First build - Battling humidity.

[printer2]

I had a top that had issues so I thought to experiment by making a laminated top. But as many projects it got bumped aside and I only got two pieces glued together, still needs a third. Being dry in winter the one ply shrunk more than the other.

[RoyBoy]

I wouldn't think the difference between 30% and 40% during build would cause significant issues. The biggest concern from what I've heard is when an instrument that is built in a more normal humidity range ends up living a very dry environment (wood heat in the winter or in the desert region of the country). That is when cracks develop and seams separate.

[Victory Pete]

Originally Posted by printer2

I had a top that had issues so I thought to experiment by making a laminated top. But as many projects it got bumped aside and I only got two pieces glued together, still needs a third. Being dry in winter the one ply shrunk more than the other.

Are the plys at a right angle?

[printer2]

Originally Posted by Victory Pete

Are the plys at a right angle?

Sorry, I should have said that, yes they are. I am guessing a drop of 25-30% RH.

[Victory Pete]

Originally Posted by printer2

Sorry, I should have said that, yes they are. I am guessing a drop of 25-30% RH.

I guess because they are different woods with different expansion characteristics is why it has curled. If they were the same wood it seems the curling would be minimal as the wood expands the most with the grain. A guy at Woodcraft told me that wood only expands with the grain and I argued that it also expands with the length even though it is quite a bit less. I went home and put an 8' length of pine trim board in a puddle on my concrete floor, sure enough it started to bow up with the length.

[charles Tauber]

Originally Posted by RoyBoy

I wouldn't think the difference between 30% and 40% during build would cause significant issues. The biggest concern from what I've heard is when an instrument that is built in a more normal humidity range ends up living a very dry environment (wood heat in the winter or in the desert region of the country). That is when cracks develop and seams separate.

Don't confuse making/assembling a guitar with maintaining a completed guitar. They are two different things.

A humidity change of 10% while building a guitar can, indeed, significantly change the geometry of thin wooden guitar parts. A humidity change of 10% between, say, 30% and 40% in a completed guitar is largely irrelevant.

[printer2]

Originally Posted by Victory Pete

I guess because they are different woods with different expansion characteristics is why it has curled. If they were the same wood it seems the curling would be minimal as the wood expands the most with the grain. A guy at Woodcraft told me that wood only expands with the grain and I argued that it also expands with the length even though it is quite a bit less. I went home and put an 8' length of pine trim board in a puddle on my concrete floor, sure enough it started to bow up with the length.

If my memory is correct the middle layer (the bottom layer in the picture) was suppose to be the same thickness as the two outer layers combined. So since the bottom layer is thicker it is the one that dictates who is going to bend. But the point is lengthwise along the grain you do not get much movement for a change in humidity, trees do not get taller or shorter with changes in moisture conditions. And our bracing does not change in length appreciably. The wood we glue them on, the tops or the backs, much more.

[RoyBoy]

Originally Posted by charles Tauber

Don't confuse making/assembling a guitar with maintaining a completed guitar. They are two different things.

A humidity change of 10% while building a guitar can, indeed, significantly change the geometry of thin wooden guitar parts. A humidity change of 10% between, say, 30% and 40% in a completed guitar is largely irrelevant.

Really??? Just what "significant changes" would you find find between 30% and 40% ?

[charles Tauber]

Originally Posted by RoyBoy

Really??? Just what "significant changes" would you find find between 30% and 40% ?

As I previously stated:

Originally Posted by charles Tauber

...Many guitar makers, including me, have experienced having thin, wide pieces of wood, such as a guitar back, radically change shape with as little as 10% or so change in relative humidity. As others have reported, a fully braced, convex-arched back can become concave-arched with as little as 10% or so change in relative humidity.

How much guitar making experience do you have?

[RoyBoy]

Originally Posted by charles Tauber

As I previously stated:



How much guitar making experience do you have?

Little construction in the flattop arena (a decent amount of repair and set-up work though). One classical and one octave mando. Mostly archtop, f hole instruments- 5 mandolins and an L5 style archtop guitar. So I deal with a lot of perfectly quartered spruce and figured maple. I build in Florida, so even with an air conditioned shop, humidity fluctuates quite a bit with seasons. I generally aim for around 50% when I close the box. I've had scraps of resawn bubinga, mahogany, and padouk laying around for quite a few years and haven't experienced the distortion your speak of, perhaps I've just been lucky.

As for the guitar back mentioned. That seems quite implausible. To go from convex to concave with just a 10% fluctuation would mean it was either improperly/inadequately braced, or the back wood was never properly dried, or both. Since guitars generally aren't sealed on the inside, putting that back on an instrument would be a gamble as it would do the same after being installed.

Granted we see that phenomena on the tops of guitars that have been severely dehydrated (>10% drop in RH) but that is a spruce top, lightly braced with spruce braces to maximize responsiveness.

I bow to your expertise in flattops Mr. Tauber. If you feel the OP is treading on thin ice building in 30-40% RH as a beginner, I think he deserves some concrete advice on how to proceed safely.

[charles Tauber]

Originally Posted by RoyBoy

I bow to your expertise in flattops Mr. Tauber. If you feel the OP is treading on thin ice building in 30-40% RH as a beginner, I think he deserves some concrete advice on how to proceed safely.

Thanks for your response.

Based on his stated range of humidity, 30% seems right. I don't recommend that he build in an environment that varies by 10% during a build. However, if that is what he can manage, he'll still be able to pull it off.

There is always the option to have a smaller area that is strictly controlled and take the components out to work on them and put them back when finished working on them.