Bamboo becoming convex after glue up?

[razorsharptokill]

I flattened a piece of boo to day to try and salvage my hickory backed hickory board bow. (Backing broke).

I flattened and sanded the boo, glued in some in reflex and check it a few hours later and the glue lines were awful.

I unclamped it and heated backing with a heat gune to pull it loose.

It appeared that the mating surface of the boo had become convex and lifted the edges away from the hickory?

Any ideas as to the cause? I flattened a second piece and checked the fit. It was spot on.

Waiting for word here before I try it again.

[rainman]

If you watch Deans video, he explains this very well.  You need to make the gluing surface concave on the bamboo, this is why a toothing plane works better than a belt sander.  If the boo is perfectly flat on the gluing surface as you tighten the clamps down the edges will raise up, so I always make the gluing surface of the boo concave.

[D]

I flatten mine with a belt sander and I've never had that problem but I don't use clamps either, not all over it anyway only where I clamp it to the form.  Then I wrap it with innertube.

[razorsharptokill]

I used spring clamps mostly. Some bar clamps too but they were not super tight.

[tenbrook]

Here is the deal with flattening Boo.

After you flatten the Boo on the belt sander (or however you do it) you need to glue it up ASAP.  

The reason is the belly of the Boo (the section you just flattened and will put the glue on) will start sucking up moisture as soon as you expose the new fibers from flattening.  As it sucks it up the belly starts to crown giving bad glue lines.

So wait to flatten up your Boo until right before gluing.

Hope this helps!

Tenbrook

[vanillabear?]

[razorsharptokill]

I went directly from the sander to the form.

[macbow]

I'm like vanillabear, I heat it then re flatten before glue up.
Ron

[Eric Krewson]

Made over 50 BBOs, never, never, never left a concave gluing surface so I don't recommend it.

I do use scrap bamboo for gluing clamps that spread clamp pressure out pretty well. I flatten the backs of the pads to get a better grip with my C clamps.

 

[horseapple]

your bamboo was to moist, flatten like you would before glue-up, put in hotbox for a few days then you'll see what I'm talking about, the belly of the boo will be crowned, then you will need to flatten right before glue-up.

[razorsharptokill]

Thanks guys.

[razorsharptokill]

Well I glued it up again. Minor glue lines this time but I believe they are due to surface imperfections in the hickory after cleaning up the glue from the last attempt.