Inverted Back Dome....

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PSmill
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Inverted Back Dome....

Post by PSmill »

I am working on an OM with bubinga back and sides. I had the wood here for a few weeks at least before starting. Glued on the back braces 2 weeks ago and everything seemed pretty good, using the 15' radius dish. Fast forward 2 weeks and the 15' dome has completely flattened and is even a bit concave :) The humidity has moved from 60 to 30, which I wouldn't have thought was extreme enough to produce that reaction. Just curious on opinions if this is out of the ordinary for that humidity range. The brace wood I have had here for a while, it could have been exposed to higher humidity, so maybe the braces shrunk and pulled it out of shape. Any good options other than removing the braces, re-making and doing over? Thanks.
Bob Gleason
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Re: Inverted Back Dome....

Post by Bob Gleason »

Yep, that's way plenty change in humidity for that to happen. If you pull the back and rebrace it, and then have that same kind of humidity change you will have the same issue. The best most of us can do is to control the humidity between 40-50%. That is still a pretty big swing, but doable. Bracing won't hold the back or top to shape in extreme humidity changes. I live in 70-95% humidity, but I have a dehumidified workspace. That is not an option for most. Maybe some builders that have dehumidied boxes that they keep their projects in will give you some suggestions. Good luck!-Bob
Diane Kauffmds
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Re: Inverted Back Dome....

Post by Diane Kauffmds »

Oh, absolutely! Wood is a great hygrometer. You might try rehydration by putting it in a trash bag, along with a plastic container with a wet (not dripping) sponge, for a couple of days. See if it helps. But you'll probably need to re-brace. I'm able to control my shop humidity at 48-50%. I live in WV. In the summer the humidity is awful. I've seen single digit humidity in winter, and the range can change amazingly in 24 hours. A 50% drop is a huge drop in your work area. Wood reacts almost immediately to any change.
Diane Kauffmann
Country Roads Guitars
countryroadsguitars@gmail.com
MaineGeezer
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Re: Inverted Back Dome....

Post by MaineGeezer »

Yep. Same thing happened to me. I think I ended up taking off the braces and re-gluing them.
Don't believe everything you know.
Anything is possible if you don't know what you are talking about.
When things are bad, try not to make them any worse, because it is quite likely they are bad enough already. - French Foreign Legion
Stray Feathers
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Re: Inverted Back Dome....

Post by Stray Feathers »

I'm curious to know how this happens. I would have thought that the surfaces of the braces that were exposed to lower humidity would dry and shrink, and increase the curve rather than reduce it? And the bubinga back should be almost equally exposed to drier air front and back? Incidentally, like PSmill I live in coastal BC, where the humidity in my uncontrolled shop varies between 40 - 60 % most of the year and I have not had these issues - at least not to the extent that I became aware of it. So I don't know what I am doing "right". Bruce W.
Bob Gleason
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Re: Inverted Back Dome....

Post by Bob Gleason »

A couple of extreme humidity stories

40+ years ago I had a shop in El Paso Texas. Humidity often 20% minus. I got new Martin guitar repairs from the stores because a new Martin would have sharp fret ends sticking out of the ebony fretboard within a few days of arrival in El Paso. Never have liked ebony since!

When I came to Hawaii to see what it might be like to live here, I brought 2 guitars from El Paso. On arrival in Hilo, it took about 2 hours to get to my destination. After 2-3 hours in Hawaiian humidity, probably at least 60-70%. both guitars were unplayable.The necks back bowed so much that the strings were laying on the frets until about the 9th fret or so.

Back in the 90's, when I thought I had my shop humidity sort of under control in Hawaii, I shipped a little koa/ spruce 000 to New York in the Winter. No humidity at all in an East Coast house in Winter! Within days the spruce top developed book matched cracks from the tips of the bridge wings to the tail of the guitar. When I got it back, I could drop dimes through the cracks. I set it aside, never fixed the cracks, and in another few days the guitar was perfectly playable again. Cracks self sealed. I played that guitar for years until it suffered an untimely death.

Lessons learned. Even though I now have relatively good humidity control, between about 42 and 50% 24 hours a day, I have no interest in sending a guitar to extremely dry places like Texas, Arizona, etc. The wet end of the spectrum is not quite so dangerous. Guitars puff up, sound like crap, and the action can get a bit high, but at least they can still be playable. They don't usually self destruct like when they get extremely dried out.

Where would we be without the lutherie gremlins to keep things interesting!
MaineGeezer
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Re: Inverted Back Dome....

Post by MaineGeezer »

Stray Feathers wrote: Wed Nov 23, 2022 7:48 pm I'm curious to know how this happens. I would have thought that the surfaces of the braces that were exposed to lower humidity would dry and shrink, and increase the curve rather than reduce it? And the bubinga back should be almost equally exposed to drier air front and back? Incidentally, like PSmill I live in coastal BC, where the humidity in my uncontrolled shop varies between 40 - 60 % most of the year and I have not had these issues - at least not to the extent that I became aware of it. So I don't know what I am doing "right". Bruce W.
My understanding is that wood shrinks a lot more across the grain than with the grain, so the back gets narrower and pulls the braces into a curve.
Don't believe everything you know.
Anything is possible if you don't know what you are talking about.
When things are bad, try not to make them any worse, because it is quite likely they are bad enough already. - French Foreign Legion
tippie53
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Re: Inverted Back Dome....

Post by tippie53 »

actually your correct that wood can move as much as 1/4 in per foot in RH swings. As the wood moves the braces light the movement. So as it shrinks this pulls against the braces and flatted or reverse the dome , as it expands it lets the dome rise so plan your RH when you glue braces. This will effect how the dome moves. I try and glue at about 45%.
John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com
PSmill
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Location: British Columbia, Canada

Re: Inverted Back Dome....

Post by PSmill »

Update....I removed the braces, using heated thin spatulas worked ok, sliding a razor blade under worked way better. The reverse dome had become more extreme since I first posted, with a straight edge side to side it's showed a 1/4" gap under at the centreline of the back. When I pulled the braces the back immediately returned to being entirely flat. I put it in the 15' dish over night with some weights on it to see if it would pre-form to that shape, and when I removed the weights it had actually re-curved back into a dome steeper than 15'.

The humidity has been around 35% steady this past week. I am wondering if bubinga, or even just this piece, is not very stable for some reason and will continue to be really reactive or problematic. Or should I just go ahead and re-brace it and see what happens...? thanks
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