Primitive Archer
Main Discussion Area => Bows => Topic started by: Red Dwarf on January 03, 2008, 10:52:09 pm
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After having made half a dozen board bows I thought that I should try a yew flat bow from some billets I harvested a couple of years ago.
About halfway through the tillering process the wood suddenly lost its strength (springiness). What did I do wrong??
Here are a couple of photos of the bow and I would welcome any and all comments/helpful advice.
Red Dwarf
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It looks really good! How much weight did you lose?
J. D.
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Not sure what you mean by that, but yew a lot of times needs to be physically thicker than other woods to get the same amount of draw weight. Did the bow take a lot of set?
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How thick did you leave the sapwood? it shouldn't be any thicker than 3/16" .
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I was heading towards 50# @ 26" but suddenly lost almost 10#.
I was wondering if things were bending a little too much close to the handle as the attempted bow took about 2" of string follow at the same time as the "weakening".
I thinned the sapwood down from 7/16 to 5/16...maybe should have taken it down further?
Here is a picture of another pair of billets for my next attempt, you can see the thickness of the sapwood (7/16 - 1/2").
Red Dwarf
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Where did the set occur? All from one spot or was it spread uniformly? That tells you if you had a weak spot versus just the normal process of training the bow to bend. 2" of set is expected with a selfbow that has a reasonable design. As far as losing springiness, I suppose if you over-stressed the wood a bunch by pulling it past the intended weight on the tiller tree, you could fatigue it unneccessarily. Did you do that? The only other thing that comes to mind, is that once you get to the "sweet spot" you can make a big defference in weight with a very small amount of wood removal. And its quite easy to blow past this spot, because the process of getting to it requires mroe agressive removal. Its because the relationship between thickness and poundage isn't linear, its more nearly exponential (twice as thick, eight times as strong, to be exact).
A fellow who is experienced with yew bows, told me you can't have too much sapwood. Only too little. So I don't think that was a problem, although with a flatbow you might have been running out of heartwood on the belly side? You wouldn't want the belly to be sapwood.
Good looking billets. Keep at it, I broke my first yew bow so your maiden voyage beats mine.
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Did you exercise the limbs as you removed wood? In most cases this will prevent such things from happening all of a sudden. Pat
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I wouldn't call that a failure. 2" is a lot of set for that bow to have taken but you ended up with a nicely-tillered, 40# bow and I'd hardly call that a total loss.
If I could guess, I would say that you possibly braced this bow very early at a high draw weight and began to take it down? That would account for the set and the rapid loss of weight--especially with Yew. Torges articulates this phenomenon in 'Hunting the Osage Bow.' I can explain in more detail if you think that might be what happened.
J. D.
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Does belly tempering work on YEW ?
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I've found that if i didn't exercise the stave at partial draws after each wood removal then the weight loss could register all of a sudden. Jawge
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What is the RH/MC? Properly dried?
Dave
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Jawge
I think that you may have nailed it. Now that you have mentioned it, I probably got a little carried away with making my 1st bow in 12 months and not exercised it enough whilst tillering.
I do not have a moisture meter so cannot comment on the MC other that to say that the short stave was left outside, protected from the weather but well aired for 22 months prior to being split into billets and worked.
JD
I didn't take it iff of the long string until the tips were moving about 9", then braced it to 2" and later to 4". I think that I probably just went at the tillering a bit on the quick side. Where's that bucket of water when you need it...??
I may shorten it a little and see what I can get; I only draw 26" so could get away with it.
I have started on another bow and will post pictures for comments as I go along.
Many thanks for the great replies.
Red Dwarf
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Red Dwarf: Just get back on that horse and ride. I'm with JD, that bow is not a failure just lighter than you wanted. Now you get the chance to infect someone else with the traditional virus and thats a good thing.
Jawge: Early on in my studies of this craft I recognized the wisdom of exercising the wood. I think that is and should be taught as foundational to proper tillering. Not only that, if you are using a tillering tree with a pulley it makes for excellent resistance exercise. Thats what I call good mulit-task time management. Lol Danny