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    Compound project

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    MRobin
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    Compound project

    Post by MRobin on Fri Sep 21, 2012 3:23 pm

    First topic message reminder :

    Hi everyone
    My first thread was about a medieval-ish crossbow characterised by a big draw length which compensated a little for the weak wooden prod. Since I can't get better material for now, I decided to start a new project that allows me to use various material with maximum draw length and minimal bending of the material in question. Therefore I went for a compound system, with four pulleys which increases the mechanical reduction, here's the first idea :



    And I've been working on this over the last couple of days :



    Those are just cheap weak wooden leaves, if it works I will have something strong.
    First I need to see if the string travels nicely through the pulleys.


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    Re: Compound project

    Post by downunderrunner on Fri Oct 12, 2012 12:51 am

    yeah ivo, lol, i'm a paleo nutter, lol, actually, just passionately curios, like yourself
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    Kale Schlegel
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    Re: Compound project

    Post by Kale Schlegel on Sun Oct 14, 2012 8:13 pm

    Where can I find steel stock like your reciever? Home depot and Lowes among other places only have 1" square and tube..

    MRobin
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    Re: Compound project

    Post by MRobin on Mon Oct 15, 2012 4:05 pm

    Hey everyone.
    My mind is quite busy these days, have to find a job and apartment and crossbows aren't gonna do that for me. So nothing new, this evening I tightened the string to the maximum, the pulleys are now spaced a few centimeters apart when cocked, I think it pulls like 45# now. Tomorrow I'll shoot a few arrows, should be pretty fast !

    As for the asked questions :

    Chaz, acorn nuts are a great idea, I haven't got any but I might buy them someday. Ivo : the crossbow will be rail-less anyway, but I want the arrow shaft to be parallel to the stock, at this time I don't know exactly what kind of arrow rest will be used though.

    A drawing of the string path :



    Nothing complicated, this is just a double-compound system.

    Kale, the rectangular steel pieces I got come from regular DIY shops, in France. For example : http://www.castorama.fr/store/Profile-acier-Tube-rectangulaire-verni-prod5480003.html

    This is basic hardware, you should have them in US too.

    MRobin
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    Re: Compound project

    Post by MRobin on Tue Oct 16, 2012 5:49 am

    182 fps (still 30g arrow)



    Would be great to find like 60# fiberglass limbs, cheap enough to saw the tips off without regrets.

    By the way, the trigger is light and smooth, no problem with that (as we had a discussion about the leverage thing)

    MRobin
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    Re: Compound project

    Post by MRobin on Sat Oct 20, 2012 1:58 pm

    the string is now on level with the nut..



    I can now add the arrow rest and upholding spring.
    Even a bipod, yeah !

    Arrow going through 18mm plywood :



    I would go hunting with this, haha.

    MRobin
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    Re: Compound project

    Post by MRobin on Sat Oct 27, 2012 8:17 am

    Of course I want this crossbow to shoot harder that this.
    So, thanks to my good friend epoxy :



    The other set of limbs is still to be sanded (kind of tedious task, I have to remove the pesky coating to make fiberglass appear) and glued..
    It might become too powerful to be cocked by hand.

    MRobin
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    Re: Compound project

    Post by MRobin on Sun Oct 28, 2012 3:58 am



    Pretty satisfied with those things, the gluing work seems rather consistent despite a very amateurish process.

    I'm hesitant whether to fill the remaining gap at the limbs base with extra glue or with some washers the same diameter as the screw shaft that will go through. First solution seems more suitable.

    MRobin
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    Re: Compound project

    Post by MRobin on Mon Oct 29, 2012 3:40 pm

    Okay..



    First two shots indoor this evening :



    Tomorrow, outdoor tests and speed measurements.
    It still can be cocked by hand but my fingers hurt !
    I need a real device to give draw weight safely
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    Re: Compound project

    Post by Nils on Mon Oct 29, 2012 4:03 pm

    okay this is just scary Shocked
    how thik is that wood?

    MRobin
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    Re: Compound project

    Post by MRobin on Mon Oct 29, 2012 5:12 pm

    18mm plywood
    Not so scary I guess if you compare with modern high performance crossbows.. I'm probably in the 80 Joules range or so. Enough to kill, that's right !

    MRobin
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    Re: Compound project

    Post by MRobin on Tue Oct 30, 2012 5:16 am

    hum, not even close actually..



    200fps, 30g arrows, that's a wimpy 56 J (still 10 more than before).
    Obviously I can increase the power by shortening the string, but before I need to make a cocking device that protects my fingers. After several dry shots due to feverish drawing, the string finally broke and I got a finger harmed pretty well haha. Time for arrow rest and retention spring, too.
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    Re: Compound project

    Post by kenh on Tue Oct 30, 2012 7:18 am

    Sorry... video was so dark I couldn't make out anything... Bow is looking good though.

    MRobin
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    Re: Compound project

    Post by MRobin on Tue Oct 30, 2012 9:21 am

    yeah it's raining dark outside, and my phone's videos are kind of .. well Very Happy But on my laptop it's not so bad, maybe a brightness issue ?

    MRobin
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    Re: Compound project

    Post by MRobin on Sun Nov 25, 2012 1:02 pm

    Hep

    Back to work. The current version of the reverse compound crossbow has some design issues : unsafe cocking when reaching over 40-45lbs due to the big draw length and string angle, impossible to get the arrow parallel to the stock if it has standard 120° feathers, and so on.

    Anyway thas was just meant to be a prototype (that's why I used fir wood for the tiller). Now serious things can begin, here are some sketches of the 1.1 version :



    Actually, the trigger mechanism should look more like this :
    (the sliding block's locking system is not shown here)



    (no english version available but the text isn't really necessary to understand)

    Some specs :

    - probably bull-pup design
    - adjustable draw length (very like an ancient ballista) but fixed modern trigger
    - easily cocked by hands, thanks to sturdy handles attached to the sliding block
    - no contact between arrow shaft and tiller
    - maybe adjustable limbs angle ?
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    Re: Compound project

    Post by Hotspur on Fri Nov 30, 2012 4:50 pm

    MRobin wrote:hum, not even close actually..



    200fps, 30g arrows, that's a wimpy 56 J (still 10 more than before).
    Obviously I can increase the power by shortening the string, but before I need to make a cocking device that protects my fingers. After several dry shots due to feverish drawing, the string finally broke and I got a finger harmed pretty well haha. Time for arrow rest and retention spring, too.


    Hi MRobin,

    How did you determine the 200fps?

    I was messing around with Audacity software and zoomed in on the ‘click-whack’ of your 10 meter shot at about 21 sec point in vid. Allowing for a 29 millisecond sound delay at 10 meters (leaving about a 103ms flight over 10m) I still calculate over 240fps. (sorry for mixing up units!). I did the same with your 27 meter shot and got about the same fps.

    Not very scientific but indicates maybe a faster bolt speed? Smile
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    Re: Compound project

    Post by Rudamchu on Fri Nov 30, 2012 6:17 pm

    Hotspur wrote:
    MRobin wrote:hum, not even close actually..



    200fps, 30g arrows, that's a wimpy 56 J (still 10 more than before).
    Obviously I can increase the power by shortening the string, but before I need to make a cocking device that protects my fingers. After several dry shots due to feverish drawing, the string finally broke and I got a finger harmed pretty well haha. Time for arrow rest and retention spring, too.


    Hi MRobin,

    How did you determine the 200fps?

    I was messing around with Audacity software and zoomed in on the ‘click-whack’ of your 10 meter shot at about 21 sec point in vid. Allowing for a 29 millisecond sound delay at 10 meters (leaving about a 103ms flight over 10m) I still calculate over 240fps. (sorry for mixing up units!). I did the same with your 27 meter shot and got about the same fps.

    Not very scientific but indicates maybe a faster bolt speed? Smile


    I believe that based upon the 103 ms over 10 meters calculation, it would come out closer to 318 fps if I'm not mistaken.

    MRobin
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    Re: Compound project

    Post by MRobin on Sat Dec 01, 2012 9:44 am

    Hotspur wrote:
    Hi MRobin,

    How did you determine the 200fps?

    I was messing around with Audacity software and zoomed in on the ‘click-whack’ of your 10 meter shot at about 21 sec point in vid. Allowing for a 29 millisecond sound delay at 10 meters (leaving about a 103ms flight over 10m) I still calculate over 240fps. (sorry for mixing up units!). I did the same with your 27 meter shot and got about the same fps.

    Not very scientific but indicates maybe a faster bolt speed? Smile

    Hi !

    So I calculated again :

    10m shot#1 : 0.129s (I think I was standing a bit closer when firing)



    10m shot#2 : 0.137s



    10m shot#3 : 0.136s



    Let's say an average 0.135s for 10m.

    Don't forget that the camera is close to the target, and 10m ahead of the crossbow, so the "firing" sound has some delay (sound speed = 340 m/s, so delay = 10/340 = 0.029s).

    Therefore the actual time is 0.135 + 0.029 = 0.164s, and arrow speed = 10/0.164 = 61 m/s.
    And that gives me 200 fps.

    Even if I go for an optimistic 0.130s and ignoring the speed of sound effect, it gives 77 m/s (253 fps)

    I expect at least 250 fps with the next version. Seeya in a few months. It's getting too cold to do proper work in my garage, and I have to concentrate on other things for now anyway Sad
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    Re: Compound project

    Post by Hotspur on Sat Dec 01, 2012 4:43 pm

    Thanks for the walk through. Math error on my part smack

    I'm going to try the Audacity thing on my bow in the next few days. I figure if the mic is equidistant from each sound source it will cancel the speed of sound error...will try not to smash my smartphone in process...

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    Re: Compound project

    Post by MRobin on Sun Dec 02, 2012 12:40 am

    Yeah, I usually put the phone half way, on the ground Wink

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    Re: Compound project

    Post by MRobin on Sat Feb 23, 2013 4:30 pm

    2-way adjustment :



    (not to scale, of course, it's just to give an idea)
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    Re: Compound project

    Post by Hotspur on Sun Feb 24, 2013 5:54 pm

    I think the reverse draw has many merits; power-stroke, centre of balance, compact dimensions, no load on the forestock. I’m thinking of trying this on my next project.

    Will be interested in your version 1.1!

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    Re: Compound project

    Post by MRobin on Mon Feb 25, 2013 2:47 pm

    Yep, there are many assets to this design.
    I will keep you posted of course, as soon as I have something concrete to show.

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    Re: Compound project

    Post by MRobin on Thu Aug 29, 2013 11:48 am

    Here I am :





    This is a bit heavy, I must admit, there will be some slimming programme at the end if it proves functional.
    Next step is the long bar that will hold the little "sledge" with the trigger mechanism, and also a stirrup..

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    Re: Compound project

    Post by kenh on Thu Aug 29, 2013 12:11 pm

    "Curioser and curioser."  said Alice.  The metallica version that is.  I must admit to liking the aesthetics of the oaken 'proof of concept' a bit more.  But "more power to you" as they say!Jam

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    Re: Compound project

    Post by MRobin on Thu Aug 29, 2013 2:01 pm

    Yeah I'll have to think of ways to make it look a bit nicer.
    First I'm gonna cut short those little screws, should look slightly better.
    And some wood is expected of course.

    Power is getting serious, gotta be careful with the first tests. Check if everything stays in place, no warp, no suspect noise, and increase the powerstroke inch by inch.

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