Official Luthiers Forum!

Hi guys,
I recently had an issue with the top radius reducing after removing from the dish.
So, I rebraced, removed it from the dish and a week later the radius in the lower bout is once again gone !!
This time I watched the humidity carefully, for a week before the rebrace the RH was 15%, I rebraced and it has never
fluctuated more that 2-3 pecent, its been very dry here.
Whats the deal ??
Why is the top back bending to a negitive radius ??
Very frustrating...starting to piss me off !!

Gregg

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Gregg

Hi Gregg,
See William Cumpiano's Newsletter #22.
http://www.cumpiano.com/Home/Newsletter ... ntytwo.htm

René

Thanks Ren'e,
However, what happens when I finish this guitar and it goes in my guitar room inside the
house which is 15% -18% RH all the time ??
I thought I should build and keep my guitars at the same RH ??

Gregg

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Gregg

Gregg,
It seems to me that you are doing this at too low RH and you need better climate control. Everything I've read about the subject indicates that the ideal RH in a shop would be about 45-50%.
Also, you will probably need a good guitar case humidor if you can't control the RH in your house.

René

I was hoping I could build at the same climate as I store....

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Gregg

Gregg-
I know how frustrating this stuff can be! Take a deep breath...
As long as the RH is reasonably constant, I can't see a problem with building and using the guitar in low humidity.
Sorry if some of these questions sound 'dumb'...
Have you measured the RH in your shop, or somewhere else in the house?
Has your wood been stored at 15%RH for a while before building? (How long?)
What sort of wood are you using for top & braces?
What bracing style are you using?
Were the braces shaped to the radius or 'pressed' into the dish?
Have you shaped the braces to final profile?
Do you have a pic of the braced top which has flattened?

BTW, don't feel too bad- I am not the only person who has had a braced back 'turn inside out' (go from convex to concave) with poor wood and poor RH control.

There are some 'quick and dirty' ways to get wood to curve, but best to try to figure out what's going on there...

Cheers
John

Gregg what are you using to check the humidity levels?

The reason that I ask is that most hygrometers are terribly inaccurate and the digital ones you might as sell take a Sharpie and a piece of masking tape and write your desired RH on them....

Not only are hygrometers inherently inaccurate their accuracy varies greatly in different ranges. For example a hygrometer may be 9% off at 45% RH and 25% off at 10% RH. I have observed some hygrometers that will not even indicate a change in RH accurately in the 20-30% range.

I am bringing this up because there is an explanation for everything and one of your variables here in my mind is your hygrometer and/or method of measuring your RH.

It may be that you do not have a reliable way to even know what the RH is and that could explain some of the mystery here.

Hi John, thanks for the reply,
I know how frustrating this stuff can be! Take a deep breath...
OK, I took a deep breath, I feel better, I dont think anyone is in danger of being killed..
As long as the RH is reasonably constant, I can't see a problem with building and using the guitar in low humidity.
I've referanced my RH from where I build to where I store and its constant (unless it rains which is rare here) 15% - 18%. I'm using two a $8 digital hydrometers, not the best but for a referance I thought it would supply the info I needed.
Sorry if some of these questions sound 'dumb'...
Have you measured the RH in your shop, or somewhere else in the house?
yes, please see above comment
Has your wood been stored at 15%RH for a while before building? (How long?)
Yes my wood has been at this RH for months
What sort of wood are you using for top & braces?
Top braces ar sitka spruce purchased from LMI, they spent appros 10 days in my shop before bracing
What bracing style are you using?
X brace
Were the braces shaped to the radius or 'pressed' into the dish?
the braces came pre radiused, however I drew a black line along the entire length
with a sharpie and lightly "re-radiused" in my 28" dish till the black line was gone
Have you shaped the braces to final profile?
Yes
Can you share with me the tricks to re-radius without dissassembling this top ??
Do you have a pic of the braced top which has flattened?

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Gregg

Hesh,
I'm using a $8 digital hygrometer from the Wal, I do know its most likely not accurate.
However my thought is this,
I am using it as a referance, I have two of these hygrometers, I put both in my shop for about a week and saw
15% - 18%, then I put both in my house for a week and saw 15% - 18% also.
Now I have one in the house and one in the shop and the RH follows.
We had a medium rain storm recently and I had the windows in the house open and the hygrometer
rose to 37%, so it does raise and lower, maybe not accurate but as a referance.
The RH here in ABQ stays dry and constant.
I agree with you "there is an explanation for everything" I just dont know it yet.
I thought by referancing the RH from my house to my shop I determined the RH is about
the same where I build and store, so I braced in a 28" radius dish and a week later all my hard work,
time, effort and $$$$ now look like S***
Very frustrating......

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Gregg

Hang in there Gregg you are doing a great job and from looking at your workmanship in the top pic it's pretty clear to me that you are a detail oriented sort.... My condolences....

I still think it's the Uncle Wally Hygrometers.... I recently tested 3 of the same brand and all of them were exactly alike - 15% off.... We do know that your hygrometer moves which is something but not much IMHO.

What you are describing does frequently happen to OLFers and we hear about it all of the time, well most of the time... It's usually caused by changes in the RH from when the cross grain gluing was done. Looking at your pic your bracing is not too light in my opinion.

You know if your RH swings just 8 - 10% in the course of a day that could cause what you are seeing. I finished bracing a top in my 45% RH shop and decided to sit on my back deck and listen to some tunes while I did the final sanding of my braces. My outside hygrometer indicated 55%, 10% difference so I moved out back and started sanding. 20 minutes later my top had flattened out.... It happened that fast....

So I still think this is RH related and the only way for you to know is to have a way to really know what the RH is at all times. You can do a wet bulb test for the price of two thermometers and that is about as accurate as you can get. That's what I would recommend and compare results to the Uncle Wally hygrometer and see what you can learn.

Well Hesh,
If your pleased with my work thats gotta be a good thing !! Thanks.
It has got to be the RH thing, I am shocked at your story, I had no idea that 10% diff in RH
would flatten out a top in a few minutes.
Heres the plan,
#1...Im going to buy/build a wet bulb hydrometer,
#2...Moniter the RH in my shop for a month or two,
#3...Determine the lowest RH,
#4...Brace at that RH so if it goes higher it shouldnt be an issue.
Yes / No ??

I really dont want to dissassemble this top, how can it be re-radiused ??
Heat lamp in the dish ??
Steam it in the dish and let it dry out ??
I'm grasping at this point...
To save time can I buy this wet bulb hydrometer ??

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Gregg

Gregg-
Make sure you search for 'hygrometer', not 'hydrometer'- different things. Some of the 'Science stores' probably have wet/dry hygrometers- also called psychrometers , but they are easy to make if you get yourself a couple of (glass) thermometers.
My guess is that dampening the 'outside' of the top would curve it enough to get back the original shape-temporarily.
Looking at your picture, I'm less sure that RH is the real issue here.
I know that 'scalloping' is quite popular - and that the straight-on picture may be deceptive - but it doesn't look like you have much 'meat' in the lower bout braces to keep the radius. I'd expect that once you string it up, you'll find that the top 'domes up' a fair bit in the lower bout as well. (Remember that the 'stiffness' of the brace varies with the cube of its height- so if you carve away 2/3 of the height, the stiffness is 1/27 of what it was...)
You've got the flat top (and probably the flat bridge plate?) working against the curved braces in the lower bout.
Cheers
John

I think the question about profiling your braces has to do also with....
Are you springing flat bottomed braces into a domed dish, or are you shaping curved bottom braces, then gluing into a curved dish? Both ways are used...

Anyway, on to my questions...

How's about letting it sit indoors in your climate controlled space for a few days or a week.. then decide on what to do..

Here's why:

Some woods absorb moisture much faster on 1 side than on the other... Why.. No idea.

You also added Glue -- which has a whole lot of water in it...

Woods of different thicknesses absorb moisture at different speeds too... so a thick piece (Brace) moves much more slowly than the thin piece (Top)

Spruce, especially thin spruce moves pretty darn fast... Here in the Hot and Wet south -- taking a glued up top outside results in an immediate potato chip... Let it sit a couple days and it flattens back out again.... Bring it back inside and you get a potato chip again... and then it flattens back out again after a day or two... Put braces on it and things happen at different speeds.... The thin top plate moves real fast, but the bracing doesn't... so it goes all whacko and then takes longer to finally settle down.

Last... I wouldn't worry so much about your top loosing a bit of its radius when you pull it out of the dish. You glued curved braces onto a flat plate -- so there is an inherent push/pull going on between the flat plate and curved bracing... Mine always spring back a bit out of the dish....

Bring it inside and watch it... When it looks like enough of your top radius is back -- glue that sucker down onto the rims!

Thanks

John

Thanks John,
Very interesting, I'll try it.

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Gregg

Ok guys, I did some research on line and I'm going to build a wet bulb tester.
This will put the RH question in my shop to rest once and for all.

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Gregg

The only other thing from what has already been covered is are you introducing too much moisture at glueup by using too much glue or cleaning up with a damp rag?

Hey Jeff,
No damp rag, just a sharpened piece of wood to clean up squeeze out.
As for too much glue.....I dont think so.?.?.?

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Gregg

Sounds like a good plan on the wet bulb testing Gregg.

I don't think that you introduced too much moisture during glue-up either. So long as you didn't introduce a lot of additional water during clean-up the moisture in the glue itself should not be a problem. I am struggling with what I just said because I have no experience at the RH levels that you live with. So maybe it's not a problem, maybe it is - don't know...

I build at around 45% and since it snows here and folks have forced air heating it can dry out down to 25% in extreme cases.... The RH can also swing to near 100% too. As you can see 45% is in the low middle of the range that I live in. As such if you have to pick a number you might want to consider building at a middle number not the low end. Just a thought.

You know we can't really know reliably what the RH was when you braced this top. If you did all the braces for the top in the same RH theoretically the top should return to the radius that was built into it when and if the RH returns to what it was when it was braced if that RH remained stable long enough to let this happen.

In the instance above that I related to you when I went inside and returned the top to the RH that it was built at and weighted it down it returned to it's shape in an hour and held it's shape with no assistance for another month or so until I closed up the box.

Sounds like you have a good plan going forward. If not you could always move....

Hey Hesh,
Thanks for the encouragement,
My plan has got to work,
With correct RH readings and bracing towards the low end
The only place to go is higher RH, I'm sure higher RH has its issues also,
However higher RH here in ABQ doesn't get close to you guys.
I'll let you guys know how it goes in a couple of months.

Thanks everyone !!

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Gregg

Hey Gregg,
I'm up in Los Alamos and lately we've had some pretty large RH swings. Even today it was 40% in the am and now its 10-15% according to the LANL weather instruments http://weather.lanl.gov/. And we'll see them again as the thunderstorm season arrives on Memorial day
A shop humidifier should take care of you if keep everything shut up.
good luck
jim

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Jim Watts
http://jameswattsguitars.com

Hey Hesh,
I read you wet bulb rest, very interesting,
I have two questions,
#1...Is it necessary to have air moving accross the two bulbs ??
#2...Is the Abbeon Hydrometer one of the best dial meters ??

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Gregg

So here's a thought. I don't know how the RH scale works, but when your humidity is swinging from 15% to 20% in a day, that's a change of only 5%. No big deal right? BUT that's not a 5% change. If the scale works this way, that's a 33% increase in the amount of moisture in the air. That's equivalent of someone building at 45% swinging up to 60%.

I don't know if that's a valid comparison, but it seems to me that building in a dry climate (which should be fine if the instrument will always live there), should be more sensitive to minor variations in RH.

As for getting the radius back into your top, have you tried humidifying it? Throw it in a tupperware tote with a wet towel. Put in some spacers so it isn't actually getting wet. If it collapsed on you due to humidity, it should go back the other way just as easily.

If you decide to rebrace, getting the old braces out with a block plane is fast and easy. Did it myself last summer. If you didn't use hide glue or fish glue, make sure you get the surface scraped clean. I got mine close and then ran it through the sander.

Good luck.

Mike

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Mike Lindstrom

Hi Gregg, I had the same problem with my first guitar and no amount of re-humidifying would bring back the radius. Ultimately I had to remove and rebrace. I realized that I had waited too long before I was ready to glue the top to the rim (about a month) which allowed the top to go through different humidity cycles causing the warping. I now finish the rim first and as soon as the braces are glued and dry I carve, shape and glue the top immediately to the rim which will stabilize the radius. If for some reason I can't glue the top down right away and especially if I see a humidity change coming I will take note of the current humidity level then seal the top in a plastic bag, weight it down in the radius dish and not remove it until the humidity returns to the original level then glue it up asap. Sealing it in a plastic bag will hold in the moisture at the level when it was sealed. I think in your wheather conditions you will have to be extra vigilant in watching the humidity levels.
Good Luck,
John

Hi Gregg, I had the same problem with my first guitar and no amount of re-humidifying would bring back the radius. Ultimately I had to remove and rebrace. I realized that I had waited too long before I was ready to glue the top to the rim (about a month) which allowed the top to go through different humidity cycles causing the warping. I now finish the rim first and as soon as the braces are glued and dry I carve, shape and glue the top immediately to the rim which will stabilize the radius. If for some reason I can't glue the top down right away and especially if I see a humidity change coming I will take note of the current humidity level then seal the top in a plastic bag, weight it down in the radius dish and not remove it until the humidity returns to the original level then glue it up asap. Sealing it in a plastic bag will hold in the moisture at the level when it was sealed. I think in your wheather conditions you will have to be extra vigilant in watching the humidity levels.
Good Luck,
John

Mike-
Minor clarification: the 'relative' in RH refers to the idea that the RH% describes how much water is present in the air, compared to the maximum that the air could 'hold' at that temperature.
The idea of air 'holding' water is not actually correct (water vapor is a common component of air ), but is a convenient simplification.
So, if you have a room at 20 deg C and 50% RH, and seal the room (so the amount of water in the room is fixed), and let the temperature drop, the RH will increase (since cool water forms a vapor less easily). Of course, if you get the room cold enough the water will condense into visible droplets.
So just changing the temperature in the room will affect the RH reading.

One way to reduce the swings in humidity is to make sure that there are a lot of 'sinks' in the room- keep a big stock of wood in the room, for instance. (That's my excuse for the wood collection, anyhow! )

Cheers
John