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    Project gone bad !!!

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    William Tell
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    Project gone bad !!!

    Post by William Tell on Tue Feb 09, 2010 10:35 am

    First topic message reminder :

    Hi Guys,!!!!

    Would someone mind telling me what the heck is wrong with my new crossbow, it is overriding the arrows and just dry shoots. here are some pictures and if you see what might be the cause please inform me.

    Thank you. Sad Mad





    Last edited by Ivo on Tue Feb 16, 2010 3:32 am; edited 4 times in total (Reason for editing : William: to add pics, Ivo: Add to the topic name, organize the IMG codes a bit :))
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    Ivo
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    Re: Project gone bad !!!

    Post by Ivo on Sun Mar 07, 2010 3:16 am

    Yes Сергей(Phoenix) is a bit of a magician when it comes to building crossbows

    I believe he used a laminating resin rather than an epoxy ... if my memory is correct he used a polyester resin that he got from RC Plane enthusiasts. Such polyester resins can have a pretty long working time in the ranges of 20-30 minutes if not more...another great feature - it's cheaper than epoxy

    Polyester resin is OK for pressing recurve limb blanks(that still need to be sanded into shape...a whole subject on that), but not for highly stressed/high flex limb designs(at least that's what I heard). It's a whole subject in itself as there are quite a few different ways of making limbs as you might have already heard from Robin. Laminating technologies used by bow makers are quire popular - where properly tapered wood core(or wood core laminate) is sandwiched in between two unidirectional fiberglass prepreg strips using a hot polymerization epoxy like Bowgrip100 or Smooth-On EA-40(There are other epoxys, I just don't know about them).

    Robin did a pretty nice article on that

    >>> Laminating Press

    >>> Building the Prodd




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    string jump

    Post by arbalest on Thu Apr 22, 2010 12:19 pm

    Gil Frey told me about a high speed photography study done of several trigger designs. It was found that the string jumps when released from a nut that drops down on release. Overclaw designs do not suffer from string jump. The cure, assuming the arrow groove is correct, is a "cowl", for lack of a better term, that keeps the string from jumping over the arrow. The "cowl" should extend an inch or so beyond the nut.
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    string-jump

    Post by Geezer on Fri Apr 23, 2010 9:01 am

    I wouldn't be at all surprised to discover string-hop or jump if you prefer, in all locks that fall away below the top of the stock. With medieval roller-nuts you can keep the hop down to a minimum by making sure the nut is lightweight and rolls very fast. (and that's why another maker referenced somewhere above, resorted to drilling holes into his bronze roller-nut... I did the same thing on my first bow) Making the lugs slightly hooked on the rear will also delay hop a micro-second or so, and every little bit helps. If you have good contact between the bowstring and butt of bolt, whatever hop remains should be no problem, but I expect it's still present... which could be one reason that modern target bows usually push the bolt with a slider, rather than letting the string do it (the slider also assures the shooter than he has pulled both limbs evenly) Locks that release from the top can rely on the bolt-track to prevent the string from going anywhere, so no string-hop, though I suppose there could be down-forces that serve to flatten the string. In some cases, you might even get the string UNDER the butt of the bolt for entertaining results.
    High-speed photos I have seen suggest substantial oscillation in the string on release isn't at all uncommon. Certainly crossbow strings with a hidden broken strand can precipitate misfires even though they appear normal otherwise.
    A substantial cowl or string-snubber should prevent hop from being a problem on most locks mounted in the stock. I would include them as a matter of course on any modern design. Medieval locks generally don't feature them and since that's what I make...
    Geezer.

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    Re: Project gone bad !!!

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