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I've noticed that contouring back bracing to say 15', results in the arch being much different than
that - particularly once the bracing has been carved.

Do others here contour their bracing to a smaller radius in hopes that the target radius is achieved
once it's glued on and the back wants to return to flat?

once you glue the braces to the back if you don't control RH the back will move quite a bit.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Good question. I saw more spring-back that I expected too (not back to "flat"). I'm confident that my braces were radiused properly and sitting flush inside my radius dishes (Top and Back), and the braces were glued overnight with the plates on the dishes. So I too am wondering how most folks deal with spring-back.

In my case, the issue is not one of RH but as Ed Haney said, one of springback.

This must be affecting other builders and if say a 15' arch is ideal, then I want
to have gotten this in the end. But because the back wood wants to return to flat
I'm ending up with a much larger radius.

I don't have too much issue
I will glue up in a 15 foot dish
Also you are putting water into the top from the glue. This may cause some of that for you.

I don't think I would loose much sleep over it.

_________________
John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

My backs keep their arch for the most part, but there's a bit if spring back once the braces get profiled. Not to worry. Put a bag of shot or a few light weights on it when you glue the rim and it should fit the dish perfectly because the weight will hold it down, conforming to the dish.

Hi James,

I do as you do: weight the back in the dish when gluing.
My experience is that when the weight is removed the back
tends toward flat again. It doesn't entirely flatten but it's
certainly nowhere near 15'.

I will add a thought

I could be wrong but, perhaps what you are seeing is spring back that is perpendicular to your bracing. Ie. The arch is maintained along the brace, but there is nothing holding the arch length wise of the guitar. So when the go bars are removed you see spring back length wise. (There is nothing yet holding that raidus, not until you glue on the sides).

Put a straight edge , or better yet a concave 15' arch template on the back side and check if that brace is maintaining its radius. If it is you should be good to go. If not I would double check you stock to be sure it is seasoned and stable.

Joel

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http://Michaudmade.com

Thanks Joel.

The arch is not holding along the brace even after being glued to the sides. The materials I use are thoroughly dry and stable and
I maintain between 38 and 42 RH. This last back - Wenge - has been the most problematic but this has always been an issue to one degree or another and I want to get it completely under control now. I'm thinking I would have needed a dish with something like a 10' radius for the arch to have held, once sprung back, to 15'.

The arch along the length of the body is not the issue: where it's falling down is across the grain, parallel to the bracing.

No one else has this problem?

I've had this problem with a thin back and wild swings the RH. How thick is your back?

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Thank You and Best To All

There's inevitably a bit of spring back, because of the flexing of the back when you glue up. Using water-based glues* and water to clean up the squeeze-out can exacerbate the problem. Also, the braced plate is quite a sensitive hygrometer, so just a few points drop in humidity will reveal itself as a flattening.

* When joining the plates, many builders will shoot a hollow glue line, revealing some light in the centre region of the joint when it is "candled". This is so that when the edge swells when glue is applied it swells to straight rather than swelling to convex, which would leave a gappy joint at the ends of the plate.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

.08 +-, wenge, just about perfectly quartered.

This may be obvious, but are you also ensuring your braces are quarter or rift sawn? If you were to apply them flat sawn they will bend easily and not hold the curve as the back timber will have more tensile strength.

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Martin Taylor
http://martintaylorguitars.com.au

There's not a whole lot of difference in stiffness (Young's modulus) between wood in the flat sawn orientation to that in the quarter sawn orientation.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

I've tested quartered VS flat sawn and it's not always the case that quartered is stiffer.

At any rate, all the bracing is quartered simply because it's easier to carve, if for no other reason.