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    Groove vs. "notch"

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    GodricSwin
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    Groove vs. "notch"

    Post by GodricSwin on Fri Mar 06, 2015 7:00 am

    I notice quite bit of discussion about bolt grooves.  Has there been any discussion on what I'm calling a "notch" - the small piece of bone (usually?) placed near the front of the tiller that has a "vee" cut out that the bolt rests on?  I've seen a couple pictures of them on medieval crossbows.  Unless there are other factors, it seems to me it would be easier to construct and it could be adjustable if need be.  I suppose one possible objection is that while reducing friction it might be less supportive of the bolt as it is leaving the bow.
    (or did I miss a previous discussion . . .?)
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    Re: Groove vs. "notch"

    Post by mac on Fri Mar 06, 2015 9:06 am

    Many "grooveless" crossbows prior to the 16th C had a notch that was part of the inlay work and not adjustable.  ( I believe Geezer has some things to say about adjustable notches and the trouble they can get you into.)

    The groove is probably more about getting the butt end of the bolt to be the right height vis a vis the string than anything else. 

    The important thing is assuring that the bolt is laying on the line that the center of the string will take when released.  That can be done with a groove or just a "rest" or "notch".  It's all the same. Once the string is released, the die is cast.  The butt of the bolt will be stuck to the string by the acceleration, and nothing will displace it.  If the butt of the bolt travels through the place where the center of gravity of the resting bolt used to be... the bolt will fly straight. 

    As a test of the above, you can intentionally draw your string about 1/4" (6 mm) off center and see what happens.  The bolt will come out of the bow at an angle, even though it had been sitting nice and straight in its groove.  Most of the time, the bolt will straighten up in flight and strike the target well off to the side.  I have seen bolts that go into a sort "flat spin" from which they do not recover.  These bolts are frequently end up broken by the oblique impact with the backstop.

    Mac

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    Re: Groove vs. "notch"

    Post by c sitas on Sat Mar 07, 2015 3:03 pm

    from the stand point of accuracy  , I like the little bone deal for an arrow support. In my archery you'll pay a high dollar for just such a thing. Where the big problem comes from is like mac mentioned ,the string is set crooked in the lock . The string has to be set dead center or ,one limb will over come the other. Thus ,missing the target and the crazy flight. A simple way to combat this would be to mark the string on each side of the lock. That would go a long ways keeping things straight.I use a real narrow catch and use a tiny loop to grab the catch . Not authentic  maybe but it sure works .

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    Re: Groove vs. "notch"

    Post by GodricSwin on Sun Mar 08, 2015 3:40 am

    I'm a long way from there but will put this in my notes as a reminder for the future . Thanks!
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    Re: Groove vs. "notch"

    Post by mac on Sun Mar 08, 2015 11:37 am

    I like to put a mark in the center of the string with a sharpy marker.  The mark should end up centered between the fingers of the nut.  I may have gotten this trick from Geezer about a million years ago.... I forget.

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    Re: Groove vs. "notch"

    Post by c sitas on Sun Mar 08, 2015 5:21 pm

    Mac ,sounds about right.  Especially the time. HeHe.

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    Re: Groove vs. "notch"

    Post by GodricSwin on Thu Mar 26, 2015 1:05 pm

    Hi Mac,
    A couple quick ones, if you don't mind.

    I'm having difficulty finding out lengths on (antique) prods; the only dimensions I am aware of so far are what you showed (antique Norwegian, 89cm) and what you told me about your own.  (I realize most antiques have decomposed away thru' the years.)

    I won't take up your time with the story but I was given three pieces of wood, two of hazel (apparently popular for self-bows here), and a small one of yew.  I want to begin work on one of the hazel staves (they are both more than a meter in length).  You made your (hickory?) prod 88.5cm. long.  What do you think of my making mine 90cm. or would you advise I go larger?  (Incidentally, I am very comfortable with metric measurements - it's all I use here.)

    I'm getting the impression that as far as scratch-built prods go wood is not very popular . . .

    I would appreciate your help with this - thanks!
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    Re: Groove vs. "notch"

    Post by mac on Thu Mar 26, 2015 2:23 pm

    I thought we had a thread going about wooden bows, but now I can't find it. 

    I did find this though.  It looks like I posted that Scandinavian bow before, but I had a citation for it that time.  http://thearbalistguild.forumotion.com/t732-wooden-prod-from-bergen?highlight=wooden+bow

    There's also the Berkhamstead bow which totals about 48 5/8" (1235mm) http://thearbalistguild.forumotion.com/t1363-the-berkhamstead-bow-fragment#12881

    The only other crossbow with a wooden prod that comes to mind is the one in the Scot collection in Kelvingrove, Glasgow.  I don't have dimensions for that one... I don't think... http://collections.glasgowmuseums.com/starobject.html?oid=242808

    The other way to come at this is to find medieval images of guys with wooden crossbows and try to make a guess about size.  That won't be very accurate, but it will get you in the ballpark, and it's a good exercise.

    Mac

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    Re: Groove vs. "notch"

    Post by GodricSwin on Thu Mar 26, 2015 4:13 pm

    Thank the gods I sent that question because I had forgotten the Berk... bow!  Wow, 120cm is long!  I will stretch mine out to the full 1m as a result.  That's longest I can get from this piece (the other is 109cm and I'll save it for later).

    The guy who gave me the wood is a real bowyer, at least from my amateur perspective; he had upwards of 40 bows either finished, being "carved", or seasoning and about 50 arrows in various stages of completion.  According to him the woods used here (Czech Republic) for self-bows are hazel, elm, yew (not legally harvested), locust, black elder and a couple others that I can't remember.  He pointed out several kinds he had made bows from. 

    The pieces I have were sawed down thru' their centers and are what I call "reversed" in their bend (meaning the convex surface is the belly); when I asked him about steaming the wood to put in deflex he very strongly replied that NO! I should not remove the curve at all - it should be retained to add strength.  

    As always, thanks for your assistance!  (I will make an attempt at extrapolating dimensions from the medieval pictures but that means I must first research average heights for medieval men.) Very Happy
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    Re: Groove vs. "notch"

    Post by mac on Thu Mar 26, 2015 5:55 pm

    What is good advise for a hand bow does not necessarily apply to a crossbow.  It all comes down to keeping the wood within its elastic limits.

    If you work slowly and carefully, you can turn that reflexed stave into a crossbow prod. But it will have to be rather thin in order to  flex far enough, and the result will not be very powerful.  It will, of course, be a valuable bow making experience. 

    As for the heights of the medieval guys...  You will not be far off if you assume that they are about 1.68m tall.

    Mac

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