Back doming gone wrong.

[jacoblinnemann]

Hi guys,

So, im in the middle of my 2nd build - a parlor based on a Stahl Style 6. I closed the box last night and everything looks good, except the doming of the back. Its dead flat over the lower bout even though i used a 15´ radius dish to brace to back. The dish is homemade and properly not very precise or i didn´t sand the correct radius into the braces !
The 2 problem braces on the back are wide and thin, opposite to "normal" braces so im guessing that is harder for them to keep the stress of the radius?
The back is alot loser around those 2 braces and the sound more boomy, than over the rest of the back. It sounds like a drum.

I´m wondering if I should be worried about this?
If I have to take of the back again and replace the braces I should do it now before thinking about putting the binding on. I already trimmed the back flush with the sides... Would it be a huge job to take of the back? Would it be worth it? Alot of questions! Ha ha :)

I can post some photos of the problem if its unclear what im talking about.

Any advise or thoughts would be much appreciated! I

Thanks alot,
Jacob

[johnnparchem]

Are the rims profiled with the dish. I am assuming that it is flat across the back not up and down.

[jacoblinnemann]

Here a few photos to illustrate what the problem is.

Im not sure if I should go through the hassle of taking the back of and rebracing it - or just leave it alone and let it be one of the "I-will-fix-this-on-my-next-build"-mistakes :) This is only my 2nd guitar, so of course its not going to be perfect.. even though that´s what I aim for!

[tippie53]

sounds more like an RH issue. The lower braces will deflect more than higher ones. The key is when gluing to glue at a lower RH. I like to glue in braces below 40%.

I don't think you would be in too much trouble if your rh is low right now. As the wood dries is will shrink, and the braces will cause this dip.

[MaineGeezer]

If you figure out what happened, let me know. The exact same thing happened to me. (See my construction blog "A more-or-less copy of a Martin OM" for details.)

I made the curved braces, glued them onto the back while it was sitting in a proper 15' radius dish, with plenty of pressure to mash everything down into the dish....and the back has ended up almost flat.

My best guess is that variations in humidity caused the back and braces to warp, but attempts to re-humidify the back had limited effect.
So I'm ending up with a flat-back guitar. Not totally flat -- there is a trace of curvature -- but a lot flatter than it's supposed to be.

I don't think it's worth taking the guitar apart to re-do it. It's your choice, but I didn't think it would be worth the effort.

You used heavier bracing than I did, so that answers one of my questions: my problem didn't happen because my braces were too light.

It's weird.

[jacoblinnemann]

I´m glad to hear that i´m not the only that has experienced this problem.

Its winter here in Denmark, so we have the heaters on in our house which should cause the RH to be low. I don´t have a hygrometer in my shop, but this should definately be on my to-buy-list!

I read some where that some builders take springback in consideration when bracing : if you want a 15 foot radius on your back, you should build it in a 10 foot disc. Then the wood will springback to be around 15 foot. Anybody that can validate this theory?

I think that I´ll leave the back on and continue. The doming on the top is perfect - properly because of the x-braces - and the top is the most importent of the 2 to get right.
Well, another reason to build a 3rd guitar in the future. So you can fix all the thinks that went wrong on the previous build :)

[MaineGeezer]

I'm not sure I agree the the "springback" theory. The braces were cut to the proper radius. They weren't forced to the curvature. The back was pushed into the curve, true, but the amount of force required to do that seems minimal to me. I don't think it should be enough to have the effect we've seen.

[johnnparchem]

I do not think you need to take the guitar apart based on what you showed in the picture. Part of the reason the dome is put in is to allow the back to move with changes in humidity without cracking. Also OK is your description of a different tap under the flatter braces. That is one of the reasons they are flatter, to allow the lower bout to contribute to the sound of the guitar.

[johnnparchem]

I´m glad to hear that i´m not the only that has experienced this problem.

...
I read some where that some builders take springback in consideration when bracing : if you want a 15 foot radius on your back, you should build it in a 10 foot disc. Then the wood will springback to be around 15 foot. Anybody that can validate this theory?
...

Classical guitar builders sometime do this when they are pressing non - radiused fan braces on the top into a dished solera while building. They expect a 50% spring back so their dish is twice as deep.

Looks like you will have a well made guitar. I like the look and size of that style. It is interesting that all of the tone bars as well as the X-brace are inlet into the linings. Are you using the plans from the GAL?

[tippie53]

The stiffness of the brace is in the height. Lower braces like this will allow the back flex and move more