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Bending..I bought some very nicely figured side wood sold as Patagonian Rosewood. Cut some side slabs,
thickness sanded and started the bending process as normal. The lower bout bent fine at 250F, but I thought the upper was never gonna 'release'.
It actually went to 425F before it suddenly released, finished bending and the next morning was beautifully curved to fit in the mold. Thickness was
about 90 thou to up to 100 thousandths. Typical wet paper sandwich. Never saw such before....?!

Flattening: I have a neighbor with some black walnut he's had for decades and was willing to share. I have two pieces both about 22" x 39"; both are noticeably cupped. One is roughly 1.25" thick, the other 11/16". I am at least three guitars away from being about to think about making use of them and wondering what best storage might help them see flatness. (in my production capacity, that's at least 2-3 years away) Right now I have them laying on concrete, both concave sides inward, heaviest on top, and an
automatic transmission torque converter (at least that's what I think it is, weighs close to forty lbs) on top sitting on a square of melamine centered.

Just looking for advice. Thanks. Roy in Az

Bending…

Remember the cubed rule of stiffness applies to your sides too. I usually bend at .080. The stiffness between .080 and .100 is significant.

Flattening…

IMO take the loss of thickness from milling the boards flat. Lots of vids on how to mill lumber true, with various machines. Trying to flatten wood that thick seems like a lost cause, but I could be wrong…

Unless you really want to make a one piece back, you’re not going to use the piece of walnut as it is but rather in 5” and 8” wide pieces. If you cut it to usable dimensions, the cupping may be much less of an issue when you resaw it.

I also usually bend in the neighborhood of .080”, sometimes a lot less for really tight bends (the bend makes it thicker at those points).

Thanks, I guess I was thinking rather than trying to laminate for better sound I'd try bending a bit thicker.

When the time comes, I'll recut into more manageable widths. For now I'm just a bit overwhelmed with such heavy
pieces of wood. Roy

Another thought: putting a weight on a piece that size might risk cracking it rather than flattening it. An uncontrolled crack might make it a lot less useful.

I agree with Bob, trying to flatten it will just crack before achieving the desired effect.

You've got the right wood for it .080" is a good baseline for most woods, but with walnut 1/8" should be fine for most guitars. I've even bent 3/16" pieces for falcate braces on a harp ukulele (I still need to finish that one...)

A one piece carved archtop back made of walnut might be kind of cool!

The first thing that I thought was 2 archtops. I made a one piece archtop, and a one piece cello out of a slab of Euro Sycamore (maple). Walnut carves nice.

But people see 5-6 guitars versus 2 arch tops.

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Why be normal?

In terms of cupping. Could the boards have been stored such that one side is drier than the other? This can certainly lead to cupping.
Try storing them so that air can get equally to both sides. With such thick boards any improvement could take a while but worth a try.

Cheers Dave