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    Compound project

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    Post by MRobin 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 :

    Compound project 3darbalete

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

    Compound project Img0224rp

    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.

    Compound project Img0222rs
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    Compound project Empty Re: Compound project

    Post by actionbow 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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    Post by MRobin 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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    Post by actionbow 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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    Post by MRobin 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.

    Compound project Img0220yzl

    Compound project Img0219zf

    Compound project Img0218u

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

    Re,

    Worked on the trigger box today :

    Compound project Img0218cj

    Compound project Img0220pn

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

    Still at work..

    Compound project Img0220ek

    Compound project Img0224mz

    I'm getting close to the real thing.
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    Post by Ivo 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:
    Compound project Img0218cj

    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.

    Compound project Revers10

    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. Compound project 524936

    Ivo



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

    Compound project Img0217uf

    Compound project Img0220mq

    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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    Post by MRobin 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 :

    Compound project Img0217ii

    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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    Post by Kale Schlegel 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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    Post by MRobin 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 :

    Compound project 4limbs

    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 :

    Compound project 4limbsmax

    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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    Post by MRobin 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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    Post by Ivo 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.
    Compound project Limb_t11

    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

    Compound project 3darba10|

    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:

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

    Compound project Barade18

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

    Ivo

    Over all...Looking good man!



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    Post by MRobin 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 :
    https://2img.net/r/ihimg/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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    Post by MRobin 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.

    Compound project Crossb4l2

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

    Compound project Crossb4l1
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    Post by MRobin Tue Oct 09, 2012 11:39 am

    Really easier to cock with a stirrup.

    Compound project Stir

    The next step will probably be an arrow guide and shoulder stock Smile
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    Post by Geezer 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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    Post by MRobin 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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    Post by chaz 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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    Post by chaz 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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    Post by Ivo 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

    Compound project Rail-l10

    Ivo Compound project 58024



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

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

    Spoiler:

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

    Compound project Kronic10



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