Pau Amarillo/Yellow heart for "Backing a bow"

[AndyTurner]

Hi, I'm fed up with trying to back bows with Bamboo. I want something more reliable that I can machine accurately. 
I believe Hickory and Lemon wood work well but the shops are shut. I have one stave of IPE and a huge chunk of "Yellow heart" that I've never touched. I hear it makes good cores but I was wondering if it would make a good backing too.

My plan is
IPE Core - 58". Tip thickness = 3mm. Centre thickness = 6mm.
Yellow heart - 58". Tip thickness = 3mm. Centre thickness = 6mm.

Any experience / opinions welcome.

Thanks,
Andy

[PatM]

If bamboo is not reliable and you can't "machine" it chances are it's not a material issue.

[AndyTurner]

Hi Pat,

On the one that fractured yesterday,

Bamboo - I sanded the entire length down to 3.8mm +/-0.05mm. Measured midpoint between the nodes, flat surface to crown apex.

IPE core - This I sanded down to 4.1mm +/-0.05mm

The first fracture occurred right next to a node. I sanded the area and offending node flat. The limb then fractured again further up the limb towards the handle. No where near a node.

Its like the Bamboo was too thin. On previous bows I had it thicker, around 6mm, but they warped on me & I read you need it thinner and no thicker than the core.

I find boo lustrating because of it's very organic shape.

I also find that it starts to split when you get up around 3mm. Last weekend I sanded about 6 lengths that all did the same. ....and I was being very gentle!

[PatM]

Let's see some pics of the bows and the raw bamboo.

[Marc St Louis]

If you are having trouble with Bamboo then it's for sure you will have major trouble with Yellowheart.  It's pretty good in compression be weak in tension.  If you are having trouble with the nodes then maybe you are too aggressive with them.  I take the sharp ridge down at the nodes but besides that I leave them alone

[AndyTurner]

Yesterdays bow...

[PatM]

Profile looks  a  bit too aggressive with that mid limb kinked reflex.  Have you done a few straight glue-ups first or simple reflex?

[AndyTurner]

....errr no. As usual, I thought better to just jump straight in head first.

I've made a few Ratan bows for the kids but that's all.  They work well.

The one's before seemed much sturdier but over powered. Some destructive testing in the vice showed the last one broke in the same place on both limbs. Unfortunately it had a twist in the middle so couldn't glue a handle on square. Looking back I should have strung it and put on the tiller board for better testing. Looks to me like
1. the glue is stringer than the wood.   
2. On one limb the walnut sheered and the boo snapped clean.
3. On the other limb the boo sheered.

Frustrating but interesting.

[DC]

What kind of glue? How did you prepare the boo, belt sander? I've only made 15 boo backed bows but the backing has been bullet proof.

[AndyTurner]

hi,
1. I sanded the Back to final finish - 200 grit. This means at the end I'd have final thickness.   
2. I sanded the Belly of the Boo square to crown
3. I used my "Boo sledge" (Plank of wood with high density foam) in the thickness planner
4. Plane down to about 5mm
5. Belt sand with 80 grit down close to 4mm
6. Switched belt to an old 80 grit and finished down to 3.8mm +/- 0.05. Measured mid point between noodles. Flat surface to apex of crown.

[AndyTurner]

oh...

EA-40.

Cured for 24 hours in w/shop. Then hotbox at 65 for 4 hours. Then left 24 hrs in hotbox for slow ramp down.

[AndyTurner]

mix was 50:50 by weight

[DC]

That all sounds god to me. I was told the other day that walnut is not very good for risers. Did you use walnut for the belly? In the pictures of the broken bow there is one that looks like the the grain runs from back to belly pretty quick. You may have had a "tectonic plate" failure.

[AndyTurner]

Yes. Used IPE / Walnut for belly. But...
1. Belly didn't fail on latest bow. It was the boo
2. On previous bow I deliberately broke the limbs to see how far they would go. Both flexed well before finally giving up.

In fact bow bows felt great. It was just that the last one failed because of the boo & I spent a lot of time on that one trying to get it spot on.

If i use another board for backing like Hickery or Lemonwood then its a lot simpler. Just chuck it through the bandsaw then the thickness planer with taper sledge.

Why is walnut bad for risers?

[DC]

https://www.primitivearcher.com/smf/index.php/topic,68249.15.html

Second page about halfway down.