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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

    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 actionbow on Fri Sep 21, 2012 4:09 pm

    Looks like a solid plan you have there. If you are planing on shooting three fletch hunting arrows off the shelf you will want to make sure that your cock fletch clears the bottom string. It looks like the arrow might not clear it from the picture but it could just be the perspective of the picture.

    Long draw length and pulleys...will the pulleys be cams or just round?

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    Re: Compound project

    Post by MRobin on Sat Sep 22, 2012 12:51 am

    I guess I'll shoot shortened arrows, yep (have a bunch of broken shafts). The cock fletch goes on the top doesn't it ? Anyway I don't really know how the string parts will organize yet, I don't really draw accurate plans (the one I posted just brings the general idea), lot of improvisation.

    Only precise thing is my formula to calculate max draw length according to the run of "bow" tips. Well it's not NASA-precise, since it counts the pulleys as one dimension points.

    No cams, just round pulleys, for now. It could change.. Those are made from brass, really "a bit" heavy to me.
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    Re: Compound project

    Post by actionbow on Sat Sep 22, 2012 3:27 am

    Well I suppose it depends on if you have a cock fletch groove as in most modern bows or if you plan on shooting three fletch upside down. You could narrow the tiller top severely so that the fins don't contact it or just deal with the arrow lift that will occur shooting with two vanes mashed into the tiller top.

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    Re: Compound project

    Post by MRobin on Sat Sep 22, 2012 11:15 am

    I really don't know for now, haha Very Happy
    As usual, changes will be made when a problem occurs, till it works well.
    Today I finished some parts, a dacron string (made it too long, though), etc.
    I noticed the string was prone to leave the pulleys so I re-carved the grooves and put good lubricant on the shafts, now it works clean and smooth.







    I can get pretty sick draw lengths from this little thing :



    The arrow is 29" long. I could have draught farther.

    Well, I guess I'm gonna get started on the trigger mechanism now Smile

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    Re: Compound project

    Post by MRobin on Sun Sep 23, 2012 1:18 pm

    Re,

    Worked on the trigger box today :





    You can see a rought drawing of the expected three axle mechanism.
    I think it won't look so bad once finished !

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    Re: Compound project

    Post by MRobin on Tue Sep 25, 2012 12:18 pm

    Still at work..





    I'm getting close to the real thing.
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    Re: Compound project

    Post by Ivo on Wed Sep 26, 2012 1:40 pm

    Nice project you got there. Always glad to see people tackle the more modern builds.

    If you don't mind me jumping into details right away, I got a few comments.

    Trigger:

    MRobin wrote:


    Noticed a bit of a quirk in the design. Considering you have such a long trigger hook lever, it shouldn't be too big of an issue, but as a suggestion > it's best to unload the sear by shifting the fulcrum on the sear lever closer to the sear. That way you'll have mechanical advantage over the forces at play and trigger pull will require less force to get things moving.



    I did a little amateur take on the leverage bit here if you're interested: LINK

    ...A few things wrong with that write up, but as far as basics go - it's alright. (Note to self: Write up a better an updated/corrected article)

    Keep up the great work, I like the neatness and how much effort you're putting into this project. I'm back on the forum for a while, so I'll be checking in to see you brawl with this thing.

    Ivo




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    MRobin
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    Re: Compound project

    Post by MRobin on Wed Sep 26, 2012 2:32 pm

    Hey thanks for your comment !
    I didn't actually drill the hole that far from the nut, this drawing was just an "idea", but yeah if my trigger hook wasn't so long I'd have made sure there was enough leverage. The final mechanism works well, maybe the trigger run could have been a little longer, but there is no disconfort whatsoever. I'm too lazy to do lever calculations and animated cinematics, so I usually go straight to the file and saw Wink

    Hopefully I will soon be able to tell you how it feels when releasing a 150 pounds draw weight or so.

    Edit : just measured the travel at the hollow of the trigger, where the finger pulls, it gives 8 mm to fully release the nut (but at 4 or 5mm it will shoot, by the pressure of the string). Don't know whether it's particularly short.

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    Re: Compound project

    Post by MRobin on Thu Sep 27, 2012 12:06 pm





    Okay now I have to find a decent piece of wood to make the limbs.
    An ash gardening tool handle may do the job..

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    Re: Compound project

    Post by MRobin on Thu Oct 04, 2012 11:00 am

    Hey, nothing really new but I've just tried with cheap 20# limbs from a beginner bow, here it is :



    It pulls about 10# at 23"1/2. Well it shoots. The 30 gram hunting arrows fly decently and stick into wood. Despite the extra weight at the tips of the limbs (I could fix it but anyway I won't have super perfs with those)

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    Re: Compound project

    Post by Kale Schlegel on Thu Oct 04, 2012 1:53 pm

    I like it. Reminds me of a spearfishing gun almost, and I'm fond of reverse draw crossbows as well!

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    Re: Compound project

    Post by MRobin on Fri Oct 05, 2012 7:50 am

    Right, that looks a bit like these fishing crossbows.. But I'm probably going to add a shoulder stock, when everything is finished.

    Today moar fun with the cheap bow limbs, I added two 30# ones :



    Hey it's getting dangerous. 25# draw weight, still at 23.5" power stroke, and it shoots 30g arrows at 158fps (average speed over 12m shots). I'm not going to check how many fps I can reach if I saw off the useless limbs tips, there are more serious things to work on before Very Happy

    Edit : this evening I had a few shots with increased limbs flexion.
    By drawing them closer from each other like this :



    I could only shoot indoor but I think it's something serious, looking at how the arrows go through thick plywood (I didn't even tried to extract them). Judging by the effort I'd say it pulls something like 35# at full draw.

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    Re: Compound project

    Post by MRobin on Sat Oct 06, 2012 6:32 am

    Outdoor speed tests : 170fps with 30g arrows.
    I wonder if I would actually lose many fps by increasing arrow weight. I don't think so..
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    Re: Compound project

    Post by Ivo on Sat Oct 06, 2012 10:26 am

    Looks like some wild stuff going on there. Very Happy

    There is a rule of thumb in Reverse Draw Bows...limb angle should be set just so when limbs at brace and at full draw would have limb tips level. Not sure if you took it on intuitively, but it seems you got the angle just right.


    Also, think yo can play with the cats cradle in the pulleys so the strings sits on the other side? This way you'll get a little more draw...but all things accounted > a little can go a long way. Wink

    |

    I'd also suggest a bit on the trigger to make it a bull pup - moving the grip forward. Kinda like AboMickey did on his "Devil's Pitchfork Flipper" here:



    Another great example is what jeep's "Rubber Powered Inswinger" here:

    http://thearbalistguild.forumotion.com/t450p45-inswinger-crossbow-rubber-powered#5200



    Let me know if you're interested and I'll elaborate.

    Ivo

    Over all...Looking good man!




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    Re: Compound project

    Post by MRobin on Sat Oct 06, 2012 12:08 pm

    Thanks man !

    As for the angle, I guess I did this way without actually thinking about it, seems pretty natural..
    Unfortunately I cannot change the string arrangement :
    http://imageshack.us/a/img407/1161/img0218u.jpg here you can clearly see why Very Happy It would be possible with a different design for the pulleys' shells.

    I like this reverse crossbow, because you can easily adjust the power, just by changing the distance between limbs at brace position, without reducing the power stroke. The drawback is this crazy sharp string angle which requires mechanical drawing assistance over a certain weight.

    I knew about jeep's work, as a french fellow, I'm looking forward to some news concerning this impressive project.

    Really liked AboMickey's design too, simple and effective.

    About the bull pup idea, yeah I could do that, but it would involve making some serious modifs, for now I stick with adding a shoulder stock haha, seems easier.

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    Re: Compound project

    Post by MRobin on Sun Oct 07, 2012 2:51 pm

    Finally, shortened the limbs. I like the current version. I just need to do some minor improvements now : shoulder stock, making the head of the crossbow much lighter since it doesn't hold any mechanical pressure (appart from the stirrup that will be set up), arrow guide, and finishing touches.. Maybe a thinner string as well.



    Pulleys axles now have nice blocking system, looks more "pro" than the bits of duct tape doesn't it :


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    Re: Compound project

    Post by MRobin on Tue Oct 09, 2012 11:39 am

    Really easier to cock with a stirrup.



    The next step will probably be an arrow guide and shoulder stock Smile
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    Re: Compound project

    Post by Geezer on Tue Oct 09, 2012 3:01 pm

    Geezer here: Not that I know squat about reverse-draw compounds, but you've now got a great big bolt sticking out of the top of your stock, just ahead of the string at rest. Is there any chance your string will hit the bolt? If so, you might want to counter-sink the bolt so there's a clear path over the top.

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    Re: Compound project

    Post by MRobin on Tue Oct 09, 2012 4:16 pm

    No risk, the string is too high. But I will lower it so that it's on level with the nut and the path will be cleared at this moment, indeed.

    As I'm getting ahead without blueprints whatsoever, things can look pretty messy at some stages, haha but everything remains under control especially when it comes to safety

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    Re: Compound project

    Post by chaz on Wed Oct 10, 2012 6:41 am

    MRobin,

    As an outsider as to what you are trying to achive ............. you might try using a bolt with a bugle phillips type head with the head counter sunk in the top if the stock barrel and the nut on the bottom.

    Just a thought

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    Re: Compound project

    Post by chaz on Wed Oct 10, 2012 6:57 am

    MRobin

    As a PS you might use a washer and an acorn nut on the bottom as a more finished look.

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    Re: Compound project

    Post by Ivo on Wed Oct 10, 2012 9:34 pm

    Or you can do a rail-less design.

    By the looks of it, this might just do the trick with a brush type arrow rest. Wink



    Ivo




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    Re: Compound project

    Post by downunderrunner on Thu Oct 11, 2012 6:34 am

    love this concept, have been pondering a way to re-use some old bow limbs i have kicking around!! how's this bow "rigged" i can't quite follow the string? any advice for someone looking to do something similar?

    also, i think ivo is refering to a "whisper buiscut" type of arrow rest
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    Re: Compound project

    Post by Ivo on Thu Oct 11, 2012 10:01 am

    downunder, you're here from Paleo, right? Check this baby out:

    Spoiler:

    Ivo in Topic: Mechanical Steampunk crossbow ideas wrote:
    http://thearbalistguild.forumotion.com/t443-finnish-crossbow-forum#3779

    Translated Build: Link



    With a little bit of fiddling, you can probably fit the spine over the back of the *staff? * and carve the tip into a claw shape. A little skull (coyote maybe) fitted over the nut would look sweet too.

    ...perhaps you can even do the trigger lever encased in bone...just thoughts.


    Enlarge this image



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    And yeh...whisker biscuit... also another one that is a bit easier to build is the hostage rest...





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