Gluing in Multilam Reflex

[mmattockx]

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.

I did some more looking at this and I want to be more accurate with what I said. While it is correct to say that gluing in deflex, then reflexing that assembly into a new glue up induces the same strains as reflexing a solid piece of wood by the same amount, that is not completely accurate because the final strains in the glued up belly lams are not the same as the strains in the solid belly piece.

What happens in the deflex glue up is the belly surface sees a small amount of compression due to bending the deflex into the thinner lams. This compression strain counteracts the tension strain induced by bending in reflex in the second glue up. The net result is that the belly ends up with the tension from the reflex glue up less the compression from the deflex glue up. Because the reflex glue up is done with a thicker belly lam than the deflex glue up you can still get significant tension into the belly surface but not as much as if you reflex a single piece belly.

Offsetting this effect is that you can pull more total reflex into the second glue up when starting with the deflexed belly lam assembly, for any given final side profile.

I guess we stole Tommy's thread. Sorry Tommy.

It seems we did. If OP wants, we can take this to a new thread and leave his thread to his questions specifically.


Mark

[Tommy D]

Keep going ... I am learning all sorts of interesting things ... it why I started the thread...

[Tommy D]

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.

So I have gone a new direction slightly ... I decided to recurve the tips first. Then pull in deflex with a core lam. Then pull reflex in with the bamboo back. I cut a kerf into my belly lam on the tips and glued in some recurved tips. I put one “wedge” in the left. I did this on a caul I have made.. I cannot get these as sharp as I would like. I have used yew and Osage before - Ipe is different. To me it feels a little like if you were comparing steels - you have steel that has springiness but will still bend once you go beyond it’s “springiness” before breaking - and then you have steels that are springy but when they reach their limit they just break. Ipe is like the later. That’s the closest comparison I can give.

Now I have pulled in some deflex and glued in my tapered core lam. It’s also “Ipe” but I think it’s a poorer quality Ipe. I know that Ipe is actually a fairly generic term. The piece I am using for cores has  made a few bows but I would rather use it up as cores than the nicer dark stuff Ipe I have.

So this it’s what I have now...



When it’s cured I will adjust the caul and pull in some reflex with the bamboo...

[willie]

Very ambitious design you have going Tommy. Will be watching with interest.

Keep going ... I am learning all sorts of interesting things ... it why I started the thread...

Perry reflex threads always seem to generate discussion, maybe because it is a concept that all of us are trying to understand better. Sometimes I think it can be simply described as a way to make the back work harder, similar to trapping.

[DC]

I like it because that initial reflex gain hides some of the set that I know is going to come later

[Tommy D]

I like it because that initial reflex gain hides some of the set that I know is going to come later

You mean like if a tree falls in a forest and there is no one there to hear it did it really make a noise?! 😂😂

[mmattockx]

Sometimes I think it can be simply described as a way to make the back work harder, similar to trapping.

Plus getting some work out of the material near the neutral axis that normally contributes very little to the performance of the bow. Perry reflex mostly transfers some of the initial belly strain into the core material that normally doesn't work very hard, making it almost like getting something for nothing.

I like it because that initial reflex gain hides some of the set that I know is going to come later

 

I wish I didn't understand what you mean by that.


Mark

[willie]

Plus getting some work out of the material near the neutral axis that normally contributes very little to the performance of the bow. Perry reflex mostly transfers some of the initial belly strain into the core material that normally doesn't work very hard, making it almost like getting something for nothing.

Mark, that's the part that went over my head. I remember perhaps an earlier thread or somewhere in TBB, a similar explanation akin to "the energy is stored in/at? the glueline."

Is there any way to quantify or even make just a ball park estimate of how much energy can be redistrubuted to midsections that would otherwise have normally expected strains?

[mmattockx]

Mark, that's the part that went over my head. I remember perhaps an earlier thread or somewhere in TBB, a similar explanation akin to "the energy is stored in/at? the glueline."

Is there any way to quantify or even make just a ball park estimate of how much energy can be redistrubuted to midsections that would otherwise have normally expected strains?

Tim Baker describes it that way in TBB V3.

It should be possible to calculate the strains for each step of the construction process at each surface of the belly and back lams to see what is going on. I have no feel for how much energy can be stored there, I suspect it will depend on the shear strength of the glue and possibly the wood itself. The other limit will be the back in tension. It doesn't get the same reduction in stresses that the belly does (unless the back is the same thickness as the belly lam and that is not how it is normally done) and will eventually end up being the failure point as reflex is increased, similar to what happens if you trap a limb too much.

I would think that 10-20% of the total energy is not a crazy estimate for what the inner wood can contribute.

I need to do some experimenting with scrap wood, gluing up lams and seeing what happens with the reflexing and how it all reacts to being pulled and pushed then glued into position before I glue up a bow for real. There's no point in wasting good wood yet until I have a better handle on how it all works.


Mark

[Tommy D]

Tim Baker describes it that way in TBB V3.

It doesn't get the same reduction in stresses that the belly does (unless the back is the same thickness as the belly lam and that is not how it is normally done) and will eventually end up being the failure point as reflex is increased, similar to what happens if you trap a limb too .

Remind me why we wouldn’t use a back  the same thickness as the belly lam in this sort of glue up?

[mmattockx]

Remind me why we wouldn’t use a back  the same thickness as the belly lam in this sort of glue up?

It is because most woods are several times stronger in tension than compression. Similar to trapping, Perry reflex adds stress to the back in order to reduce the stresses on the belly (along with getting some work out of the core wood).

For same wood bows (all lams the same material) Dan Perry recommends the belly lam be 2/3 of the stack and the back 1/3. For combinations with a much stronger back than belly he recommends the belly lam be 3/4 or more of the stack depending on the wood combination.


Mark

[Tommy D]

Right ... enough theory ... where do I start with this thing!!

Have I created a monster!!!!

Not quite sure where to begin! The bow is 70 inches around the curve so I could take some of the tips off and shorten it if I struggle to get a string on ... it will definitely be a recurve if I leave them as is but my tips are fairly narrow - though they are lined up!!

Was thinking of starting to tiller from the outer limbs first on a long string and see if I can brace her but I think the strings will pop off...

[DC]

String alignment will be key on that one Tommy.
 I've had lots of problems getting the weight on my RD's. Go slow, meaning 4-5 scrapes tops and then exercise to full draw weight and look it over. Out around 18-20" mine have a tendency to relax or something. Not sure how to describe it but you will all of a sudden get the the bow going from 20" out to 24"(approx) without any scraping. In my case I think it's set but just be aware that it may happen.
 Use your eyes a lot. Look for areas in the belly that are getting thin. Compare side to side looking for the same thing.
Make a tracing of it on something and check for set regularly. It's easy to do when you're on the long string but a PITA once it's braced because you have to unbrace it every time but do it. A quick check for set is to measure the reflex. If the reflex is dropping you're getting set.
 For the first little while when tillering you may gain some reflex. Keep track of that so you can watch for set.
Did I say go slow By that I don't mean take lots of coffee breaks. Look it over very critically. Watch for those thin spots. These things will hinge in an instant.
Good luck

PS it looks very cool

[mmattockx]

Right ... enough theory ... where do I start with this thing!!

Have I created a monster!!!!

That thing almost looks like a Duo-Flex profile if the recurves work a decent amount. Good luck with the tillering.

Out around 18-20" mine have a tendency to relax or something. Not sure how to describe it but you will all of a sudden get the the bow going from 20" out to 24"(approx) without any scraping. In my case I think it's set but just be aware that it may happen.

Where does the set tend to occur?


Mark

[DC]

Just inside mid limb for me. It has crossed my mind that when I have the bow in the vice for scraping that the mid limb is the highest part because of the reflex. I've wondered if I'm scraping harder mid limb because of that. I try to keep that in mind but my mind ain't what it used to be.