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    In-Progress German Crossbow

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    stoneagebowyer
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    In-Progress German Crossbow

    Post by stoneagebowyer on Fri Jan 14, 2011 11:57 am

    First topic message reminder :

    Hi, everyone. I have never posted photos on this forum, and still am not sure about if I am doing the posting correctly, but here goes.
    I've mentioned already in the newcomer greetings that a primary goal of mine is to build a Roman arcuballista for a 4th century Late Roman living history unit I am a member of, Exculcatores Iuniores Britanniciani. I am grateful, as probably many of us are for Kurt's build-along on his Knight's Armory site. I am pretty much following his instructions religiously, and thus far, it has been a joy to build.
    I am not going to post individual descripitions for each photo (if I can figure out how to post photos, of course), but just include photos for each major stage of the building process. Maybe that will give others ideas or reassure them that they too are doing something correctly (or, err, incorrectly in my own case).
    And so, I will post this intro and then subsequent responses with each additional section of all this. Everyone, feel free to jump in, give me hints, tell me I am an idiot, or whatever. It seems very civil for a forum to design and build pretty killing machines, and I love that kind of thing.
    Dane
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    DARIVS ARCHITECTVS
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    Re: In-Progress German Crossbow

    Post by DARIVS ARCHITECTVS on Thu Jan 27, 2011 7:01 am

    I've blown bindings on a 5/16" thick 180 pound Alchem prod on a German crossbow before. No big drama, the bow fired and the bindings blew, and the prod fell swinging gently back and forth, hanging from the front of the bow. Those bindings were actually white cotton string, with lots of passes to make it thick, and it was stretched VERY tight, so the cords were pretty close to the breaking stress as wrapped. Cotton string isn't very strong, but it worked for 6 months. I went to hemp as soon as I could get some. Problem solved.
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    Re: In-Progress German Crossbow

    Post by Ivo on Fri Jan 28, 2011 3:28 am

    Re-reading my post and realizing just now how I put Geezer's work in the crosshairs . As unintentional as it was... Geezer, you have my apology.

    Darius, Todd,

    Great pointers guys...I'm beginning to understand what Pavise meant when he mentioned "Resources"...another man who deserves my apology.

    Being an amateur bowyer turned amateur crossbow builder, not even in my mid twenties, driven by ambition rather than experience, just chilling with you guys here is a real eye opener.

    Ok, I'm done jacking the man's thread.




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    Re: In-Progress German Crossbow

    Post by mac on Fri Jan 28, 2011 1:26 pm

    Tod,

    I think those bindings blew for a number of reasons.

    -First... Latigo straps are not the best material for the job. (but I'm sure you knew that)

    -Second...The bow (lathe,prod) is probably "working" too much in the center. I have seen this before in modern bows. The last time I had to deal with it, I retillered the bow by grinding until it worked more in the limbs and less in the center. This reduced the draw weight, but it was worth it.

    -Third....The tiller is wider than it strictly needs to be at the business end. This exacerbates the second problem. If the bow seat were narrower, the bindings would "feel" less of the bow's flex. We moderns tend to overbuild our tillers.

    Mac
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    Re: In-Progress German Crossbow

    Post by mac on Fri Jan 28, 2011 1:42 pm

    DARIVS ARCHITECTVS wrote:One only has to imagine the forces of compression, tension, and shear in the 3-D volume of the tiller to understand what the effects of the geometry changes would be. This comes with study of stress mechanics.

    A super heavy prod is really hard on the bindings, which in turn tug forward on the volume of wood between the binding hole and the rear face of the prod when the crossbow is fired. If you imagine a huge sudden pull on the binding hole in the forward direction, you can see why there would be two shear flat horizontal planes between that section and the wood immediately above and immediately below. Since this in along the grain, cracks form over time. Imagine you grabbing the front end of the tiller at the binding hole and tearing that volume of wood right out. As a result, the upper and lower volumes of wood in the nose of the tiller were tied to the central volume with a steel rod. The rod transfers the force on the central section to the upper and lower sections, so no shear cracks form.

    Now this rod is required, but ANYTIME you drill a hole in wood, that GEOMETRICAL IMPERFECTION from a SOLID piece of material introduces a STRESS RISER in the material.

    In this case, the drilling of a hole, installation of a rod, and the introduction of the resulting stress riser is a necessary TRADEOFF to eliminate the problem of the shear stress of that "tugging" on the binding hole which will crack the wood. Sometimes you have to add a negative design feature in order to get rid of a GREATER negative design feature. The trick is known WHEN to do this!



    As with siege engines (which I LOVE to build), a crossbowmaker MUST have a good understanding of the strength of his materials, especially ones like wood which are stronger in some directions and not in others, and a good understanding of:

    1) The directions and magnitudes of the forces in 3-D space within the materials,
    2) How sharp changes in geometry, like holes, inside corners and outside corners, can concentrate stress.
    3) How these more concentrate stress areas will reach the breaking point first, before other areas of the material do, as you apply more and more force on the object.

    So if you want to really design good crossbows, study basic stress mechanics. Then you can choose the best materials, best shapes, and best dimensions to withstand the forces without overbuilding the shit out of everything like a novice.

    Darius,

    I'm with ya', but with one observation. Since there are basically no surviving siege engines, we must do exactly as you say, and understand the stresses we are dealing with and build to handle them with a safe margin. On the other hand, with crossbows, there are a great many survivors to base our reconstructions on. If we simply copy what professionally trained crossbow makers of the Renaissance did we can not go far wrong. Although it is better if we understand *why* they did it that way, we don't really have to do any stress analyses. All of that has been worked out by "trial and terror" hundreds of years ago.

    Mac

    p.s That's a nice "home defense balista" ya' got there!
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    Re: In-Progress German Crossbow

    Post by DARIVS ARCHITECTVS on Fri Jan 28, 2011 3:58 pm

    mac wrote:
    Darius,

    I'm with ya', but with one observation. Since there are basically no surviving siege engines, we must do exactly as you say, and understand the stresses we are dealing with and build to handle them with a safe margin. On the other hand, with crossbows, there are a great many survivors to base our reconstructions on. If we simply copy what professionally trained crossbow makers of the Renaissance did we can not go far wrong. Although it is better if we understand *why* they did it that way, we don't really have to do any stress analyses. All of that has been worked out by "trial and terror" hundreds of years ago.

    Mac

    p.s That's a nice "home defense balista" ya' got there!

    Did I say we HAD to do a stress analysis? Nope. Only said you have to understand basic stress mechanics. I hate doing math too! I agree we have plenty of examples of crossbows to use as a basis. That's helps us out a great deal!
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    Re: In-Progress German Crossbow

    Post by Ivo on Fri Jan 28, 2011 4:31 pm

    mac wrote:Tod,

    ***
    -Second...The bow (lathe,prod) is probably "working" too much in the center. I have seen this before in modern bows. The last time I had to deal with it, I retillered the bow by grinding until it worked more in the limbs and less in the center. This reduced the draw weight, but it was worth it.

    -Third....The tiller is wider than it strictly needs to be at the business end. This exacerbates the second problem. If the bow seat were narrower, the bindings would "feel" less of the bow's flex. We moderns tend to overbuild our tillers.

    Mac

    Mac, interesting observation!

    Back in the day, looking at Digitarc's prods, I was wondering about the mid section being specifically made flat and stiff...


    I thought to myself "how lame"...they could have made the prod so much more compact by making it bend in a perfect arc...turns out I was so far from the truth... Darius's words ringing in my ears drunken ...
    Sometimes you have to add a negative design feature in order to get rid
    of a GREATER negative design feature. The trick is known WHEN to do
    this!



    A small thought...I've seen crossbows with a smaller mounting block with deeper grooves and binding laying closer to the center and also gripping a steel ring(I had a feeling they were more than just for hanging on the wall hook)...perhaps it is a way to use a slightly more flexible prod( limbs beginning to flex a bit closer to the middle)




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    Re: In-Progress German Crossbow

    Post by JMC on Fri Jan 28, 2011 5:04 pm

    the bow of a crossbow does not bend at the center, which is why the central part is generally thicker than the branches (limbs)


    Last edited by Ivo on Fri Jan 28, 2011 5:08 pm; edited 1 time in total (Reason for editing : help with terminology)
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    Re: In-Progress German Crossbow

    Post by mac on Fri Jan 28, 2011 10:15 pm

    Darius,

    I guess I got a bit hyperbolic there with the implicit accusations of stress analysis. Sorry!

    Mac

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