Thinning wide boo backing

[Flyonline]

I'm about to start a couple of wider than normal bows (for me anyway) paddle style bows and wish to back them with bamboo. The issue I have is that thinning it down enough leaves me with with one of two problems: 1) Either the center is left unthinned so I get the edges nice and thin and correct width at it's widest but I'm left with a hollow from the centre of the boo or 2) If I thin enough to get a flat surface, I end up narrowing it too much and it no longer fits.

Is it possible to either split a pair of boo strips, thin them and then splice longitudinally to fit the wide back of the bow, or use something like a round plane/end of belt sander etc. to remove pith somewhat consistently across the whole arc.

If the latter, is there any issue with bubbles forming underneath when gluing up a convex surface onto a flat plane, or does the clamping compress it enough that it's not an issue.

I'm limited to using ~8" boo culms at the moment, so going bigger diameter to give me less arc to work with isn't really an option at this stage.

Cheers

Steve

[Tommy D]

I am also curious about this. I know that longitudinal cracks in bamboo have almost no effect on the bow, so my gut tells me you would be fine with two trips side by side.

[Del the cat]

I think the side by side strips is the most practical and elegant solution.
Del

[Ben.A.M]

If you cut the strips down the length about 3/4 across the width and married the thicker edges together as the middle you’ll get a stronger joint and the centre of the bow should have a stronger structure also, getting the thickness right could be a challenge though. I’ve seen thin strips of bamboo floor boards done this way as a backing before and it held fine, tips were as thin as my pinky too, guy was using smooth on and there was even small gaps up the centre where the strips didn’t marry up properly, didn’t effect it one bit.

[bownarra]

Even sinewed paddle bows end up really thin. Sinew has low stretch resistance.
Bamboo has high stretch resistance so a boo backed anything paddle bow would end up being too thin...really thin bows are a nightmare to tiller as even a thin scraping is proportionally a decent amount of thickness. 
I would suggest making a semi paddle bow, reducing your width but keeping the front profile.
What belly wood are you planning on using? Length? Planned width at present?

[Flyonline]

If you cut the strips down the length about 3/4 across the width and married the thicker edges together as the middle you’ll get a stronger joint and the centre of the bow should have a stronger structure also, getting the thickness right could be a challenge though. I’ve seen thin strips of bamboo floor boards done this way as a backing before and it held fine, tips were as thin as my pinky too, guy was using smooth on and there was even small gaps up the centre where the strips didn’t marry up properly, didn’t effect it one bit.

Are you saying to back only 3/4 of the width? I'd be hesitant given most eucalypts tendency to splinter - this is ironbark which may not, but I've never used it before so can't comment. I"ll be using thickened epoxy again, so shouldn't be any issues with small gaps as the same glue has held up fine on a couple of other boo backings, one of which could have been better!

Even sinewed paddle bows end up really thin. Sinew has low stretch resistance.
Bamboo has high stretch resistance so a boo backed anything paddle bow would end up being too thin...really thin bows are a nightmare to tiller as even a thin scraping is proportionally a decent amount of thickness. 
I would suggest making a semi paddle bow, reducing your width but keeping the front profile.
What belly wood are you planning on using? Length? Planned width at present?

I'm not aiming to replicate anything in particular, just a paddle style to practice/play with so design is flexible to a large degree. I'm intending to use ironbark boards @ ~42mm/1.7" as I know ironbark has been used very successfully for paddle style bows before, but I have other options for belly (osage, lemonwood and red oak would be the three I'm guessing you're familiar with plus others). The consensus here seems to be that Aussie woods are more suited to wide/lenticular designs. I hear you on the thin is hard! I've started (and broke) a couple of paddle style stave bows already and quickly noticed the caution required!!!!!

Thinking further on the boo, I wondered if it would be possible to steam and clamp flat with back down or pressure stip down onto back to flatten it somewhat.

Steve

[DC]

I tried steaming and flattening some boo way and it split almost immediately. That was early in my bow making life though.

[RyanY]

I've seen a few bows over the years where the bamboo backing is narrower than the full width of the backing. If it has to be backed with bamboo then this might be the simplest option. Just taper the back so it's trapped a little bit where its not covered with the bamboo. The issue will be that depending on the length and width of the bow, the limbs could end up quite thin and the bamboo could overpower the belly.

[DC]

You don't say how wide you want it. You can cover almost 2" with a 4" dia boo. With 8" dia you should be able to get better than 4" wide. What is the wall thickness of the boo? Or do you mean 8" circumference?

[Flyonline]

I tried steaming and flattening some boo way and it split almost immediately. That was early in my bow making life though.

Was that pre or post thinning?

You don't say how wide you want it. You can cover almost 2" with a 4" dia boo. With 8" dia you should be able to get better than 4" wide. What is the wall thickness of the boo? Or do you mean 8" circumference?

Honestly I could probably cover the 42mm board at the moment with a bit of care, but it would be narrow margins on the edges - I was thinking as much for the future for wider style paddle bows as much as anything as I'm not set on any particular width for the first bow. Wall thickness is ~9-10mm. Are you talking post thinning it's flattened somewhat to reach that width?

Err, just had a senior moment - should be I have 4" boo sorry, NOT 8"

I've seen a few bows over the years where the bamboo backing is narrower than the full width of the backing. If it has to be backed with bamboo then this might be the simplest option. Just taper the back so it's trapped a little bit where its not covered with the bamboo. The issue will be that depending on the length and width of the bow, the limbs could end up quite thin and the bamboo could overpower the belly.

Yes, that had crossed my mind it's just that I've had one bow throw splinters on the edges even when trapped and backed 2/3 - 3/4 of the width. Given that this timber has proven to work well in this format (though I don't know if stave or board form), I will probably go with narrower backing and see how it goes.

[Ben.A.M]

Sorry, when I said to strip a quarter off the edge of each and marry the cut sides together I was working under the assumption that 2 strips together would be more than you needed and by doing this you could strengthen the back a bit more, structurally speaking, avoid a big dip at the centre of your backing, especially as you get toward the tips, alternatively you could have 1 strip straight up the centre and put thinner strips either side.

[DC]

I tried steaming and flattening some boo way and it split almost immediately. That was early in my bow making life though.

Was that pre or post thinning?

You don't say how wide you want it. You can cover almost 2" with a 4" dia boo. With 8" dia you should be able to get better than 4" wide. What is the wall thickness of the boo? Or do you mean 8" circumference?

Honestly I could probably cover the 42mm board at the moment with a bit of care, but it would be narrow margins on the edges - I was thinking as much for the future for wider style paddle bows as much as anything as I'm not set on any particular width for the first bow. Wall thickness is ~9-10mm. Are you talking post thinning it's flattened somewhat to reach that width?

Err, just had a senior moment - should be I have 4" boo sorry, NOT 8"

Too be honest I'm not sure but I assume it was pre thinning. It wouldn't get any wider if I'd already thinned it.

[Flyonline]

Sorry, when I said to strip a quarter off the edge of each and marry the cut sides together I was working under the assumption that 2 strips together would be more than you needed and by doing this you could strengthen the back a bit more, structurally speaking, avoid a big dip at the centre of your backing, especially as you get toward the tips, alternatively you could have 1 strip straight up the centre and put thinner strips either side.

Roger David! Just gotta get in the shed and make it happen now....

Too be honest I'm not sure but I assume it was pre thinning. It wouldn't get any wider if I'd already thinned it.