Primitive Archer
Main Discussion Area => Bows => Topic started by: upstatenybowyer on July 23, 2018, 07:13:08 am
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I changed the title.
So here's the problem:
I keep braking these highly reflexed wood-backed bows. >:( I get them braced with a pretty even bend, then begin removing wood equally on both limbs to bring them to the target draw length. Somewhere between brace and full draw they just explode. :o
What am I doing wrong? Grain runoff? The breaks appear to be happening mid-limb on the back where most of the reflex is. Am I trying to glue in too much reflex and therefore over stressing the wood?
The first 5 pics are of a maple-backed BL. The last 3 are from a maple-backed lilac.
I need to figure out why this is happening!
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From what I heard, very. How bad is it?
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Run off is inevitable as the width profile tapers. Its the sharp run offs that will peel up or blow. Take a picture.
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last ones
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Notice the breaks are nearly in the same areas? That's clue #1 your tiller isn't matching your design.
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Could you elaborate a bit Pearly? Do you mean that the wood isn't working enough or too much in the wrong places?
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Its working too much where it broke. If you had grain or run off issues the areas that break would be all over, nit in the same place every time.
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That makes sense. The problem is the thickness is even through out the limbs w/ the width tapering. Do you think the limbs are too narrow or not thick enough?
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Hard to say without seeing it pre-break. The 3rd to last pic. Your reflex start just off the fade and stays straight after that. That whole limb needs to bend, if you tillered that to predetermined shape you had in mind and not to what that limb needs, it will break, like it did, every time.
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What type of glue?
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Smooth on bushy.
I think I understand what your saying pearl. Thanks a bunch for your thoughts.
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I could explain it easily in two sentences if I was looking at you and the bow.
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They look like tension breaks to me. Where are you getting the maple backing from? are you cutting it yourself? Was it cared for properly from the stump?
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They look like tension breaks to me. Where are you getting the maple backing from? are you cutting it yourself? Was it cared for properly from the stump?
I know what you mean Pat. I've been questioning the quality of the backing. It's quarter-sawn maple I got from a local lumber store. I cut them from a board I bought there. I'm sure it's kiln-dried. No way to know how it was cared for I guess.
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My best guess is a combo of what was stated and to much pressure at glue causing a staved joint.oh my I have broken so many backed bows!
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I don't believe the breaks would be so consistently placed if it were a backing quality issue. You'll note the breaks all happened where all the reflex was introduced. Too early and too abrupt in my non-professional opinion. If you start the reflex that early then it needs to carry out to the tips evenly and bend accordingly.
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One of the things I can't figure out is that the limbs appeared to be bending pretty evenly prior to the bows breaking. It felt like the backing just couldn't handle the stress. That's why I suspected grain runoff.
It's hard to tell from the pics cause they're after the breaks, but I put a lot of thought into reflexing evenly across the limbs, but maybe not enough. I think you're probably right pearly. Your explanation makes the most sense.
This is driving me crazy!
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My best guess is a combo of what was stated and to much pressure at glue causing a staved joint.oh my I have broken so many backed bows!
I did put a lot of pressure on them during the glue-ups. What's a staved joint?
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Sorry I meant starved joint,meaning squeezing out all the glue.
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Got it bush. Thanks :)
I'm adding the reflex w/ dry heat on a caul designed to make the reflex even across the whole limb. Then putting the spliced bow's tips on blocks and pushing the center down and clamping to glue in more reflex. Should there be support under the entire limbs when I glue in the reflex?
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A starved joint is where you squeeze the glue out of the joint with too much pressure.
Have you built straight limbed backed bows successfully? Can you show up a pic of a bow that you have glued up and before it breaks.
By adding reflex you add more stress to the limb. If that stress is concentrated in one area it can cause a break.
Are you pre-tillering the belly before glue up? are you allowing the heated areas to cool completely? Are you heat treating the backing?
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Here's a pic of what I use to glue in the reflex...
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That's similar to what I use. I doubt that's the problem.
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Are they straight sided pyramids? I just looked a a few of mine and it looks like I left them a little wider mid limb. Still tapering all the way. It's strange because I never put a lot of thought into it. The wider mid limb was to minimise twist. It's hard to tell after the fact but they really don't look like too much reflex. My money is with bad backing but I won't argue with Pearly's ideas. :)
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It's highly possible that it's one of the things you mentioned Pat. I've build a backed bow successfully with not nearly as much glued in reflex. I have been allowing it to cool. I haven't been pre-tillering the belly cause I'm afraid it will break without the backing. Maybe I should splice and pre-tiller before heating in the reflex?
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Here's mine. It looks like your's has more bend on the inner limb. Maybe where they're breaking.
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Are they straight sided pyramids? I just looked a a few of mine and it looks like I left them a little wider mid limb. Still tapering all the way. It's strange because I never put a lot of thought into it. The wider mid limb was to minimise twist. It's hard to tell after the fact but they really don't look like too much reflex. My money is with bad backing but I won't argue with Pearly's ideas. :)
Yes DC, and they do taper pretty narrow. I start at 1.5" down to 3/8. Maybe a more gradual taper till close to the tips, then more abrupt right at the end.
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Agreed,maybe Marc will chime in as he is the resident expert on this style of bow!
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This hand sketch better explains my thoughts on your issues. Just my thoughts is all.
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If you hold the broken bows up to the caul maybe the break will point to the problem.
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That scetch by Pearl is a good explanation of the failure, too much tension stress concentrated on a short area.
What I'm always thinking of looking on those construction is: where is the tension zone, neutral plane and compression zone. I've seen diffrent proportion of belly wood and backing, sometimes the backings are thin react probably like a rawhide backing (most of the tension is in the main wood). But sometimes, like yours the backing strips are thick and tension is defintely there. For me (don't have access to hickory) boo is the only option, as there are strong running through fibers on the high stressed area.
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I agree with Simon and DC, try bamboo on your next one and leave the higly stressed midlimb area a little wider. My last few glue ups I made them a little wider mid limb also because I noticed the previous ones took a little set in that area. The transition from deflex to reflex is usually the most stressed area I guess
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Thanks guys. I'm pretty sure it isn't wide enough in the area(s) of highest stress and that like pearly said the curves need to be spread out as much as possible. I'm gonna try making my own backing strips out of hickory.
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I don't have much to add to the already great advice given except that I've had bad luck with maple backings from boards on these designs. I know Marc uses maple with good success in high stress designs, but I believe he is harvesting his own trees. Same exact design, maple backed bow exploded, bamboo back made a nice bow.
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If it were me...I'd be wondering if the time and effort I'm putting into the project is worth kiln dried board backings vs. biting a bullet on good bamboo backings. Just sayin...good luck.
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I feel you guys big time. It may not be the quality of the backing, but if there's even a chance, Parnell's right, with all the time and effort I'm putting into this, the kiln-dried stuff may have to go. :-\
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I've never had problems with kiln-dried Hickory backings. Before I glue them on I always take them and bend them in a complete circle or close to it. An eighth inch piece of wood 6 ft long should not break. considering both of them broke at the same spot I would rethink your glue up methods and or your tillering process. Bring it to Elm Hall next week and we'll take a look at it >:D
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I think PD hit on the main problem. I'm just wondering why you would back a wood like BL which is already strong in tension but medium in compression, upstate. Maybe the maple should be a belly wood. Don't know as I avoid wooden backed bows. Jawge
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+ 1 what Bushy said its hard to tell from the pics but it looks like a delam in some of the pics , how did you glue up ?
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Jawge, I used BL cause I had billets. I saw DC made a successful maple/BL bow so I thought I'd give it a shot.
Stick, I did the glue up by wrapping in innertubes, then bending into reflex w/ the rig pictured above. I used Smooth on. The lines looked good, and I don't think I starved the joint cause I can's see the innertubes doing that. :-\
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Just out of curiosity, how thick are your backings?
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Everyone above gave good advice on things to look at.
One thing I make sure of with any bow is that the back is in nearly finished condition. No dings, scratches and especially no square corners. I round mine off pretty good. If narrow the limb while tillering go back and radius the corners again......as many times as it takes.
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Hmm, hadn't thought of that Mark, thanks. :)
DC, 1/8"
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I agree if you use rubber band/tubes ,I can't see enough pressure to starve it providing enough epoxy was used ! The mystery continues ::) I have been using less and less smooth on at glue up the more bows I do but I use clamps or fire hose !
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It still looks like delam in the pics.how do you prepare your surfaces?
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That's a excellent question Bush smooth on doesn't like to adhere to smooth surfaces , I grind my lams to a minimum 50 grit preferably 36 grit and clean the surface with acid tone, I agree looks like a de lam in the pics if the glue line is right and surfaces right it shouldn't separate like that, looks at least in the pic like the backing separated clean .
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Thank you guys so much for your continued diligence in helping me figure this thing out. :)
For surface prep, I flatten everything out with my jointer and then sand w/ 50 grit glued onto a wood block.
Hold on, I'm going out to look at the broken bows...
Okay I just went and looked very closely. It definitely wasn't a de-lam. I can see how it looks that way in the pics, but upon close inspection I'm confident we can rule that out.
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Jeff I'm afraid I'm probably not a big help here. I've never tried a wood backed bow, but if I understood you correctly you heated prior to gluing up on caul? Did you heat the backing to? Any chance the heat is damaging your backing? Could maybe explain why breaking in same area to. Just a question I had.
Bjrogg
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Sorry to beat a dead horse on this one!seems Chris was right all along!
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Pearly may be right but its good to verify all the bones , Bow CSI...,lol , I personaly dont trust kiln dried lumber for backing , just because you really dont know how it was treated , I make my own air dried backing , core wood I have no problem with kiln dried but Im selective where its coming from , Jeff your getting a belt sander you have the jointer & band saw its pretty easy to make a make shift thickness sander out of one , Ben on here did a effective simple set up like that & the backing strips dry quickly if you dont have seasoned wood for it , at least eliminates the bone problems in the exspermentle world !
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Good question BJ. I kept the heat away from the backing 100%.
I think Chris is right as well. I'm also a bit suspicious about the backing. The only way to find out for sure is to redesign the reflex w/ Chris' suggestions in mind and back w/ a homemade hickory strip (as you suggest SB), which is precisely what I'm in the process of doing.
Can't thank ya'll enough for your input. Bow CSI at it's best. )P(