Gluing in Multilam Reflex

[mmattockx]

I have found that using a caul like that(with a board and a post) seems to concentrate the bend around the post(s).

It certainly does that.

In order to get a nice even curve you have to cut a curve in a 2x6 or 8 and then clamp it to that.

You can also use multiple posts to approximate a smooth curve and get quite close while maintaining a very flexible, adjustable caul set up.


Mark

[Tommy D]

Ok back on this project... been reading on another forum (though the link came from here) on Dan Perry’s differentiation of “Perry Reflex” vs glued in Reflex.

http://leatherwall.bowsite.com/tf/lw/thread2.cfm?forum=23&threadid=188683&category=

It seems that the thicker the piece of belly wood you are pulling into reflex the more effect it will have - so long as it all holds together. It’s all about the difference in tension and compression that occurs the further apart these forces are based on the wood thickness... or as best I understood it.

He describes a neat way to get ones head around this with foam pieces...

I am slowly getting my head around this one over gluing up lots of multilams... And how it differs ...

Now ... if I glue two lams into deflex until cured and then pull this one Laminated piece  into reflex as per the “Perry Reflex” would this be considered the same principle as a two lam Perry reflex only with 3 lams or would it be  better, or would this simply be like a multilam glue up...

[willie]

It seems that the thicker the piece of belly wood you are pulling into reflex the more effect it will have.....if I glue two lams into deflex until cured and then pull this one Laminated piece  into reflex as per the “Perry Reflex” would this be considered the same principle as a two lam Perry reflex only with 3 lams

this is how I understand it to work from reading Dan's posts, and the recommendations of avcase.

[DC]

If I can stick a question in here. Does glued up reflex allow you to get more draw weight from less wood or does it actually improve the springyness of the bow? Or is there such a thing as springyness?

[PatM]

Dan's theory seems to be the opposite of what everyone else thinks.  Not sure his foam evidence holds up with wood.

 Of course springiness exists.   Compare a bunch of materials.

[mmattockx]

If I can stick a question in here. Does glued up reflex allow you to get more draw weight from less wood or does it actually improve the springyness of the bow? Or is there such a thing as springyness?

This is quite the topic, with a lot of different views/theories/understandings of the concept. I don't know if you can get more draw weight but since it supposedly lowers stresses on the belly wood that seems to indicate it is possible. The springyness is usually more a material property than a geometry one, but I'm open to someone convincing me different in this case.

Dan's theory seems to be the opposite of what everyone else thinks.  Not sure his foam evidence holds up with wood.

What is his foam evidence? Can you point me to an explanation of that? I don't recall foam being mentioned in the TBB article.


Mark

[Tommy D]

What is his foam evidence? Can you point me to an explanation of that? I don't recall foam being mentioned in the TBB article.


Mark

Mark if you read through this post he explains his foam theory http://leatherwall.bowsite.com/tf/lw/thread2.cfm?forum=23&threadid=188683&category=

What he is saying - as I understand it - is that when you bend a thicker piece of foam the outside stretches more and the inside compresses more than a thinner piece - think I Beams here. By gluing this in, you are harnessing this potential energy. This is compared to say a multilam glue up where each thin strip - because it bends much easier - cannot harness or hold that energy. You are able to bend them into whatever shape you like, but they are not being stressed as much in doing so and are therefore not storing so much energy.

One advantage he states - which I can understand in I Beam lingo - is you can get a narrower stronger and therefore lighter limb ...

I haven’t fully wrapped my head around this ... but that’s my take so far...

[mmattockx]

Mark if you read through this post he explains his foam theory http://leatherwall.bowsite.com/tf/lw/thread2.cfm?forum=23&threadid=188683&category=

Thanks for that, Tommy. I will go have a read.


EDIT - I have had a read through it. What Dan is talking about with his 'surface travel' concept is really strain as it is defined in mechanics of materials theory in engineering. His explanation is consistent with how Tim Baker describes Perry reflex in TBB V.3.

PatM, Dan's foam explanation holds up for all solid materials, including wood. It is basic mechanics of materials stuff combined with a bit of beam theory. Foam is used as the material because it will tolerate large enough strains for them to be seen by the naked eye, but all solid materials adhere to the same rules.

One thing I am wondering about is Dan describes using very thick pieces of wood for the belly lam, upwards of 3/4" thick for his bows. He also mentions that he likes to be close to finished dimensions when he does the glue up. I can't imagine having a finished bow be over 3/4" thick anywhere on the limb. What wood species and type of bow would that apply to? My last pyramid bow is made from hard maple, draws 40lb@28", the limbs are 2" wide at the fades and the finished thickness is around 0.385" at the fades. Doubling that thickness would make it 8 times stiffer for a draw weight of 320lb@28" (not that the maple would survive that). That seems crazy to me. Even if you halve the width to 1" that leaves you with a 160lb bow.

One thing that Dan seems to not understand is the idea of gluing two thinner lams into a deflex shape, then pulling that assembly into reflex during a second glue up with the backing lam. He says that wouldn't be the same as pulling a single piece belly lam into reflex as he does, when it actually would accomplish exactly the same thing. He seems to think that having a glue line in the first assembly somehow renders the response of the glued up pieces to be different than bending a single piece of wood. As long as the glue is as strong as the wood is (which is typically the case for the glues we use) then there is no difference in the end.


Mark

[DC]

I got the impression when I read it was that the 3/4" was just for the glue up and then he tillered it down to whatever thickness. I've noticed that when I used a thicker belly I get more reflex gain as I tiller. I'm not sure if that's what he was after or not. Something you may answer is about the reflex gain. Basically I think that all that is happening is that when you glue them together, reflex them, and then release them the forces balance out. As you tiller, the belly gets thinner and weaker and the back starts to win the tug of war. Presto, reflex gain. What I'm wondering is would the results be the same to if you used a thinner belly and just reflexed it more at glue up. Hope you can follow that.

[mmattockx]

I got the impression when I read it was that the 3/4" was just for the glue up and then he tillered it down to whatever thickness.

Hmm. If that is what he is doing then he is gaining nothing over just gluing up with the finished thickness in the first place. Strain is proportional to the distance from the neutral axis, so if you bend a 3/4" thick piece and then tiller it down to 1/2" thickness that surface will see the same strain as if you just bent a 1/2" thick piece to the same radius to start with.

The only possible gain with Dan's method is the neutral axis shift as the belly lam is removed during tillering. I can't say if that affects anything or not without looking at it in more detail, but it is possible it could affect the final result. My gut feel is that it makes no difference.

Basically I think that all that is happening is that when you glue them together, reflex them, and then release them the forces balance out. As you tiller, the belly gets thinner and weaker and the back starts to win the tug of war. Presto, reflex gain.

That is correct.

What I'm wondering is would the results be the same to if you used a thinner belly and just reflexed it more at glue up. Hope you can follow that.

Should be the same in the end, based on the engineering principles I mentioned. The strain a material sees is only dependent on the deflection it sees, not how that deflection is achieved.

Since we are talking about gluing in reflex, how much spring back do you typically see with your boo/yew bows (or whatever you are using these days)?


Mark

[DC]

If you are talking about springback when I take it off the caul after the epoxy has cured it depends on the thickness of the belly again.

[mmattockx]

If you are talking about springback when I take it off the caul after the epoxy has cured it depends on the thickness of the belly again.

What kind of range do you see? I'm just trying to get a feel for how much to plan for in future cauls.


Mark

[DC]

About an inch

[willie]

one thing that might come into play when making comparisons, is the stiffness of the backing relative to the stiffness of the core.

It is not specifically stated in this thread, but I think Tommy D has been using boo over ipe, while Don is relating to his experience with boo over yew.

I could well imagine that a bamboo backing, glued in perry reflex, would cause a boo/yew bow to go into more reflex as it is tillered, than a comparable boo/ipe bow.

Since Don's basic question is how he might utilize perry reflex better in his application. Might there be ways to evaluate relative stiffness's when considering how much reflex to glue in? (or in the case of a pre-glued deflexed core, how much deflex to glue in to the core? 

[DC]

I guess we stole Tommy's thread. Sorry Tommy. Thinking about this I was wondering about Ipe. Since it's so hard to bend would it even lend itself to glued in reflex. Especially if you tried gluing deflex in two belly lams and then tried to reflex that piece. Would it even go. I should try some Ipe so I can at least have some knowledge about it.