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Concave top!!
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Author:  cgal_1 [ Mon Feb 14, 2011 10:08 am ]
Post subject:  Concave top!!
Can anyone help? I've just braced a top and left it overnight before having the time to fix it to the rim. When I've looked today it's gone concave, so I can't glue it to the rim.
The humidity is reading 49%, though I wouldn't guarentee how accurate this is.
Is there any way of recovering it?
Charles.
Author:  WaddyThomson [ Mon Feb 14, 2011 10:15 am ]
Post subject:  Re: Concave top!!
The problem is not what the humidity reading is now, but the difference between this reading and the one when you braced the top. It's the reason everyone who builds preaches humidity control in the area of the shop where assembly is done, or keeping your materials in a humidity controlled environment between work sessions. The top changed because of a humidity change. You need to return it to the reading it was at when you braced it(terrible sentence). It should then return to it's original shape.
Author:  enalnitram [ Mon Feb 14, 2011 10:20 am ]
Post subject:  Re: Concave top!!
you will want to make sure that the unbraced side is receiving air flow, so that moisture is reaching it at the same amount as thru the braced side.
Author:  Hupaand [ Mon Feb 14, 2011 12:25 pm ]
Post subject:  Re: Concave top!!
The trouble is, even if you get it to it's original r/h to attach it, whenever the r/h goes down to 49ยบ the top will try to go concave again.
Author:  Laurent Brondel [ Mon Feb 14, 2011 12:54 pm ]
Post subject:  Re: Concave top!!
cgal_1 wrote:
The humidity is reading 49%, though I wouldn't guarentee how accurate this is.
It would be better for your luthiery success to actually know accurately your RH. A concave top is the sure sign of bracing at much higher RH than what it is now (when it is concave). Where are you located? The best course of action is to take the braces off and do it again (with the "right" RH).
Author:  meddlingfool [ Mon Feb 14, 2011 12:58 pm ]
Post subject:  Re: Concave top!!
Indeed. If it is concave at 49% you've got a real problem. Most of us try to build at around 40-50% at the most. I do my work at 40%. I sexing that you need to rebrace it at a lower RH.
Author:  Howard Klepper [ Mon Feb 14, 2011 3:18 pm ]
Post subject:  Re: Concave top!!
meddlingfool wrote:
Indeed. If it is concave at 49% you've got a real problem. Most of us try to build at around 40-50% at the most. I do my work at 40%. I sexing that you need to rebrace it at a lower RH.

Where is Freud when you need him?
Author:  Bob Garrish [ Mon Feb 14, 2011 3:26 pm ]
Post subject:  Re: Concave top!!
Howard Klepper wrote:
meddlingfool wrote:
Indeed. If it is concave at 49% you've got a real problem. Most of us try to build at around 40-50% at the most. I do my work at 40%. I sexing that you need to rebrace it at a lower RH.

Where is Freud when you need him?


He visits his mom on V day...strange cat ;)
Author:  J Hewitt [ Mon Feb 14, 2011 6:14 pm ]
Post subject:  Re: Concave top!!
Bob Garrish wrote:
Howard Klepper wrote:
meddlingfool wrote:
Indeed. If it is concave at 49% you've got a real problem. Most of us try to build at around 40-50% at the most. I do my work at 40%. I sexing that you need to rebrace it at a lower RH.

Where is Freud when you need him?


He visits his mom on V day...strange cat ;)

Freud...sounds like a DOG to me!...lol
Author:  Bobc [ Mon Feb 14, 2011 6:39 pm ]
Post subject:  Re: Concave top!!
Ahhhh humidity rearing it's ugly head again.
Author:  cgal_1 [ Tue Feb 15, 2011 3:08 am ]
Post subject:  Re: Concave top!!
I'm in North West England.
I build in a concrete garage which has been insulated and lined with ply.
The reading on my dehumidifier itself is 35% which sounds incredibly low to me.
A separate electronic gauge reads between 48% and 52%.
If I turn off the dehumidifier the gauge reads around 80%.

Thanks for the advice, but I'm still not sure what's happening.
Charles.
Author:  meddlingfool [ Tue Feb 15, 2011 3:12 am ]
Post subject:  Re: Concave top!!
Hrkz! Dang auto correct in the iPhone...
I second, repeat second, that you should rebrace at lower RH.
Ahem.
Author:  Alexandru Marian [ Tue Feb 15, 2011 7:05 am ]
Post subject:  Re: Concave top!!
From my experience it sounds too like a top that was braced in too high humidity. It only takes some 10-15% lower and half a day to mess it up nicely.
In any case you first need to get a reliable hygrometer. Caliber III is good and cheap, except they often seem to read 5% lower than what is true. Mine reads 10% lower when very dry, 5 lower in the mid 40-55 interval, and about right in the wet interval, this checked against a wetdry test. An Abbeon/Lufft mechanical hygrometer is pretty much the nicest price/quality ratio out there.
Author:  nickton [ Tue Feb 15, 2011 5:46 pm ]
Post subject:  Re: Concave top!!
At least it's easier to remove braces before gluing your top on. You gotta look at the bright side.
Author:  cgal_1 [ Wed Feb 16, 2011 6:40 am ]
Post subject:  Re: Concave top!!
Just ordered a caliberIII. Hopefully this will help.
Am I right in thinking that if the top is flat before bracing, and the humidity is around 49% and is kept there while I brace and then fit to the rim I should be OK?

This is the second time this has happened. The first time was last year and I damaged the top getting the braces off and had to throw it.

I don't really understand what I'm doing different. I've made 5 mandolins (flat top) and 3 guitars with no problems with the tops and don't think I did anything different with each of them.

Charles.
Author:  wbergman [ Wed Feb 16, 2011 7:13 am ]
Post subject:  Re: Concave top!!
If by chance your brace wood was too dry before using it, this could happen.

I'm not as dexterous as some people, so when I had the problem you have, I removed the braces by trimming gradually with a chisel and leaving a thin amount of the brace still on the top, so as not to accidentally damage the top. It worked fine.
Author:  arie [ Wed Feb 16, 2011 3:29 pm ]
Post subject:  Re: Concave top!!
i'd suggest stripping off the bracing and doing it again. get decent hygrometers and calibrate them. let your bracing and top acclimate to the proper rh and do it again.
Author:  runamuck [ Wed Feb 16, 2011 4:48 pm ]
Post subject:  Re: Concave top!!
cgal_1 wrote:
Just ordered a caliberIII. Hopefully this will help.
Am I right in thinking that if the top is flat before bracing, and the humidity is around 49% and is kept there while I brace and then fit to the rim I should be OK?

This is the second time this has happened. The first time was last year and I damaged the top getting the braces off and had to throw it.

I don't really understand what I'm doing different. I've made 5 mandolins (flat top) and 3 guitars with no problems with the tops and don't think I did anything different with each of them.

Charles.


When you say your top is concave, which side is concave - the braced side or the top? My guess is the top.

The concave side is the relatively dry side. If you used a go-bar deck, your braced side was exposed to humidity while
the glue was drying.

When you say that your top was flat before bracing, was it flat while stickered, or laying on your bench
with one surface exposed to the humidity? If you take a dry top and lay it down on a bench in a humid environment,
it will concave against the bench.

My suggestion at this point is not to plane off the bracing just yet. Hang it up for 24 hours or so, so that all surfaces are exposed to an equal environment. It may flatten out.

But if you've glued your bracing on and the humidity was indeed quite high, you're going to
run into problems later which would require you start again.
Author:  cgal_1 [ Wed Feb 16, 2011 5:55 pm ]
Post subject:  Re: Concave top!!
The top surface is concave.
I've just read an article about calibrating a hygrometer using salted water in a sealed bag. I'm trying this out with mine to see what sort of reading I get.
I did use a go bar to brace the top.
It really feels like a black art at the moment!

Charles.
Author:  cgal_1 [ Thu Feb 17, 2011 9:45 am ]
Post subject:  Re: Concave top!!
Don't believe it!
I've just received a mahogany back and sides set and the back is curved!
This problem is driving me mad. I've now separated the back half plates, the front half plates (which have been a few months in my workshop and are still flat) and the braced plates which are still curved and placed them vertically so that air can get to both sides. The braced back is still on the go bar deck, I can bring myself to take it off in case it too bends.

The salt and water gave a reading of 71%, so I'm guessing my current meter is reading about 4% low.

Based on this, the lowest I can get my workshop to is 53%.

Would putting in a second dehumidifier help?

I feel that I'm grasping at straws which are moving further away from me.

Thanks for all the suggestions, any more gratefully received.

Charles.
Author:  arie [ Fri Feb 18, 2011 1:01 pm ]
Post subject:  Re: Concave top!!
cgal_1 wrote:
Don't believe it!
I've just received a mahogany back and sides set and the back is curved!
This problem is driving me mad. I've now separated the back half plates, the front half plates (which have been a few months in my workshop and are still flat) and the braced plates which are still curved and placed them vertically so that air can get to both sides. The braced back is still on the go bar deck, I can bring myself to take it off in case it too bends.

The salt and water gave a reading of 71%, so I'm guessing my current meter is reading about 4% low.

Based on this, the lowest I can get my workshop to is 53%.

Would putting in a second dehumidifier help?

I feel that I'm grasping at straws which are moving further away from me.

Thanks for all the suggestions, any more gratefully received.

Charles.


...I've just received a mahogany back and sides set and the back is curved!
if the material is oversize you could plane the curvature out by flip/flopping the wood as you plane/sand etc... once your humidity issues are resolved. you didn't say how much -do you have a pic?

...placed them vertically so that air can get to both sides
IMO this isn't a good idea unless it is hanging freely. the weight of the wood when standing up vertically upon something can buckle the plates. it's said that Romanillios did this on his clothsline out in the yard

The salt and water gave a reading of 71%, so I'm guessing my current meter is reading about 4% low
the salt test should give you 75% rh. adjust your hyrgometer.

Based on this, the lowest I can get my workshop to is 53%
is this adjusted or actual? if actual it's still a bit high IMO

Would putting in a second dehumidifier help?
not knowing your shop layout, sq ft. area, local weather, and the capacity of your current dryer then it couldn't hurt to try -if so far your best efforts yeild 53% actual RH

if you can, it's really, really important to get the RH under control first IMO and then it'll be one more variable out of the way. if you can do this and are still having problems, then we can take a further look into what's going on

good luck!
Author:  cgal_1 [ Fri Feb 18, 2011 7:15 pm ]
Post subject:  Re: Concave top!!
Starting to see light at the end of the tunnel.

Managed to get the humidity down to a reading of 42%, which is probably about 47%.

The new mahogany has straightened out after overnight at this humidity.

I removed the bracing, and the top went flat at the above humidity, so I re braced and have attached it to the rim. It seems ok!!!

Going to use the mahogany for an octave mandola based on the M'cDonald bazouki book.

Many thanks for all the advice.

Charles.
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