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

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    Post by African Archer Wed Jul 25, 2012 2:27 pm

    First topic message reminder :

    Ok so here goes, iv built this stock from plans available at Alchem and modified it to suit my taste. i haven't cut the slot for the prod yet as im not sure what distance from the nut to cut it to.Think im working a bit backwards but i intend to make a prod from car leaf spring. my question is how do i work out the dimensions for the prod at a given power stroke to get about 300lb draw weight. my intention is to hunt with this so would prefer the heavy draw weight. By the way the stock is made from Kiaat.First Build - Page 2 _DSC0501_600x402Photobucket" alt=""/>Photobucket" alt=""/>Photobucket" alt=""/>
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    Post by downunderrunner Sat Aug 11, 2012 4:28 pm

    also, if you want to cold form your prod, mark it at 1/2 inch intervals and bend one side, then mark out the profile onto a timber buck, complete a form for both sides, then beat it to shape, then heat treat, normalise, and temper as for blade steel
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    Post by African Archer Wed Aug 22, 2012 1:07 pm

    Well its been some time since i last posted, got stuck in to the prod and bent it into its deflex and added a small amount of reflex to the tips, not sure if it was a good idea but got carried away. i have made a make shift scale type thing using a bathroom scale and a car jack between the string and the prod works ok just wont go up enough in weight. at about a 1/4 way through the draw its at 110 kgs which worries me think its going to be a bit stronger than what i was hoping for. and i do think it will meet my draw length of about 300mm including brace.

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    Post by Rizzar Wed Aug 22, 2012 10:29 pm

    Oh.

    That really seems to be a bit strong...(really strong)

    So either way there must be something wrong with my formulas or something with the prod.

    It is hardened, I think? Do you have values?? How was the exact draw at 110?

    I like the bending design, looks elegant but would have tried to keep the deflexed area out of the center to the limbs.

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    Post by African Archer Wed Aug 22, 2012 10:53 pm

    Hi rizzar ya the thing is strong the guy at the place that hardened it asked if I was hunting t rex. I'm not sure wat the exact draw at 110kg cuz I had to use a longer string to get it over the jack going to mess with it abit and c what happens but was thinking if I don't come right might grind a taper into the thickness of the prod to try get the weight down not sure if it will work tho
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    Post by Rizzar Thu Aug 23, 2012 9:31 am

    My questions:
    How hard is it hardened (HRC)?? Did you mention to harden it for spring purpose or not?
    How strong is the deflex (string rest should be close to brace height, because on the picture it looks quite low)?

    You can measure the moving distance at the string rest of the limb (generally this is the point to refer on, not the draw lenght, since this is the parameter to be calculated, draw lenght is just predicted from this value) and get a clue on the draw lenght.
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    Post by African Archer Thu Aug 23, 2012 12:32 pm

    The deflex set is 2in befor the string is pulled into brace hight the prod was hardened the same way as the would do car springs. Rizzar I'll do some mesurements just now on how high the tips move in relation to the 2in set and will let u no
    Thanks kindly
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    Post by Rizzar Thu Aug 23, 2012 10:28 pm

    the tip move is relevant for the draw and the power since it is the movement of the "spring" so you can recalculate.

    In my opinion the deflex and resulting brace height is to low and a waste of energy (in conclusion a too high weight at this point) and deflex should be almost at brace height so the string is just preventing the bow from dynamic bending in the opposite direction during firing process with "low" tension.
    If your brace height would be 3inch i personally would deflex the bow the string rest as close as it is possible to 3inch.

    You could get trouble reaching your draw lenght (either by material issues and/or when finally dealing with those weight during string mounting and spanning).

    You didn´t answer your plans for spanning the crossbows yet, do you use mechanical help?
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    Post by African Archer Thu Aug 23, 2012 10:46 pm

    Rissar I'm going to have to drop the weight of the prod there is no way the nut will hold it without spliting the tiller . I was toying with the idea of cutting the taper down to 30mm in the middle maybe make a differance if that dosnt work I'll make a new one. As far as spanning I was thinking about fiddlers 600lb crossbow and his first class lever its simple and strong or doing somthing similer but with pullys
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    Post by African Archer Wed Sep 12, 2012 2:57 pm

    Well its been a while since iv added to this post, been sorting the prod out, thanks to rizzar iv got to about my desired draw weight and power stroke. the prod draws at about 202 kgs just over 400 lbs. We getting close to finnishing now.

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    Post by Todd the archer Wed Sep 12, 2012 3:22 pm

    With a high draw weight prod you might want to reinforce the tiller behind where the prod mounts by putting lag bolt up from the bottom between the prod and binding hole. This was done in medieval times as well. Check out some old ones in museums and online.

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    Post by African Archer Wed Sep 12, 2012 11:00 pm

    That will be going in todd I wouldn like to have the tiller split when its drawn , iv got some 6mm thread bar and structural epoxy. Should do the trick.
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    Post by jeep Thu Sep 13, 2012 4:34 am

    isn't the band a bit short ? And the prod angle to (lot of friction) I suppose the rope we see on the picture is not the definitive one ?
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    Post by African Archer Thu Sep 13, 2012 4:49 am

    Jeep not sure what you mean by band but the prod is just loose and not set at the correct angle. The line is just to check its if the prod is 90 deg to tiller
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    Post by actionbow Tue Sep 18, 2012 4:35 pm

    It's looking great so far. I am excited to hear how it shoots. Should be a beast...hopefully you get the trigger/release right. The difference between a beautiful build and one that is a joy to shoot is in the trigger. 300lbs is a load to put onto a trigger.

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