Core Lamination???

[Wooden Spring]

OK, stupid newbie question... What is the purpose of a core lamination in a tri-lam bow?
If I've got plenty enough material for a hickory backed Ipe longbow, would a core lamination actually do anything?
(Incidentally, I only want about a 100# to 120# bow)

Furthermore, will this design work to produce a 100# bow?
Note that the bow arches from nock to center, there are no straight line cuts in the bow. I used to be an architect, so I shaped this like I would have drawn a pre-stressed beam. (holds more load) But I'm not sure how that would transfer over to bowmaking.
HOPEFULLY I can get the picture to post...

[adb]

Your dimensions look perfect for a reasonably heavy bow. You 'only want' 100 - 120#? That's ambitious. Especially if this is your first heavy bow. Making a 100+# bow is not easy. Well... a good 100+# bow, anyway, with good cast and little set.
Yes, a core material is structural. With a tri-lam construction, you end up with 3 layers, and 2 glue planes. If done properly, the glue joints are stronger than the surrounding wood. Each glue joint creates 2 opposing forces at the joint of tension and compression. The core material is important. I use only strong hardwoods, like purpleheart, yew, osage, cherry or bloodwood. Some people think the core material doesn't matter, but that is not true. If you're making a FG bow it is true... the core does nothing more than hold the FG, and the FG does all the work.
Also, the tri-lam is a good looking bow IMHO, if the 3 woods used have nice contrast. I make a lot of tri-lams, especially for heavier bows. I use parallel lams, usually 1/4" thick for back and core, and then add belly wood depth to accomplish weight (usually with overall thickness of 1 - 1.25").

[}|{opukc]

 This is just to reduce the overall physical weight. What I gave in size to the other theme will work well for that weight.
I have 3 lam warbow that is of similar size and pulling 136 # @ 32 " for 74"

http://www.primitivearcher.com/smf/index.php/topic,33227.msg438561.html#msg438561

With hickory/Ipe maybe you should start a little thicker and wider - 1 3/8" x 1 1/4"

[Wooden Spring]

Thanks for your help adb... You're right about this being an ambitious project - I didn't know how ambitious of a project this would be until I got onto my third try - I'm not new to archery or woodworking, but I am new to putting those two hobbies together.

So far my first bow attempt suffered from a dramatic suicide... I had an Osage Orange belly that I didn't score deep enough to prepare for the BowGrip100 - I had tillered the thing beautifully, then when I finally got it to pull down to 32", an 8" piece of Osage snapped off and embedded itself into a nearby 2x4. The next bow that I put together was apparently with the wrong materials (1/8" hickory back, 1/4" Ipe core, 3/4" hickory belly), it tillered nice, but my 210 pound body and gravity's help couldn't pull it down past 30" with the extra long tillering string, so it sits in the corner of my shop now collecting dust... My third attempt got the materials right, but when I put it into the hot box to cure the BowGrip100, my hickory backing dried out and curled up away from my core wood leaving a gap....

SO... I've learned that I need to use a glue that I'm familiar with and have used for decades - Titebond 3. AND I need to use my materials a little different - in this case, a 1/4" thick hickory back, and an ipe belly. Maybe #4 will produce a shootable bow! So far this little adventure of mine has been nothing but a comedy of errors worthy of a Marx Brothers movie!

[}|{opukc]

With hickory/Ipe maybe you should start a little thicker and wider then to reach final dimension - 1 3/8" x 1 1/4"

[adb]

You're right... go with titebond 3. No hot box required. Wrap your glue up layers with bicycle inner tubes, leave it overnight to cure (at room temp), and you're good to go. DO NOT score the lams with TB3. The surfaces need to be smooth, clean of dust and oil, and perfectly matching. With TB3, the bonding occurs at the molecular level. I use a thickness sander with 80 or 100 grit to finish my lams for glue-up.
Use a former to wrap your lam layers, otherwise your thin backing layer may end up with waves. I sandwich the thin backing and core layers between a former and the belly wood for a nice even pressure. Apply a thin layer of glue to BOTH surfaces of all layers for a strong glue joint. The excess will be squeezed out with wrapping. Better a wee bit too much, than to starve the joint of glue. You can add a bit of reflex if you want after wrapping your glue-up, by blocking up the tips and clamping to a bench at the middle.

[mikekeswick]

Aim for a thickness of 1 1/8th and width of 1 1/4 at the handle.
A core can be used to slightly increase performance. Maybe to start with you would be best using a straight hickory backed ipe blank.
Forget the curves as it will make it harder to keep the stave exactly equal either side of your centerline. A little more material on one side WILL cause the bow to start drifting off square and trust me you don't want a heavy bow that twists in your hand!!
With bows like this it's pretty much vital to get the tiller perfect very early on in tillering or else you will find chasing good tiller and maintaining weight is tricky...
Good luck.

[Wooden Spring]

Well, the hard part about being an ex-architect like me is that I can't build anything unless I've got dimensions. And since I've been running autocad for over 18 years, everything gets drawn up first, then I print off those plans for the bow full size, I transfer that over to plywood and cut out a pattern, then transfer the pattern over to my laminated stave. When I'm done cutting everything out, the shaped stave is accurate to within the thickness of a printed line from an inkjet printer. 

But here's the problem, and it's a big one... The one thing that I've looked for desperately, but could never find, is some material that gives me a step-by-step build along for a backed bow like this. And one that assumes nothing - I've got no experience with making bows, so I need to know not only what is being done, but why it is being done. The "why's" help me generate the math that makes this make sense.

There's a LOT of people who can make one of these beautiful pieces of art by feel, and they know what looks good and what will work - yeah, I'm not one of those guys. My world is ruled by math, geometry, physics, and drafting... It may be my handicap, but that's my world.

Anyway, thanks for all the help! I hope to have my ipe in from the lumberyard either today or tomorrow, so hopefully I can start gluing up this weekend!

[adb]

I understand your analytic mind. However, you're working with natural materials, which don't accept exacting dimensions without exception. Sure, if you're making FG bows, it's a recipe, plain and simple. But, this is not.
You've been given some excellent advice regarding dimensions for a heavy bow... 1.25 - 1.5" wide at the handle, 74 - 78" long, and 1 - 1.25" deep. Taper both profiles continuously to 0.5" tips, and away you go. Even the same species of wood will vary in density, so this is a basic starting point. Let the wood guide you. Use the mass theory, it works well.
Making bows from wood is part science, but also a part feel and art... especially the tillering.

[fishfinder401]

hlstanley i completely understand where you are coming from, i am similar in many ways to that, just had not gotten set in those ways before starting to make bows luckily, with bows you can have a general idea, but never exact, that takes some experimentation..... maybe that's not the best word for that, you need to get to know the wood and see how it reacts by working with bows and tillering them, eventually you will be one of tise people doing it by feel, it just takes time for most people.
-now a question for you abd, i keep hearing about the mass theory, what is that?i feel like its something i should know

[Wooden Spring]

Thanks for everyone being so helpful. I'm taking everyone's advice, so hopefully something decent will arise from the pile of sawdust that is my shop... If nothing else, I'm REALLY gaining some experience using my cabinet makers rasps!

[adb]

hlstanley i completely understand where you are coming from, i am similar in many ways to that, just had not gotten set in those ways before starting to make bows luckily, with bows you can have a general idea, but never exact, that takes some experimentation..... maybe that's not the best word for that, you need to get to know the wood and see how it reacts by working with bows and tillering them, eventually you will be one of tise people doing it by feel, it just takes time for most people.
-now a question for you abd, i keep hearing about the mass theory, what is that?i feel like its something i should know

The mass theory was developed by Steve Gardner (Badger) and Tim Baker. Do a search under archive library, using the title: 'little history behind the mass theory.' Badger's thesis explains it better than I can. He also covers the no set tillering method, which I also use. Yes, very valuable info.

[fishfinder401]

ok, thanks, sounds like something i should learn

[adb]

I think basically what the mass theory says is:  for less dense woods, like perhaps maple, the limbs need to be wider and flatter. For more dense woods, like ipe or osage, the limbs need to be thinner and less 'massive.' So an ipe bow of the same draw weight will have thinner narrower limbs than a maple bow of the same draw weight, and they will have the same mass. I think it's a guideline to match bow design with the wood of choice, to achieve a good performing bow with minimal set.

Steve (Badger)... if I'm way out to lunch here, please correct me, but this is my understanding of the mass theory.

[mikekeswick]

I hear what you are saying about printing/plywood etc...but trust me i've made over 100 elb's and have been right around thew houses with them! Althought your 1st bow is likely to be overbuilt (a good idea) and not therefore as(almost!) unstable as a high performance narrow elb. It is not a good idea to use more steps laying out the width profile when one will do the job. It's so important to get your layout totally straight with laminated elb's. I finally got a laser cutting company to make me up a stainless tremplate that is accurate to +/- 0.05mm. If you make a few of these bows and start using the mass principle to push the performance of them you will see what I mean