mode is one that we need to be especially cautious of. )

    If the bowyer starts with a stave that is deflexed, it will be difficult for him to tell when the limbs are flexing properly. He needs to look, not at the flexed shape, but at the difference between the resting shape and flexed shape. An experienced bowyer does this all the time, but it is an acquired skill that builds on an innate ability to "see" the shape.

    The inexperienced bowyer might find it easier to employ a "tool" here. That tool is the model of which I spoke. While a powerful (wood or steel) crossbow stave must have deflex, a light and wimpy one does not. If the stave is modeled in a thin springy material, there will be no need for deflex. The model can be tillered to a nice graceful curve, and the resulting shape or "plan" can be copied directly in the thicker stave.

    The model removes the need to visualize the difference between the final curve and the initial deflex. Instead you only have to see the difference between the final curve and straight.

    The model also lets you do the tillering on something that only requires a few pounds of force to flex. This is much easier than having to flex the leaf spring you are starting with. Also, if you overdo it and destroy the model, there is little harm done and not much investment lost.

    It is a difficult thing to write clearly about a process that happens in such a "wordless" place in the brain, but I have done what I can. If I can clarify anything, I hope that you will ask.

    Mac
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    Re: My crossbow project

    Post by Zardoz on Fri Dec 21, 2012 12:18 pm

    Yeah. What ^^ he said.
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    Re: My crossbow project

    Post by ferdinand on Fri Dec 21, 2012 2:21 pm

    Now that is what we call usefull info! Wow, having a hard time really understanding it all.
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    Re: My crossbow project

    Post by hullutiedemies on Sun Dec 23, 2012 5:08 am

    ZigiMan wrote: can this calculation tells how to tiller the spring to get a prod with certain strength.

    It is fairly easy to determine the energy storing capacity of a spring. Especially a solid metal one.

    In your case the spring material can take a safe maximum load of 1,5 J / cm^3. And the bow limbs are approximately triangle shaped.

    So you have average 2cm wide limb. 0.6cm thick and 87cm long. This means ~100 cm^3 working spring. Maximum load it can take is about 150J . The bow should be stressed to at least 100J to be efficient. ( Means maybe 200# weight , depends about length of string and initial curve)
    String the bow and test its strength for a couple inch draw. Then it is easy to determine safe power stroke.

    If you want lighter bow make the limbs proportionally narrower.
    Make sure to leave the area around the central hole widest in actual material, to make sure it is the stiffest point of your bow.
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    Re: My crossbow project

    Post by ZigiMan on Mon Dec 24, 2012 2:44 am

    Mac,

    I want to thank you for taking the time and answering my question. Your post is more comprehensive and through that I could have ever asked. It make me think and try to understand the issue completely. I also would like to thank Zardos, Nerd Flintstone and all the others who contribute to the topic.

    After doing a lot of thinking, re-reading your posts, and using a small steel ruler I think I managed to understand it. Let me know if I got it right.

    With my thoughts regarding the model I even got to the thought that maybe I have to understand something basic. I got to the question - Why the prods are curved? Why the crossbows weren't build with straight prod?

    First, regarding the arc form: When taking a straight thin material such as steel ruler and drawing it by little it curved nicely. I assume that most of the stress is concentrated in center and gradually around it, where there is no stress near the tips.



    By doing this, you get just a portion of the energy the material can provide.

    If you want to get more strength you have to pull it harder but this causes much more stress on the center deflexing in such way:



    So you don't get what you want - more power.
    In order to exploit more from the elastic capability of the stave you have two ways:

    One, to put the stave in an arc form. Drawing a prod that its' resting position is already in arc form, allows the stress to spread more evenly on the prod because more part from the limbs can also bend.



    When pulling hard a prod that's already in arc form when resting the result will be more curved like circle:



    The second way to get more power from the stave will be
    to weaken the limbs - tapering toward the tips so they can also join the "party" and bend - Tillering.

    Combining the two, i.e. arc form and tillering, exploits most of the energy from the prod without breaking or bending it in the center.

    And for the question of the model: The point that makes it hard to understand is how come an aluminum, and not only that, a thinner stave can simulate a steel thicker one. And how we implement that on an already arced form stave...
    This is how I understand it (and it took me a while scratch to figure, and I'm not sure I'm right):
    What we what to achieve is to see how an elastic stave act when drawing it. Let's take for example two flat staves, one is steel and the other is aluminum. Both are in same width and height (but not the same thickness). The curved form that is created when drawing them to the same length will be the same (surely you won't implement the same power). So after tillering the aluminum stave in order to spread the stress evenly (and the result will be visually a nice circle shape) we can know how to do the same for a steel stave. And it doesn't matter if the stave is already bend into arc.

    Let me know what you think.

    ZigiMan
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    Re: My crossbow project

    Post by Geezer on Mon Dec 24, 2012 10:12 am

    When making your steel crossbow limbs, remember it's possible to make them too skinny out near the ends... if the stock isn't about twice as wide as it is thick, the tips can torque when you draw them. About three successive shots will be sufficient to throw your string off one end of the prod, if not break it outright. Geezer.
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    Re: My crossbow project

    Post by ZigiMan on Wed Dec 26, 2012 5:51 am

    Geezer,

    So for example, if the thickness of the prod is 6mm then the width at the tips should be at least 12mm?

    ZigiMan

    p.s. For those of you who have read my previous post, I didn't meant to teach you... If it's (as Mac said) "old-hat" I apologies, I just arranged the things for me by writing them (and to check that I'm right) and also for those who are newbie like me who also need to understand it...
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    Re: My crossbow project

    Post by ferdinand on Wed Dec 26, 2012 6:06 am

    happy to learn zigiman!

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