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    first crossbow design questions

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    paliden
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    first crossbow design questions

    Post by paliden on Wed Nov 03, 2010 11:04 am

    Hello all,
    I am new to the forum and to crossbows. i have had the idea of building a crossbow for a number of years but have put it off until now. I'm working on what could be called a hybrid medieval crossbow. i got the bug after seeing an article in a primitive archery mag a few years back. i finished the prod yesterday, spring steel, it pulls somwhere in the 260lbs range. i designed it so that the centerline of the prod allows the string to ride just even with the rail to elemenate string wear. I have a pic on photobucket.

    a couple of design questions include: what string material would be best / safest considering the draw weight and that i would like to keep the string diameter to a minimum. the string that i have on there now is only temporary and is made of 6 strands of 1/16th in aircraft cable, i do not plan to shoot with this string but used it only for tillering pourposes.
    and secondly; nut size, dimensions. i plan on useing steel for the nut due to the heavy draw.
    any advice will be greatly appreaceated.
    thanks,
    J
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    Pavise
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    Re: first crossbow design questions

    Post by Pavise on Wed Nov 03, 2010 11:45 am

    Hi paliden,

    Welcome to the forum.

    I suggest that you visit the following website before making your string. Please see:
    http://www.thecrossbowmansden.com/

    If you'll navigate through those pages you will find under "Tips" how to make a string and from what material. Dacron B is but one, and available from most good archery suppliers. The "jerk strain" on your prod is 5 times the draw weight and the string must be made to withstand that total weight. In your case 5 x 260 = 1300 lbs.

    Then I respectfully suggest that you spend some more of your time going through the many good tips and suggestions on "this" forum. That's what it's here for.

    Good luck with your project.

    Pavise
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    paliden
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    Re: first crossbow design questions

    Post by paliden on Wed Nov 03, 2010 6:57 pm

    Thank you,
    since finding this site i have been pouring over the post and have found many very helpful and interesting things. i also took your advice and checked out the crossbowman's site, very interesting.
    However i still have questions that i haven't found answers to. though i found several references to nut dia. i haven't found any dealing with its width. how do i calculate the proper dimensions of the nut taking into consideration draw weight, string dia, nut material ect...
    Additionally having watched the tutorials on Robin's site i noticed that he didn't reinforce the string eyes except by serving them. i found that curious given the draw weights of most crossbows. shouldn't the # of strands in the eye be equal to if nut greater than the # of strands in the main body of the string?
    i also haven't found a definitive answer to the question of which string material is best for the application, there seem to be a no of synthetic materials from which to make a string but which one would be best considering draw weight, and prod material. i have read where some stretch too much and some not enough. thus which one best fits my application?
    I certainally look forward to more input especially from those of you who have far more experience in this than i. i am here to learn and thus may ask the occasional dumb question. hoever i beleive that really the only dumb question is the one never asked.
    thanks again,
    and rest assured i will continue to search this site to see what gems await.
    and again any input into the questions i am asking will be appreciated.
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    Pavise
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    Re: first crossbow design questions

    Post by Pavise on Wed Nov 03, 2010 8:30 pm

    Hi paliden,

    I'm glad you found the late Robin Allen's 'site interesting; he was a dear friend to many of us around the world.

    Your observations on the string loops are astute and there are some who will reinforce them with extra threads to avoid having the string cut into the nocks of a prod. This cannot happen with a steel prod though, and although divided into two, they are, by design, (times 5 remember) strong enough. You have asked about making a thin diameter string and I assume this is to minimize the mass that has to be propelled by your prod. Dacron B is as good a material as any and I wouldn't hesitate to use it to make a string for your prod. You will need a suitable jig and a serving tool to make the job easier too. Serving thread can be bought at the same place as where you get the Dacron.

    As for the "nut", it should be as light as possible in order to give a quick and satifactory release with minimal intertia. But it must also be of sufficient width to provide a pair of claws strong enough to stand, in your case 260 lbs of held weight. The width between those (polished) claws must be wide enough to accommodate the butt thickness of your arrow or bolt. Typically this gap is five to seven sixteenths of an inch wide with each claw being perhaps three sixteenths to one quarter inch thick; enough to prevent bending or breaking under load. Add those measurements togther to arrive at the total width of your nut. Two hundred and sixty pounds is not really heavy as medieval crossbows go and Delrin or antler (see other related topics) will suffice as will brass or steel. The diameter of the nut is around one and one half inches or less, making for claws that are tall enough to retain the cocked string of perhaps a quarter inch thick at the centre. And again these guides can be found by following similar questions on here and looking at Geezer's book, or the ones by Sir Ralph Payne-Gallwey. I am willing to help you all I can on here but I'm not going to re-type everything for you just to make things super easy. This forum is perhaps one of the most informative available on the subject, and sorry to say so again, but most questions have already been covered, illustrated, or well answered by others.

    Good luck,

    Pavise
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    Re: first crossbow design questions

    Post by Geezer on Wed Nov 03, 2010 8:37 pm

    Harumpf! My post just disappeared, so will try again.
    Concerning string materials: There is no Best material for bowstrings. Each offers a compromise for strength, durability and cast. I recommend Dacron B-50. It's strong, slightly stretchy and stable. There are other good materials, but if you use B-50 for your first string, you won't be sorry.
    As for roller-nuts. You can safely build your roller-nut of moose or axis-stag horn up to several hundred pounds of draw if you remember to put a steel sear-plug into the bottom for the trigger to work against. I drill a hole through from top to bottom, tap, and thread in a bit of steel allthread for this purpose. It's easy and strong. If you experience heavy wear on your sear, you can easily unscrew it and put in a new piece.
    As for nut size: typically medieval roller nuts range from 1 inch to two inches in diameter, but the average seems to run from 1.25 to 1.5 inches diameter. Widths average about 1.25 inches. Payne-Gallwey's useful book suggests 1.5 diameter by 1.25 inches wide. That's a bit large for the average bow, but should work fine. He suggests putting the roller in a steel socket and burying it only about 3/5 of its diameter. At the weight you're anticipating, a bone-reinforced socket will do fine, but figure on burying the nut about 2/3 of its diameter. 3/5 is too shallow without the close-fitting metal socket. If you make the socket as deep as 3/4 diameter, you won't be able to remove the roller from the top of the socket, but will have to go in through the side. So what are you waiting for? Get to work. Geezer
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    Re: first crossbow design questions

    Post by paliden on Wed Nov 03, 2010 9:25 pm

    Thanks guys,
    there is alot of great info here and am still learning to navagate the site but am looking forward to digging in. the idea of useing moose antler for a nut makes sense, i just read (tonight actually) in an old archery mag that it has a similar density to brass and thus is a favored tool for flent knappers. i may have to see about aquireing some.
    again, thanks,
    J
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    basileus
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    Re: first crossbow design questions

    Post by basileus on Thu Nov 04, 2010 9:58 am

    If you have not yet visited my site, you'll probably find it very interesting.

    paliden wrote:
    Additionally having watched the tutorials on Robin's site i noticed that he didn't reinforce the string eyes except by serving them. i found that curious given the draw weights of most crossbows. shouldn't the # of strands in the eye be equal to if nut greater than the # of strands in the main body of the string?
    I serve my loops, but also use additional leather protection as mentioned in these articles. Leather protection is especially useful if the loops of the bowstring have to bend in acute angle around the nocks.


    i also haven't found a definitive answer to the question of which string material is best for the application, there seem to be a no of synthetic materials from which to make a string but which one would be best considering draw weight, and prod material. i have read where some stretch too much and some not enough. thus which one best fits my application?
    Personally I prefer Dynema when using steel bows. The reason is it's way superior strength compared to, say, Dacron B-50. As an example, I managed to break two nearly identical, well-made, Dacron bowstrings after shooting quite light bolts a few (3-5) times. After this I made a bowstring from Dynema and it could shoot the same bolts with no issues whatsoever over extended period. The Dacron bowstrings weighed 22 grams (~1oz) whereas the the Dynema bowstring weighed only 6 grams (~1/4oz). So Dynema is at least 4 times as strong for it's weight compared to Dacron. Also, the lack of stretch in Dynema seems to be no issue for steel bows, meaning the bow does not easily break/bend because of dry-fires.

    So, if you don't mind using a synthetic fibre, I'd definitely go with Dynema. Linen is the best easily available natural fibre, if you get good quality thread, e.g. bookbinder's linen thread from here.


    Last edited by basileus on Thu Nov 04, 2010 9:59 am; edited 1 time in total (Reason for editing : Fixed a typo)
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    Re: first crossbow design questions

    Post by paliden on Thu Nov 04, 2010 11:09 pm

    Thanks Basileus,
    i found and book marked your site early this morning. it definetly has some great info, and many good toutorials.
    i made my first string tonight, (had to first make a jig). its not the prettiest but should do for now. i used 36 strands of B50 but ran out of serving so served it with some waxed linen thread i had in my leather kit. seemed to work well. i used B50 cause that is what i had. i did reinforce the eyes with an additional 18 strands each so they are as thick as the main string. i served them with the regular serving material. i made the string about 1 1/4" shorter than the cable that i had on there because so many of the toutorials recomended it since the string will stretch some.
    when i put the string on the prod it curiously revealed some tillering issues i will have to go back and deal with. one side is slightly stiffer than the other. what i don't understand is why this didn't show up with the cable. ? i plan on letting the string strech for a few hours then remove it and re-tiller the offending side.
    i hope to make the nut and its surrounding assembly tommrow. i have a piece of 1 3/8"dia steel that i can cut a 1"wide piece out of and was thinking of drilling some holes along the primeter to reduce weight. additionally if i drill one hole a little larger i can utilize it for a sfaty.
    i am considering useing some corian countertop material to make the houseing. i have some white left over from another project. its tough and kind of slick which might help reduce friction.
    so what do you think does it seem workable?
    one another note; i really liked the toutorial on forging bodkin points, i've been looking for a simpe way of making them that did'nt involve having to forge the socket too.
    thanks alot,
    i'm still going through the site and finding all kinds of stuff.
    J
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    Re: first crossbow design questions

    Post by paliden on Fri Nov 05, 2010 2:39 am

    couldn't sleep so i went out to the shop and re-tillered the prod. all the measurements look good, 7/32" nearest the center tapering to 9/64" nearest the tips. still one side wants to bend more radically than the other. when i cut it out from the truck spring i used a cut off wheel on a 4.5"grinder watching closely for any change in color. took my time and didn't have any evident heat discoloration. same with the tillering process. though i used my big disk grinder keeping a hand on it to monitor heat. when it began to get too warm to comfortably hold into the water bucket it went. i suspect that despite trying not to, i may have ruined the temper.
    i may attempt to re-heat-treat but it is a bit larger than anything I've attempted before.
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    Re: first crossbow design questions

    Post by basileus on Fri Nov 05, 2010 2:49 am

    paliden wrote:Thanks Basileus,
    i made the string about 1 1/4" shorter than the cable that i had on there because so many of the toutorials recomended it since the string will stretch some.
    I think bowstring stretch is more of an issue for handbows. Also, I've heard that heavily twisted bowstrings stretch more, apparently because the outer threads are stressed more. I have not noticed any noticeable stretch in any of my crossbow bowstrings, whether made from Dacron, Dynema or linen. That said, most of them have been shot <100 times. I nowadays try to m ake my bowstrings as long as possible to minimize brace height; it can make a very big difference to the crossbow's performance. If some stretching occurs, it's always possible to twist the string slightly to shorten it.


    when i put the string on the prod it curiously revealed some tillering issues i will have to go back and deal with.
    Just curious... are you tillering your bow like one would tiller a wooden bow? I've tapered my steel bows only in width to get a roughly circular tiller. That gives excellent performance with least amount of work.


    i hope to make the nut and its surrounding assembly tommrow. i have a piece of 1 3/8"dia steel that i can cut a 1"wide piece out of and was thinking of drilling some holes along the primeter to reduce weight. additionally if i drill one hole a little larger i can utilize it for a sfaty.
    i am considering useing some corian countertop material to make the houseing. i have some white left over from another project. its tough and kind of slick which might help reduce friction.
    so what do you think does it seem workable?
    I think your plan should work just fine.


    one another note; i really liked the toutorial on forging bodkin points, i've been looking for a simpe way of making them that did'nt involve having to forge the socket too.
    thanks alot,
    i'm still going through the site and finding all kinds of stuff.
    J
    Thanks! I've used three different methods to forge the points, each having some distinct advantages. The third method is most universal, meaning that you can use it for almost any bolthead sizes and shapes.

    Regarding ruining the temper of the steel... I use an angle grinder to cut my bows and never had had any issues with overheating, even though I've been much sloppier with the waterbuckets than you Smile. If you're 100% certain that both width and thickness taper are equal, then I suggest using a very coarse (hand) grinding stone to get rid of the possibly overheated outer surface of the bow. Grinding just enough to get rid of the old grind marks should suffice.
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    Re: first crossbow design questions

    Post by paliden on Fri Nov 05, 2010 10:27 am

    i think i'm going to pitch the original prod. it was tillered more like a hand bow with predomenetly straight limbs. (see pic earlier in the thread), the original idea was to raise the centerline of the prod just above the top of the rail to eleminate string contact and thus wear and friction. i'll start over with another spring and go with a pyramidal design such as is found on Basileus' site. still would like to get the center-line high enough to eliminate contact with the rail, may not be able to completely.
    thanks for all the encouragement, i'll keep posting as to the progress and perhaps(barring any more missteps) will be able to post some pics soon.
    thanks
    j

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