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    Plans and materials for my first build

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    chilihook
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    Plans and materials for my first build

    Post by chilihook on Wed Aug 08, 2018 8:08 am

    If this is in the wrong spot or in anyway not in line with forum policies, my apologies!

    So I am an absolute beginner crossbow builder, but I have some wood working and bowyering experience as well as decently fitted out little wood shop in my basement. I think this is gorgeous and is what I'd like to try to build:



    Am I out of my gourd? I think I'm at the reach of my abilities here... The parts where I have real concerns are the trigger mechanism (more on that in minute) and mounting the prod.

    I am starting to gather materials and draw out plans. So far I have red oak for the stock and some steel banding to replicate the mounting plate as seen in the picture above. 

    I am going to keep this fairly low draw weight at between 80 and 100 lbs and was wondering if delrin would be strong enough to make a nut from. Never having worked with it before, I'm not sure of its limitations.

    The trigger/cam would be delrin, the lever steel, the nut delrin. All critiques are welcomed on this as I'm really not sure what I'm doing, lol.

    The prod I'm considering would either be a manufactured one like on the crossbow pictured or using the tips of cross country skis to make my own. I'm not 100% how the prod is held in place in the photo, or in the series of photos this pic is taken from. Anyone have any guesses? Thanks in advance for any and all help!
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    Re: Plans and materials for my first build

    Post by Geezer on Wed Aug 08, 2018 11:25 am

    Delrin will work fine for your nut.  I have used it successfully for many-many shots on bows up to @ 200 lb, though most of my stuff is 70-150 lb.
    Geezer.

    chilihook
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    Re: Plans and materials for my first build

    Post by chilihook on Wed Aug 08, 2018 2:09 pm

    Good to know, thanks Geezer.

    I tried to post a picture  of my planned trigger assembly but the site said that I need to wait 7 days before I can posted a site-hosted image. So I will post that image when I am able to!

    chilihook
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    Re: Plans and materials for my first build

    Post by chilihook on Fri Aug 10, 2018 6:36 am

    Plan of the design of the trigger I have in mind.


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    Re: Plans and materials for my first build

    Post by drawknife on Wed Aug 22, 2018 12:37 pm

    Hi chilihook,
    Just a couple of observations.
    The catch (sear) on the nut end of the pivot arm needs to work diametrically opposite the string hooks/claws.
    The length of the trigger arm looks quite long so you must make it strong enough not to bend or have any give.
    The cam on the trigger only needs to be long enough to move the arm up as far as needed to release the nut, it looks quite long on your drawing. You could have a two stage trigger pull on that mechanism.
    If you are using Delrin (I have found it very good in the past on some of my crossbows) you will need a steel wedge where the sear connects to stop wear.
    What are you intending to use as an axle for the nut or are you running it in a cradle?

    The prod fixing is a whole new issue.
    Good luck
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    Re: Plans and materials for my first build

    Post by Gnome on Sat Aug 25, 2018 12:37 pm

    Hi Chilihook, and welcome to the forum! I did a bullpup design some time back, 
    http://thearbalistguild.forumotion.com/t571-number-two if you're interested in checking it out.
    I learned a lot on that one, and it changed a lot from design stage to finish.
    It's not nearly so long an action as you're showing us here, however.

    This is a diagram of the trigger mechanism I made:


    I like the compact length, particularly with a swiveling foot stirrup, and it is very well balanced, too.
    What I don't like about the bullpup configuration is having the lock mechanism so close to my face.
    You may want to give that some serious thought before committing to such an extreme version.

    Good luck and keep us posted!
    Gnome

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    Re: Plans and materials for my first build

    Post by globalmark on Sun Aug 26, 2018 12:27 pm


    globalmark
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    Re: Plans and materials for my first build

    Post by globalmark on Sun Aug 26, 2018 12:30 pm

    Dont know where my writing went with above pic - 

    Hi 

    I just finished my crossbow short of a few finishing details and a sight - made my trigger from Aluminium and oak was pretty easy with standard woodwork tools , rasp , file hacksaw and holds a 160lbs prod no problem - based on chinese one - happy to give details if want ..
    i scaled it bigger to fit thickness of my timber using printer settings then cut out paper stuck to aluminium cut out - just filed and sanded until happy ..
    will post finished Xbow on new post .
    Mark

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    Re: Plans and materials for my first build

    Post by chilihook on Thu Aug 30, 2018 2:52 pm

    Revised trigger mechanism for you consideration.


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    Re: Plans and materials for my first build

    Post by Andy. on Thu Aug 30, 2018 3:42 pm

    Nut sear face needs to be directly radial to nut pivot. Would suggest having it directly below the pin.

    Consider having connector arm pivot closer to nut, and/or a longer trigger. Trigger travel will be more, but not as heavy.

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    Re: Plans and materials for my first build

    Post by chilihook on Fri Aug 31, 2018 7:19 am

    Thanks for the advice Andy. Moved the nut sear under the pin, moved the arm's pivot point closer to the nut for a little advantage, and redesigned the trigger itself to (hopefully) make for a lighter trigger pull. Revision here:


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    Re: Plans and materials for my first build

    Post by chilihook on Fri Aug 31, 2018 2:51 pm

    Side view of the plan. Length is about 28 3/4".


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    Re: Plans and materials for my first build

    Post by Andy. on Fri Aug 31, 2018 3:40 pm

    Looks good!

    You could also incorporate a spring of sorts to keep the nut in the loading position so you don't have to fiddle trying to reset it for cocking.
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    Re: Plans and materials for my first build

    Post by Geezer on Mon Sep 03, 2018 7:16 pm

    Some high-end medieval/renaissance crossbows had a stop to prevent roller-nuts from spinning past a certain point.  Ideally you either want the nut to roll right over and reset, or roll down 90 degrees, leaving the flat behind the lugs upright.  Then drawing the string back should reset the whole shebang.
    The potential downside of having a stop: If your nut has substantial weight/intertia on release and it hits a stop after only 90 degrees of rotation, you may end up beating the stop right out of the stock, or beating up the sear-plug where the trigger engages.  So it might be a good idea to spring-load your stop, to reduce the shock of stopping.  Of course if you're making a simple falling-gate lock that's lightly built, you shouldn't have this problem.  But with a robust medieval roller, one might be better off just making the roller-nut spin freely and learn to reset the nut as a matter of course after shooting.  It depends... geezer.

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    Re: Plans and materials for my first build

    Post by chilihook on Wed Sep 05, 2018 7:40 am

    Thanks for all the posts everyone!

    Been collecting my materials, red oak for the stock, bamboo for the deck (? no idea what the top of the stock is called where the bolt sits prior to shooting, lol), steel bar to make the bracketing, bowstring, 150 lb fiberglas prod (I ordered one, but they sent me three - double checked my order and I only asked for one, so yay!), multiplex, and some walnut for details.

    Still need threaded fasteners, chicago bolts, wood inserts, aluminum for the trigger and arm, picatinny rail and scope, and a couple of small pieces of 1/8" steel for the sides of the roller nut housing. I'm sure I'm forgetting some things... oh yeah, TIME. I need some time. :-) Other than that, things are moving forward.

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    Re: Plans and materials for my first build

    Post by stuckinthemud1 on Wed Sep 05, 2018 1:26 pm

    The plate the bolt rides on is called the table.  Have you considered holly as a suitable timber in place of the bamboo - it is called false ivory sometimes and has a lovely colour...

    chilihook
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    Re: Plans and materials for my first build

    Post by chilihook on Wed Sep 05, 2018 1:55 pm

    stuckinthemud1 wrote:The plate the bolt rides on is called the table.  Have you considered holly as a suitable timber in place of the bamboo - it is called false ivory sometimes and has a lovely colour...
    Sounds nice. But I already bought the bamboo... I'll remember it though!

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    Re: Plans and materials for my first build

    Post by chilihook on Tue Sep 18, 2018 8:22 am

    Still gathering materials and finalizing plans, but it's coming together! Only thing I'm waiting on is the nut which I am having 3-d printed.

    Here's the plan as it sits now: 
    Laminate construction of the stock, 3/4" red oak-1/4" baltic birch multiplex (recessed for trigger mechanism) - 3/4" red oak sandwiched together

    Butt plate and riser for picatinny rail of stock - maple

    Table - bamboo

    Trigger guard and bolt holder - schedule 40 PVC

    Stirrup - aluminum
    Stirrup mount - aluminum
    Stirrup mount and prod holders - aluminum

    Prod - 150 lb fiberglass

    Trigger - aluminum
    Trigger arm - aluminum
    Roller nut - delrin



    I'm sure I'll have many questions as the build gets going, any and all help is greatly appreciated!

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    Re: Plans and materials for my first build

    Post by Andy. on Tue Sep 18, 2018 9:11 pm

    Looking good! Stock design is great.

    Personally I've strong reservations in using alli for the trigger link sear. Much better to go hard steel here, and similarly for the nut sear (wedge or pin in the Delrin). Aluminium is too soft for these high load points... especially with a 150lb prod providing the force.

    Would also use steel side plates on side of stock where the link and nut are, and run the pivot pin right through. Better still, box the entire trigger mech in steel, and attach within stock.

    Would also make the trigger mech removable, and not permanently sandwich in the stock laminates.

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    Re: Plans and materials for my first build

    Post by chilihook on Wed Sep 19, 2018 6:18 am

    Andy. wrote:Looking good! Stock design is great.

    Personally I've strong reservations in using alli for the trigger link sear. Much better to go hard steel here, and similarly for the nut sear (wedge or pin in the Delrin). Aluminium is too soft for these high load points... especially with a 150lb prod providing the force.

    Would also use steel side plates on side of stock where the link and nut are, and run the pivot pin right through. Better still, box the entire trigger mech in steel, and attach within stock.

    Would also make the trigger mech removable, and not permanently sandwich in the stock laminates.
    Hi Andy! Thanks for the feedback. You're right, I completely forgot about the side plates, which I have since drawn up and will use similarly to how the original design used them. 

    I have heard many differing opinions on the the strength required of the internals - from Joerg Sprave using multiplex for a trigger and release (wasn't a nut, don't know what to call it) on a 300 lb crossbow to folks here leaning towards steel for everything. Stronger IS better, but I have no good way of working with steel - I just don't have the right tools. I can work with aluminum though, which I thought was a decent compromise... guess I'll have to experiment a little.

    You're right again, sealing the components inside the stock has been bothering me since the get go. The guy who built the original used a solid piece of wood and made a hollow for the mechanism (not a boxed trigger) to work in. This looked to me to be a time consuming effort with potentially a lot of chances to make mistakes and I am not as good a wood worker as the original builder - so I don't quite trust myself to not make things go crooked . Again, I'll have to think on this some more.

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