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Crossbows - Everything about Building, Modding, and Using your Crossbow Gear

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Gnome
jocky
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    Compact repeating compound project

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    Post by jocky Fri Feb 17, 2017 2:19 pm

    I finally found some time to expose my latest in-work project, I would be interested to read any observations, criticisms or suggestions.

    My design criteria were : 1) useable indoors, with no shoes on 2) burst shooting rate of at least 15 per minute 3) at least 6 shots before reload 4) powerful enough to stop your average zombie in his tracks.

    I was very happy with my 150lb fibreglass bow, so my first thought was the 80lb version. However, a single 80lb bow (Cobra types) looked too feeble for criterion 4). So, why not twin 80lb bows in a compound configuration ? (I have since seen that Gnome did exactly the same thing https://thearbalistguild.forumotion.com/t1124-pseudo-compound-pistol-slurbow#10401).

    To obtain a high firing rate, my plan was to reload without moving my hands from the firing position. To achieve this, I decided to make the entire trigger/grip block moveable. The tiller is composed of two planks separated by 1/4" in which the trigger slides. Two recessed drawer sliders are used as the basis of the trigger/grip module. It was something of a challenge to design the trigger using the available in-line holes in the slider, but I think it turned out OK.

    So this is how it works : there are two grips, a forward one, just below the bows, and the pistol grip.  The trigger is pushed forward until it is armed by the tension of the braced string. The grip is then pulled back until it engages in a catch/hook at full draw. After shooting, the grip is pulled back and tilted down to disengage from the catch, and the cycle can recommence. The action in all is very similar to that of an archer.

    Here are a couple of photos which should make things clearer.


    Armed position :
    Compact repeating compound project 104_2810

    After firing, disengagement from the catch :
    Compact repeating compound project 104_2811
    Gnome
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    Post by Gnome Sat Feb 18, 2017 9:22 am

    Interesting! I like any drawing aid that keeps my fingers away from the string. If you get the geometry right between your two grips it should be workable, but without any mechanical advantage to the arming motion, your arms could be pretty shaky with exertion by the time you were taking that 6th shot. I don't have a good way to measure the actual draw weight of my builds, but have no reason not to believe that two 80# prods would be roughly equivalent to one 160# prod.
    So with the firing rate you are looking for, I guess you plan on some type of box magazine for auto feeding your bolts? I'm looking forward to seeing how this turns out.
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    Post by Andy. Sat Feb 18, 2017 6:54 pm

    I've often thought how a version of the Daisy Mod 25 pump action bb gun could be incorporated into a moderately powered, repeating crossbow.
    The "elbow" pump mechanism efficiently pushes back the guns mainspring via a clever, and proven system of levers...

    https://www.youtube.com/watch?v=orGt7LBrjG0

    Will be interesting to see how you go with this Jocky Wink
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    Post by jocky Sun Feb 19, 2017 8:18 am

    Gnome wrote: I don't have a good way to measure the actual draw weight of my builds, but have no reason not to believe that two 80# prods would be roughly equivalent to one 160# prod.

    I assumed this at first too, until I started to look at the mathematics of crossbows (surprisingly complex). In fact, to compare the power of prods, you have to take into account the draw length. So, if you have a 150 pound bow with a draw length of 11", an 80 pound bow with a draw length of 6.5" will have (80x6.5)/(150x11)= 0.315 of the power. So, at best, with two 80 pound bows, you have roughly two thirds of the power of the 150 pound bow.

    On the issue of shaky arms, that is indeed something of an unknown. If my calculations are correct, the peak draw weight will be around 55 pounds. I'm not an archer, so I have no idea how hard that will be. However, I don't have to maintain it at full draw, thanks to the catch.

    @ Andy I have seen pump action crossbows on youtube, however my inutition is that it is easier to pull something apart than to force it together. Also, the pump action is generally effected with the weaker (forward) arm, whereas in my system, the stronger (rear) arm is pulling and the weaker arm is essentially a brace.
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    Post by jocky Sat Mar 11, 2017 11:10 am

    OK, I have found some time to work on my project. I cast the prod holder and eccentric pulleys from aluminium (photos below). The prod holder was cast directly in sand using the lost-foam method, but I had to use a different technique for the pulleys, because the aluminium cools too fast in sand to get to the extremities. So, for that, I made a plaster/sand mould, then heated it up to 800°C before pouring the aluminum in. The result is not perfect, but useable. Next time, I will use more air vents.

    I seem to have slightly miscalculated the string length, so the brace height is too low, and the trigger does not engage. Damn ! Another problem : the aluminum track, which I had simply glued to the tiller, detatched under the upward pressure of the cables. So, it will have to be screwed down. Third problem : the prods tilt slightly downwards when the string is pulled (I seem to remember having already seen this problem somewhere on the site). The string is only very slightly above the centre point of the two pulleys, so maybe the cable pressure is to blame ? Also, I used circlips to keep the pulleys in place, but they popped off under the downward pressure when the string was drawn. Maybe spacers would be better.

    Now for the positives : I am strong enough to pull back the trigger. That was a big unknown. Globally, I think I have got the geometry right. I used the smallest possible pulleys (to keep the weight down), with a three-quarter operational turn, which I think is the practical maximum.

    In summary, with a bit more work, I think this will be a useable weapon.


    Compact repeating compound project 104_2816


    Compact repeating compound project 104_2815
    Compact repeating compound project 104_2813
    Compact repeating compound project 104_2812
    Compact repeating compound project 104_2817
    Compact repeating compound project 104_2814
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    Post by Anatine Duo Tue Mar 14, 2017 6:00 am

    Those are pretty nice castings!  For the pulleys did you carve the patterns by hand?

    I've done some lost foam but had limited success on anything more complex than a knife handle.
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    Post by jocky Thu Mar 16, 2017 3:25 pm

    @Anatine Duo

    I fell in love with casting because I have no specific tools for machining metal. I have had a lot of failures, but I kept trying.

    After various trials, this is what works best for me :

    1) bone dry sand (dry it in an oven), as fine as you can find. This flows almost like a liquid, and avoids air spaces around the foam model. Also, any water in the sand will cool down the molten metal very quickly. The rate at which the aluminium cools is a major factor in the success or failure of the casting. Another advantage : gas (air and foam vapours) is easily evacuated through dry sand, more so than wet sand.

    2) as much head as is reasonably possible, I would say around 4 inches minimum. to achieve this, I use a funnel made from a tin can opened at both ends and cut down the middle (that makes it easier to remove). This also means melting a lot more metal than the moulded piece requires, say 5 to 10 times more.

    3) very hot metal. People on youtube talk about 660°C, but I would say 800°C is a minimum. Otherwise, the metal freezes as soon as it leaves the crucible.

    I don't carve the foam by hand, as such, not "freehand" in any case. I use a combination of a hot wire cutter in a jig, and a dremel in a small drill press. You can get pretty accurate models using this method.
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    Post by Crossbowmen Fri Mar 17, 2017 3:36 am

    Like your creative thinking.
    I'm working just mostly preparing to make a compound bow. What I found was that you need to make the pulleys of centre to get the compound effect. But on a bow that's probably not necessary because you don't need to hold it, the trigger does.
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    Post by jocky Sat Apr 22, 2017 10:46 am

    A quick update on this project. I had to make new eccentric pulleys, because I had made the mistake of using the aluminium direcftly as a bearing surface, which almost immediately failed. So, this time, the bearings are brass.

    The crossbow is now reliably shooting. I was pleasantly surprised by the performance : 187fps with a 430 grain bolt (out of the barrel). That means around 45.5 Joules in the bolt. If my calculations are correct (I can't measure it), I put around 50.5 Joules into the bow, so the overall efficency is 90%. This amazes me, as the pulleys, axle and clamps weigh around twice the weight of the bolt, but there you go...

    A problem I have encountered is that, although I can pull back the trigger, it gets very difficult close to my shoulder. I had thought that I could pull it back using the front grip as a brace, exactly like an archer, but no, I have to pull it back against my shoulder, and the distance between the armed trigger position and my shoulder is too short for comfort, so I may have to lengthen the butt and shorten the draw length. This would mean a new cam design.

    Overall, I probably was over-ambitious with this project. Too many new things at once : aluminium casting, cam design, moveable trigger design etc. So, it is taking a lot of time, but I am slowly getting there.
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    Post by Phil Abrahams Mon May 08, 2017 2:10 pm

    A very interesting design and look's very powerful you have done a great job there keep going as i think you have a very interesting project going on,Phil.

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