Two lam board bow

[8Charles]

I want to build a two laminate boardbow.  I have built 1 Hickory and 1 Oak boardbow.  I think the Hickory is the better of the two. I want to try and put a little reflex in this bow.  I have glued on the handle in the past and tapered the fade into the limb material so the starting limb was 9/16" thick.  I'm thinking of Hickory for the back and Maple for the belly (good, bad?).  My question is, out of the total 9/16' starting limb thickness, how thick should the back (Hickory) be?

Thanks

Emmett

[canopyboy]

I'm working on a maple belly, hickory backed board bow right now.  I made them equal thickness roughly.  Just starting to tiller it now, I made it a bit thick to start with.  I think my hickory is about 5/16.

Not that I know what I'm doing, but so far it seems to be going well.

 

 

[vanillabear?]

[8Charles]

Canopyboy,

Keep me posted how it works out for you.  What did you use for glue?

Emmett

[4est trekker]

I second vanillabear...Cut the hickory at 1/8" thick and then work down from there if needed.

[canopyboy]

Originally posted by 8Charles:
Canopyboy,

Keep me posted how it works out for you.  What did you use for glue?

Emmett

I used TB3.  I'm new at this and just experimenting to see what would happen.  I'll keep you posted.

I've spent my career trying to pretend to be a structural engineer.  This concept of over overpowering the belly I think can be taken a bit far.  If there is a mismatch in the modulus of elasticity or flexural modulus of the two materials, then you start to shift the neutral axis accordingly.  And you can tend to overpower the weaker side (think bending wood with a steel strap, or glass and balsa).  But the numbers I found for hard maple and hickory show hickory to be only about 10% higher.  So I think the load sharing should be reasonable between the two.  

I was actually aiming (for no good reason) to have the glue line at the neutral axis with the hickory in complete tension and the maple in complete compression.  But we'll see.  It's a fairly wide pyramid style, so I don't think anything will be too close to the edge of the envelope here.

[Scott Beitzel]

conopyboy did you make your own form or did you get the measurements from someone else.
am thinking of another bow build this winter when i get layed off work and kinda like the looks of that style . i built a tri-lam and tried to put some reflex deflex in it but didn't come out as well as i would have liked ( it's almost flat)
so am looking for more info on this shape or form till then .

thanks,
Scott

[canopyboy]

Scott -  

I made my own with dimensions that seemed about right to me.  I splined the curves to what seemed about right, a little more reflex than deflex.  Allowed for a little spring back.

 

I like the shape that came out, but I'll have to finish tillering it and see just how it shoots before I'd recommend it or not.

I really don't know what I'm doing here, I'm just an engineer who was raised in a cabinet shop.  My theory is that I'll just try something and then analyze and learn from the results.

[Mike Most]

Just for more info here, I just finished a 48" kids bow, using entirely red oak, Belly was plain sawn, with the grain running horizontal on the belly and the back approx .120" the grain ran vertical. Makes for good compression and tension according to the bowyers bibles.

Good Luck

Mike