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    RDT crossbow project

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    Post by jrdallmann on Tue Oct 28, 2014 6:10 pm

    Hello folks!
    First of all, I'm from Brazil, so sorry my bad English. I'm planning to build my first crossbow on next vacation, which begins within about a month. I've been doing a lot of research about it for the last few months. Since I own a car shop, I got everything I need to build my crossbow. 
    I'm inspiring myself on patandjali's rdt model. Today I started to search for components for my crossbow. I found a spring leaf from an old Willys which will be my prod and removed the valves and the valve guides from a VW engine head to use as axle and sleeve for the cams.

    RDT crossbow project Img_2011

    RDT crossbow project Img_2010
    The leaf is 40" long

    It's not a project I wanna do in a hurry, so I'll take my time. I'm open for suggestions.
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    Post by Hermit on Wed Oct 29, 2014 9:01 am

    The valve guides might work for cam bushings,if they are made to be a press fit in the cams.The valves I'm not so sure about,as they will need to be worked,either drilled,or threaded to keep them in position in the cam axle bearings,and the valve stems are hardened.
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    Post by jrdallmann on Wed Oct 29, 2014 3:36 pm

    thanks for replying Hermit!
    I know the valves are hardened but I think I can work them. I have a lathe, an angle grinder and many other tools as well. I thought that the valves would be great because they are made to just fit into the valve guides. Maybe I'll use some kind of latches to hold the valves in the position, or I'll weld them on a holder and then fasten this holder to the prod.
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    Post by Hermit on Wed Oct 29, 2014 6:22 pm

    One thing I learned as a machinist,is that you can't work hardened steel with ordinary machine tools.The only way to work it,is by grinding it.To do so with accuracy requires special machines(surface grinder,universal grinder).If you weld the valve stem,you will destroy the temper in the metal and soften it.As an auto shop owner,you must have spare grade8 bolts lying around,if you don't have one the right size you can machine one to size on your lathe,they already have a head, are threaded,are not hardened,and are tough enough to make a suitable cam pivot.
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    Post by jrdallmann on Wed Oct 29, 2014 6:53 pm

    I've been thinking of doing that as well, but don't you think that a welded valve, even not being hardened anymore, wouldnt be tough enough? I'm insisting with the valves because it'd be such a pitty not using them, and my lathe is kinda old, not as accurate as the modern ones. But when the time to start building it comes, I'll try many ways of doing it, I wanna build this crossbow calmly, seeking perfection. I really appreciate your replies and I'm sorry that I'm so stubborn, but everything you say I'm considering. Now I'm starting to think that the big problem for me will be the string. I was able to find everything to build the crossbow, but I don't know about the string. Will I have to buy one or are there methods to make one?
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    Post by Hermit on Thu Oct 30, 2014 10:04 am

    You will have to make the string yourself.There are videos on you tube showing how,and what materials to use.The important thing is string length,which you will have to find by trial and error.You will need to make a rough experimental model of the bow using much weaker bow limbs out of wood, the same dimensions exactly as the steel limbs and using the cams for the finished bow, with the correct draw length(patandjali's draw length is 20inches,51cm's metric)You can then string the bow with thread or string as the finished bow will be strung,and determine the string length.
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    Post by ali.j on Thu Oct 30, 2014 1:03 pm

    its my home made rdt crossbow.  Cool it is not complate. comolated soon. ( 95% is complate )


    RDT crossbow project 30_1414146963181021


    RDT crossbow project 30_14141469631831042


    RDT crossbow project 30_14141469631837843

    RDT crossbow project 30_14141469686574471RDT crossbow project 30_1414146968658922
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    Post by jrdallmann on Thu Oct 30, 2014 7:56 pm

    Thanks for the info hermit and nice job ali.j
    Ok, now i'm not worried about the string anymore. And about the cam pivots, I found some old brass sleeves today and also a 12mm screw which fits very nicely in them. But I also gave a try to the valves an worked one on the lathe to see if I can do it, and I could. Now I'm in doubt, but I guess I'll follow hermit's advise and use the screws. Do you think 12mm is too thick? Because there are other sizes of sleeves there, I just took this ones because they fit very well with the 12mm screws. I'll post a pic of them, the screw in the pic is too large, of course I'm not going to use one so large, it was just to demonstrate.

    RDT crossbow project Img_2012
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    Post by Hermit on Sat Nov 01, 2014 5:51 pm

    6mm would be better if you have them............................
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    Post by ali.j on Tue Nov 04, 2014 1:33 am

    yes 6 mm is perfect. and if u use al aliaj 5000 or 6000 or 7000 u dont need belbring or other
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    Post by jrdallmann on Sat Nov 08, 2014 10:40 am

    there is something between the limbs and the pillars where the limbs are fasten to the pillars, I guess it's a piece of leather or nylon or smt like that. I've seen that in almost every decent crossbow project. What and why is that?
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    Post by kenh on Sat Nov 08, 2014 1:28 pm

    Leather 'compression washer' to allow the fastening bolts to be driven tightest.
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    Post by jrdallmann on Sun Nov 09, 2014 12:24 pm

    What do you think about my cam's project? (quotas in mm)

    RDT crossbow project Cams10
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    Post by ali.j on Sun Nov 09, 2014 1:09 pm

    this is my crossbow cam sistem. this cam need 42 cm distance betven hed lims and 50 cm ril
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    Post by jrdallmann on Sun Nov 09, 2014 1:17 pm

    what do you mean with "ril"? Does these cams work well? I indeed inspired myself in your model, just made a few modifications and draw it using autoCAD.
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    Post by jrdallmann on Tue Nov 11, 2014 7:58 am

    what did you mean with "50cm ril" ali.j? sorry my bad engish. and what could I use to make my own string? I can't seem to find a proper nylon serving material to make it...
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    Post by Hermit on Wed Nov 12, 2014 7:42 am

    The material you want for your bowstring,is called Dacron B.It comes in the form of thread,and each strand has a breaking strain of 50 lbs.It can be obtained from Archery supply shops,the internet,and is used in the manufacture of sails for sailboats,so if you live on the coast,you may be able to obtain some from a sailmaker
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    Post by jrdallmann on Sun Nov 16, 2014 1:28 pm

    i just can't find dacron b50 in my country, so i decided to buy it from amazon. my question is: in the product's description it says that the spoon is of 1/4lb, as far as i know lb is a mass measurement unity, how am i supposed to know the lenght of the thread to evaluate if one spoon is enough for me?
    i don't understand why americans keep using imperial unities, they are so strange (at least to me). OK, nevermind, i've watched many videos on youtube to figure out how to make my own string, and i got it, but one question remains: if i'm not misunderstood, i will need only dacron b50 to make the "essential" part of the string, then i'll need another product to make the "serving" which would be the "serving material" (another type of thread), right?
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    Post by Gnome on Sun Nov 16, 2014 2:41 pm

    Just checked on that, you're right, it says how much the spool weights but not how long it is! I've made about a dozen strings from one spool and it hasn't run out yet- just buy it, it's enough to get started.
    Yes, you'll need to buy serving material separately, and, importantly, a serving tool to wind the serving string. It's a simple device you could probably make yourself, a frame with an axle you can adjust the tension on with a lock nut so that you can wind the serving material tightly.
    Serving material is a thicker, woven thread, wrapped around the string to protect it at the critical points- the loops that attach to the prod tips and the center.
    I know string making can be intimidating at first, but it's pretty essential to learn. Don't hesitate to ask about it.
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    Post by jrdallmann on Sun Nov 16, 2014 2:47 pm

    thanks Gnome, very insightful response. Im going to buy the materials today but they'll should arrive within 1 month i guess, so when i have them in hands ill probably ask smt. Smile
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    Post by Hermit on Mon Nov 17, 2014 11:39 am

    Hi Jrd,
            A spool of, Dacron has I would estimate about 300 metres of thread.If it's any help,one pound imperial measure is equal to 600 grams metric if you can memorise that,it will make calculating between the 2 systems easier.The imperial measurement system has served us Anglo saxon types well for hundreds of years,you know what people are like,give them a system that works,and most won't want to change it.
                                      Regarding a serving tool,I have never used one,I served my bowstrings by hand,with the same results,so it is not something you really must have.From what I have seen of serving tools,they probably make the job a little quicker but you can manage without one.
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    Post by hullutiedemies on Tue Nov 18, 2014 1:40 am

    Hermit wrote:If it's any help,one pound imperial measure is equal to 600 grams metric

    1 lb = 453.6 g

    Or approximately "half minus 10%".
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    Post by Hermit on Tue Nov 18, 2014 1:15 pm

    I stand corrected Nerd,I was sloppy,I opened mouth before engaging brain and checking.Thank you for the reminder.
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    Post by ali.j on Thu Nov 20, 2014 3:31 am

    jrdallmann wrote:what do you mean with "ril"? Does these cams work well? I indeed inspired myself in your model, just made a few modifications and draw it using autoCAD.
    yes is work well. 

    this png file  is orginal  in size jus print this and dont resize or  chenge elemnts.



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    Post by ali.j on Thu Nov 20, 2014 4:01 am

    jrdallmann wrote:what did you mean with "50cm ril" ali.j?.

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