Primitive Archer
Main Discussion Area => Bows => Topic started by: MtDent on June 18, 2012, 12:15:54 pm
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Ive been tillering my first bow, a 68" ttt red oak board, and ive been getting a weird bend in the limbs. They bend smoothly/evenly except when you sight down the limbs. When you look down them on true tillering stick, both of them, one side bends more than the other, like so:
(http://www.deviantart.com/download/309113454/limb_torque_by_princeypoo0099-d541dgu.jpg)
My dad, being an engineer, taught me simple beam theory and told me to start taking wood off the lower side, the left in this picture. but I wanted to see what you guys think. I've been working on it for days and just cannot get either limb to stop bending like that. (And no I'm not just holding the camera crooked ;) ) also, it gets worse as I bend it farther and farther.
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From what I can see that is a very acceptable amount of bend or torque in the limbs. You can use a caliper and check the thickness of the limbs on both sides. I've seen worse in store bought fiber glass long bows. More pics may help also, maybe a belly shot from tip to tip with tiller stick.
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I'll go get some more pictures now
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Where is the torque?
I keep a piece of swirly grained maple that I bend and look at the torque and limb twist. It's easier to "feel" where to remove in order to fix the problem. So find a section that bends like my twisted bow and look at what is going on there and how I'd fix that spot then do that on the actual bow. It's made a big difference.
Some is ok.
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Well the picture doesn't really do it justice. Plus I don't have it bent very far on the stick.
(http://www.deviantart.com/download/309123422/bow_by_princeypoo0099-d541l5q.jpg)
(http://www.deviantart.com/download/309124935/bow2_by_princeypoo0099-d541mbr.jpg)
Yes the tiller isn't very good.. still working on it but I'm trying to get it to stop bend like that before I go any further. It isn't even at brace yet. Plus my stick isn't very straight and it throws me off sometimes...lol.
This is the swirl in what will probably be the top limb
(http://www.deviantart.com/download/309124992/bow3_by_princeypoo0099-d541mdc.jpg)
(http://www.deviantart.com/download/309125036/bow4_by_princeypoo0099-d541mek.jpg)
The string does track down the center, I just took the picture at an angle.
(http://www.deviantart.com/download/309125135/bow5_by_princeypoo0099-d541mhb.jpg)
(http://www.deviantart.com/download/309125893/bow6_by_princeypoo0099-d541n2d.jpg)
Sorry my pictures aren't that great. It's kinda hard to hold my not-very-straight stick, take a picture and take care of my little cousin with a bloody nose at the same time :P
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PS ignore the very messy "Workshop"... lol I have to use my breezeway because there's no room in my garage or shed for this kind of thing.
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PPS, it just blew up. So never mind :P
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Lemme guess - she blew by that "swirl" on the top limb? That's some serious runout, and that board was probably destined to blow regardless of how you tillered it.
But for future reference, your dad is right. The limb will always twist toward the weaker side (which in your photo would be the high (or right) side, therefore you'd remove wood from the low (left) side to get it to come back around.
But that looked like a pretty minor twist.
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Lol that's exactly where it blew. I knew it would go. I wanted practice on the bad wood though instead of going straight to the expensive stuff. It had bad grain all around so it was destined to crack. Maybe I'll turn the surviving limb into an atlatl! :P
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The first picture I saw the runoff and knew it was a goner. As I scrolled down , the prediction was fulfilled.
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You guys beat me to it. I was gonna say that swirl looked real bad!
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Looking at the pic the twist started at the severe grain run off. Sorry it blew on ya but that one was destined to do so. I don't think your tiller had anything to do with the limb twist. You will find that almost every stave or board bow will have some twist, some more some less.
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Lol that's exactly where it blew. I knew it would go. I wanted practice on the bad wood though instead of going straight to the expensive stuff. It had bad grain all around so it was destined to crack. Maybe I'll turn the surviving limb into an atlatl! :P
that is poor reasoning, never buy wood that you know will fail, give yourself the best chance to succeed, allways, Bub
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Mee too. I saw that run out and thought.... don't even waste your time!
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If you can't find better boards next time back it with thick rawhide. As already said that board had zero chance un-backed. Also when you are still using the long string the outer limbs should look stiff or else when you brace it they will be weak.
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I have some old jeans...but no hide or any kind of backing that's ready to be used. Would this old denim work?
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No on the denim, I mean, you could use it- it would be done same as with silk/linen- but cotton is no good for bow backing, too much stretch to offer much protection. Go to a thrift store and get some cheap old silk ties, or even a silk dress. Silk is my go-to for when I am unsure of the bow's back (iffy grain especially). If you are not sure something is silk, test burn a strand with a lighter- if it stinks a bit like burnt hair, its silk. If it curls up and smells plasticky?- its likely a poly blend.
I prefer to get silk from things other than ties, since alot of ties are bias cut, which is the direction in silk fabric that "gives" the most. It is more preferable to get your length cut with the direction of least stretch (you can determine this by pulling the fabric in different directions).
Mike
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Sorry about your blow up,I don't know much about board bows but that board looked bad to me from the start,I would have been very surprised if it stayed together even with backing. :) Run off is like violating the back of a stave bow,maybe worse. :) Better luck on your next one. Practice is a good thing,but I would never practice on a piece of wood I was sure was going to fail. :) :)
Pappy
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Thanks Mike. We have a lot of old clothes that have to be gotten rid of at some point so maybe I can rummage through them and steal something out of them.
Pappy, I had bought two boards and only after the fact had I realized that they had bad grain (seeing as I didn't really know what I was looking for). So I decided to go ahead with it anyway. I have an Osage board now (that probably has just as bad grain) so I'm going to take some precautions with this one.