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    Reverse limb "Test" crossbow

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    Kali
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    Reverse limb "Test" crossbow

    Post by Kali on Wed Feb 17, 2010 6:33 am

    During last weeks I started to build a "test" crossbow (in fact I just plan the design, gathered the materials and prepared some spare parts):
    The design to be tested: Reverse limb configuration + full compound system.
    To speed the process, I've built 2 straight limbs, using 2 layers of clear fiberglass and 2 strips of action bamboo (the laminated limbs are made using 4 laminations totaling 7 cm thickness, 4cm wideness and 45 cm in length). I will say that the limbs are pretty soft (I estimate a max draw weight of just 20kg), but, for testing purpose will be just perfect
    I also designed a trigger system and made calculations regarding limbs alignment : the limbs will have to be oriented 30 grades away from the stock in order to achieve a draw length of 50cm. Aiming a draw length longer than the limbs, I had to place the trigger in the back of the riser - from this perspective, I had to design the riser a little wider, each limb being 10cm away from the stock (to be able to cook the bow by hand and not to get the rope passing over the riser)
    I also built 2 pulleys (round and with the hole in the middle). Here I made a mistake, building the pulleys with 2 x 6mm thick ball bearing inside - this leading to a 15mm overall thickness (I think that it will be too much to cut a 15mm hole in the 40mm limbs tops). This week i will build another pulleys using just 1 ball bearing, leading to just 10mm thickness.
    Unlike Ivo, i choose to build the metal spare parts to a machine shop to get better accuracy (especially for the riser that will have to be perfect aligned and for the pulleys ...). Up to the end of this week I will have to have ready the trigger mechanism components and the new pulleys. Next week I should get the riser and to start assembly the crossbow. I will start to take and display pictures as the works will progress.

    Ivo
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    Re: Reverse limb "Test" crossbow

    Post by Ivo on Wed Feb 17, 2010 11:51 am

    Hey we are rooting for you Kali!

    When you said totaling 7cm in thickness did you mean the thickness of "two" limbs added and that the total was 7cm? I got a bit confused there. cyclops

    Also about the hole in the limb for attaching the cams...it isn't really necessary...all you would need is a small bracket that you can cut,drill, and bend into shape very easily. Let me know if you need a better description as I believe I have a few pictures of a crossbow with reversed limbs that has uses the bracket mount instead of just hole in the limb tip.

    Everything else including being able to "order out" Wink and get the parts made professionally is great and should speed you up to the testing phase instead of dropping stuff on your foot like I did. Mad Laughing

    I have a shop near me that makes custom motorcycles and those guys have the most insane equipment and have been inviting me to come in if I want to do something...but I said NO until I finish it on a knee and present it in step by step tutorial form I am not moving a finger in their direction. pirat

    Good Luck and keep us posted on your progress Kali!

    Ivo :frank:


    Last edited by Ivo on Fri Feb 19, 2010 9:49 am; edited 1 time in total

    Kali
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    Re: Reverse limb "Test" crossbow

    Post by Kali on Wed Feb 17, 2010 4:50 pm

    Hi Ivo,
    Thank you very much for your advices ...
    My mistake about the limbs - they are 7 mm thick : 4 layers / 2x 1.3mm of clear fiberglass + 2x 2.05mm of laminated bamboo.
    Regarding the pulleys - I've seen several simple ways to mount the pulleys on the brackets, but, without considering the easier way to install, i can see 2 disadvantages:
    1. the weight - the pulleys that i have are weighting about 50 grams each (made from aluminium alloy + 2 steel ball bearings inside). Using just one ball bearing i can reduce in half the weight of each pulley; the brackets will also add more weight rather than some steel bushings glued on the limbs tops.
    2. using the brackets i will have to increase the axle to axle distance, reducing the available power stroke
    (i think that this a subject itself to be debated - how much the weight attached to the limbs tops does matter - my personal believe is that it counts a lot)

    In terms of DYI parts - i also own a large array of tools, including a Dremel 400 Digital will all accessories, a vertical drilling/ milling machine, hand drilling/ sanding/ cutting machines, a vertical saw, etc, etc - the problem is that machining metal parts at home (i'm living in an apartment) produce to much noise for my neighbors ears Smile). Anyway, the precision of parts machined on CNC is by far greater than anything i can produce handmaded.

    Ivo
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    Re: Reverse limb "Test" crossbow

    Post by Ivo on Fri Feb 19, 2010 9:53 am

    Yeh I can write a book about coming home late and not being able to make noise for weeks at a time. ninja vanish

    Got some inspiration for you...a guy from a Russian crossbow forum is doing a similar one...all wood and nothing serious...just fooling around, but it does look good.

    chopp wrote:

    Oh and the bracket VVV cyclops

    Image: Courtesy of Moonkryket


    Kali
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    Re: Reverse limb "Test" crossbow

    Post by Kali on Fri Feb 19, 2010 12:20 pm

    It looks very nice your's friend all wood crossbow - it is very good inspiration for a real one Smile
    Now i see what you mean about the small brackets - these are looking like really small and low weight.
    Today I got the steel triggers' parts from the machine shop - hopefully, tomorrow i will have the trigger assembly and I will have some pictures to display
    I also sent to production the riser plans, that will be made from dural :

    sorry for the bad drawing ...

    and this is my hand-made Assyrian bow - 46lb (sorry for the off-topic):

    the bow was not made by me - is produces by a local bow artisan from Romania, leather covered and painted following some Assyrian "traditional" models Smile


    Last edited by Admin on Sun Feb 21, 2010 12:03 pm; edited 1 time in total (Reason for editing : Reposting pictures in a clickable "Thumbnail" format.)

    Ivo
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    Re: Reverse limb "Test" crossbow

    Post by Ivo on Sun Feb 21, 2010 10:08 am

    Nice! 46lb is a good hunting weight. I believe I've seen a similar bow somewhere on the net before...Perhaps it was even yours that I saw. Very Happy Do you mind enlightening me a bit on who this bowyer might be? I'd love to see more of his work.

    There is a very nice book on the construction of traditional composite bows by Adam Karpowicz. I recommend it in any bow lovers personal library... link

    Ok...back on topic

    About the plans...I see you have planned lightening of the load a lot by milling the riser practically hollow...did you just eye out the design having a feel for how strong the duraluminum you are using or have you done some calculations? By the way, what mark of dural are you using for the riser and what are the trigger parts and housing is being made from?

    More inspiration Very Happy


    This one was made by a Ukrainian guy...don't ask...I was not able to understand what this guy was talking about as he was talking in "shortened" heavy duty street slang...how much do I regret for busting out on that guy for the way he was talking, but I and many others just couldn't understand what in the world this guy was yapping about...thou he did make some wicked "hard" cams on this apocalyptic styled crossbow. >>> More Pics

    PS: click to open
    Spoiler:
    I've updated the pictures in your previous message to the Clickable Thumbnail format...you can setup your code list that shows up under the picture (or on the side when expanded) by going to "Link options" and activating by checking the "Clickable Thumbnail" code to be displayed in the code list...by using(copying and pasting) thumbnail code into your post you won't have to resize the pictures you upload - keeping all the detail of the original picture while keeping your post nice and compact.Smile

    Kali
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    Re: Reverse limb "Test" crossbow

    Post by Kali on Mon Feb 22, 2010 6:30 pm

    About the riser : there are designed milling holes just from upside-down, the milling on sides is just 3mm deep. There is no precise calculation, but I bet that will easily hold 100lb Smile - the dural is some kind of aircraft aluminum - i don't know the code, but is much harder than ordinary aluminum ... we will see next week how will looks like Smile
    The machined trigger piece is made of carbon steel. The housing is from 2mm thick steel.
    The trigger assembled/ dismounted/ housing :
    ;;;
    Still, there is a lot to work on it: first, to find a way to include a safety system, then - to make some mounting rail to be attached on the top of the mechanism, providing support for the scope, and to finish the trigger pull (I will left this after I will finish the stock.
    An idea about how the mounted scope will look like:
    LE: now i'm seeing that i placed the scope reversed Smile)
    ;
    These are the machined trigger parts made from carbon steel:

    Kali
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    Re: Reverse limb "Test" crossbow

    Post by Kali on Mon Feb 22, 2010 6:52 pm

    The limbs (7mm thick) and the pulleys that were machined initially (too thick and too heavy) - i already ordered 2 sets of lighter pulleys : 1 in 45mm diameter and another one in 70mm - I will see which fits better

    the limbs are 2 fiberglass + 2 bamboo strips laminations, glued together with Bizon Epoxy Universal "Slow hardening" (shown in the picture)
    This is my current idea about the fire rail + first part of the stock:
    ;
    As the works progress, i start to realize how difficult actually is to build a reliable/ strong and decent looking crossbow ... in this stage, after some 2 months of gathering pieces, design and try and miss , still, there is a LOT of things to do:
    1. to build the new pulleys
    2. to design/ build a reliable way to attach the pulleys to limbs (here, i'm still debating with myself if to use 2 limbs with holes at tops, or to cut the limbs in 2 and to use 4 narrower limbs)
    3. to build the fire rail
    4. to build the stock (here I think that i will go with a very simple idea - some aluminum tube with a shoulder rest attached
    5. to build the rope (I think that dacron is the most recommended)
    6. some solution to cover the 1000 screws in the trigger mechanism Smile
    7. painting, etc, etc - most probably there are a lot of things that i didn't thought about yet Smile)

    Kali
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    Re: Reverse limb "Test" crossbow

    Post by Kali on Mon Feb 22, 2010 7:31 pm

    By the way, what way of clearing the excess epoxy/ smoothing the edges of the limbs do you recommend ?
    I cleared one limb with this (the gross parts with the drilling machine and finishing with the dremel):
    ;
    but is very time consuming and is making a lot of dust around

    Ivo
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    Re: Reverse limb "Test" crossbow

    Post by Ivo on Mon Feb 22, 2010 7:35 pm

    Ah thanks for the info on the riser...by the way...did you check out this program? I', sure you will like it a lot...just watch the tutorial videos and you will see that planing your project in CAD is much more fun than on paper. Very Happy >>>Link

    Sorry...I've run out of time at the moment and have to get back to work, so I'll check in a little later...so far I can only say - great job!...and the comment on decent looking crossbow Smile >>> if it's done more or less correctly it will "shoot" and thus look GOOD! Very Happy I'll be back to this as I've much to add. :idea:

    PS: I see people use belt sanders to clean up as well as shape limbs...what kind of fiberglass are you using here...unidirectional prepreg like in laminated bows?




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    Kali
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    Re: Reverse limb "Test" crossbow

    Post by Kali on Tue Feb 23, 2010 6:35 am

    Thank you for appreciation and advices,
    I did downloaded the program (Google sketchup) - it looks very interesting. I will try these days too see if i can plan the riser 3d.
    Indeed, the fiberglass is unidirectional. I bought it from 3rivers archery:
    http://www.3riversarchery.com/Bow+Building+Fiberglass++Clear_c49_s133_p0_i4475X_product.html
    they are selling some very good quality stuff.
    I tested the trigger with some 25 kilos hanged up, and it works incredibly smooth - the ball bearing is reducing a lot the weight to be applied on the trigger pull to release. Is not clearly seen in the picture, but all pieces are well locked in place - the permissible movement of the trigger being 5mm - just enough to release the rope.

    Ivo
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    Re: Reverse limb "Test" crossbow

    Post by Ivo on Tue Feb 23, 2010 7:59 am

    I'm glad you like it...it will save much of you time...thou I sometimes sit and draw on paper as well. Smile

    If you don't mind I go by your list here...


    1. to build the new pulleys
    2. to design/ build a reliable way to attach the pulleys to limbs (here, i'm still debating with myself if to use 2 limbs with holes at tops, or to cut the limbs in 2 and to use 4 narrower limbs)
    3. to build the fire rail
    4. to build the stock (here I think that i will go with a very simple idea - some aluminum tube with a shoulder rest attached
    5. to build the rope (I think that dacron is the most recommended)
    6. some solution to cover the 1000 screws in the trigger mechanism Smile
    7. painting, etc, etc - most probably there are a lot of things that i didn't thought about yet Smile)

    1.Speaking of bearings...I know a place where you can get some really nice tiny bearings in practically any size and variety...from regular ball bearings to ceramics used in engines...here is a section where I hang out(look at the price and you'll see why Smile )

    http://www.bocabearings.com/main1.aspx?p=searchWebstore&s=3&where=%20storeid%20=3&colnames=Categoryvalues$4%20as%20[ID%20%28d%29],Categoryvalues$5%20as%20[OD%20%28D%29],Categoryvalues$6%20as%20[Width%20%28B1%29]

    2. You are right in this moment. I know from looking at some crossbows that have a split tip are usually reinforced by thickening that area. If limbs are pressed into a laminate - a wooden "wedge" is used...sort of how the man did here(scroll down a bit to the compound bow)>>>Link

    ...if you decide to make split limb made up of two narrower limbs...I can see that you will most likely have to make some big or complex mounting brackets to keep the limbs from twisting. Let me know if I'm not making any sense. Wink

    3. Well the arrow track or rail or whatever the world calls it today(some people don't like to use term "fire" with crossbows Wink ) You have chosen a pretty good design...in this type of crossbow I don't see an extreme need for rail rigidity as it doesn't need to support the draw weight of the crossbow when it's loaded...so what ever you do just make sure it is strong enough to support a foot stirrup without bending. Also a good note might be to fill the hollow space in the rail to quite down the crossbow as shooting it will send vibrations down the crossbow body making it humm like a tuning fork. Very Happy

    4. Sounds good to me. sniper shot

    5. There are better ones out there, but Dacron is recommended simply because it stretches a bit compared to the more expensive string materials that have much less stretch. The stretch is good for a reason...those stronger and less stretchy strings are very hard on the crossbow as each shot is a very hard stop that sends more destructive vibrations down the crossbow limbs and and other components.

    Then there are also a few different ways of making the string...there is the "endless loop" method and the "Flemish twist" method of making a string. Both are widely described by bow builders. Advantages of The Flemish string(even thou it takes longer to make) is that it has additional amortization and can be adjusted for length by twisting much better than the endless strings...thou Endless strings are not bad and the speed and simplicity of making them also makes them an excellent choice. Don't forget to Pre-stretch your strings. Wink

    6. Cast, machine, or layer(riveting one layer to another) a dedicated housing perhaps Smile

    Interesting trigger you've got there...to be honest I don't exactly understand the principal of it's operation other than what you've already mentioned with the bearing retaining the latch...thou it does looks very familiar. cyclops

    7. Well painting is certainly an interesting topic...for example I'm currently getting ready to spray my brothers stock to a carbon fiber cloth effect yeh

    PS: 3rivers sells it a bit more expensive than where I get it...what do you think about this guy...I've bought black glass as well as fast flex bamboo laminations from him before with out any trouble >>>Link




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    Ivo
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    Re: Reverse limb "Test" crossbow

    Post by Ivo on Fri Feb 26, 2010 7:45 am

    Excellent gallery full of useful detail...If I wasn't building a crossbow already...I'd be building one of these. Very Happy

    http://crossbow-review.com/Slideshows/ArmcrossLeopro/index.html

    Also an interesting thing I've noticed after looking at various crossbows of reversed limb design...the cams are slightly angled up towards the rail....I assume they should flatten out a bit upon full draw. Suspect



    I hear that connector has an interesting use...If I'm not mistaken it makes possible changing of the string in the field. Smile




    * *
    ~ "I don't have any special talents. I'm only passionately curious."
    * * *
    ~ "All Genius is Simple"
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    Kali
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    Re: Reverse limb "Test" crossbow

    Post by Kali on Fri Feb 26, 2010 12:01 pm

    To be honest, i started to think about my reverse limb design after I've seen LeoPro first time ... On the other hand, Scorpyd RDT looks a bit more attractive for me - especially considering the low draw weight involved that can be easily achieved using "classical" bamboo + fiberglass limb construction
    I hope that the "test RDT crossbow" will convince you that the next crossbow that you will build will also be reverse limb configuration ...
    Thank you very much for your very useful advices regarding my "to do" list
    This morning i went to the machine shop - just to find that i will get the riser sometime during the next week ... i can't wait to see the final product Smile
    On the other hand, i found a very interesting solution for the trigger - how to make it work using just one steel spring ... Later today I will take some pictures to let you see what i'm talking about ...

    Kali
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    Re: Reverse limb "Test" crossbow

    Post by Kali on Fri Feb 26, 2010 5:42 pm

    Ok, so this will be my trigger idea:
    uncooked:
    cooked:
    and a little movie showing how is working (I had to screw both housing's plates to make it work properly ...)

    any ideas about how to include a safety system there ? so far i'm pretty much out of ideas ... mainly because the lack of space inside of the mechanism ... as you can see, the whole thing is just slightly bigger than a pack of cigarettes
    Mention that the mechanism (including housing and everything) is weighting precisely 365 grams - that i will say is pretty light Smile

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