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I have a vexation I wish to share...
I have noticed for almost all of my builds that the back dome after bracing and assembly, almost always flattens out much less than the 15' radius I put in the braces - to the point where most of my backs are flat when checking the lower bout brace after assembly.
My woods are well shop seasoned for RH, and I build between 35 and 45 Rh.
Most alarming is my current build, using some $$$ ziricote, and after I braced the back, after taking it out of the radius dish, it developed a strong reverse dome....to where the back was 1/8" concave....(what the ?!?)
My hypothesis was the fish glue and fastidious damp mopping cleanup caused this, so I let it sit for a week to adjust - no change.
So I removed the braces and started again, this time with no damp cloth glue cleanup, wax paper over the radius dish, etc....
All seemed perfect - when I joined the back, a lovely dome was present.
2 days later - flat as a pancake across the lower bout - everything still in the mold.

Any suggestions?

Rob

1. Check your hygrometer.

2. Build at a lower humidity. Then when you raise the humidity you will get more doming.

Hi Rob!

You know what you are describing to me sounds like RH issues and I know that you indicated that your RH was in a good range but what I wanted to ask you is what kind of hygrometer do you use?

Digitals suck and regardless of what some may believe we have purchased and tested most of them out here/there and they just don't give accurate readings. Some of the better ones such as the Caliber three will read fairly accurate in the 40 - 50 range but outside of this they can be 10% off and more... And with an older battery we have tested Caliber III's to be over 15% off in the 40 - 50% RH range. I have one now in my shop in the city that is reading 30% at present when the standards that we use indicate 46% RH.

For us the only way to know what the RH really is give or take a few percent is a sling cychrometer (spelling.... ) or similar type device that is then used to calibrate an Abbeon.

From your post it sounds like you are doing everything correct so I suspect your hygrometer.

Also RH is very dynamic and constantly changing in small amounts simply by people moving around, breathing, etc. Since we can't stop moving... or breathing we seek averages in our range of desired RH.

PS: Even though we build to a specific radius things change, wood moves, etc. but the dome is still present unless the instrument dries out. This time of year we see lots of cracks, no jokes please and no we don't repair refrigerators... because our forced air heating systems are now on drying the heck out of anything wooden and people as well. This also leads me to believe that you are doing the right things but something that you rely on, the hygrometer... is not reading accurately. You can do wet bulb test with two lab thermometers, some gauze, and something to move air such as a shop vac. The tables can be found on-line. Wet-bulbs are very accurate if done correctly and a few times in a row. Perhaps get yourself some lab thermometers (the mercury ones are the best but harder to find) and see how your hygrometer stacks up?

Hope this helps.

I run RH to 35 - 40% when I glue braces.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Once the back is braced it should be stored so as to allow air to circulate completely around it. If you lay it flat with the braces up it'll flatten out.

I do all the bracing/box assembly as quickly as possible.

the assembled box is more stable.

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Old growth, shmold growth!

Sorry to hear that my friend. Do overs aren't fun.

What is the brace wood Robbie? I've only had that happen once and it was the last time I used Mahogany. I've only used Spruce since. I don't pay too much attention to humidity myself. Around 45 - 50% seems to work for me but like GW said, getting it glued on to the rims ASAP is probably wise. If possible it wouldn't hurt to leave it in the go bar deck with one bar in centre to keep it shaped until you can get it glued to the rims.

Best of the Season,
Danny

After bracing at 40% RH, I put tops and backs in a thick plastic bag to insulate from room RH changes. The dome stays put for a long time that way.

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David Malicky

Yes, Robbie, It is all good info here. Most importantly is that the back needs to come off the radius dish after gluing the braces and allow air to get to both sides.

Since you control your humidity well you will only need to make sure as the water comes out of the glue that both sides of the plate also equillibrate to the environment. It is a luxury to have an accurate RH reading. The most important thing is to glue the braces during low humidity so when the RH rises, the tension will cause a higher dome and when the RH goes dangerously low, it will flatten but not too much or to the point of cracking.

You will be better off by building a jig to keep the dome while letting the air get to both sides. A simple plywood board with the center cut out will be good. Clamp with spring clamps the perimeter and let it dry. Then for a final test, put it in the heater room or a hot area or lower the shop RH with a dehumidifier to test it. A more complex jit that has a post in the center of the dome is perhaps an upgrade, but I tend to want to see where the dome will go without it.

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Ken

Great input all, much appreciated!
I have two mechanical hygrometers, a Barigo, and a Lee Valley unit, that I calibrated using the salt water in a bag routine.
It is quite cold here, which is a good thing, as I have a killer Honeywell TruSteam humidifier that can pump up to 18 gallons daily of H2O into the forced air feed if need be.
Meaning I can control the humidity - when I assembled this particular back, I kept the humidity stable at an indicated 35 percent - the lowest I let it get to.
Danny, the bracewood is Lutz spruce, which has been seasoned in my shop for over 3 years, and cut to nominal brace dimensions a year ago.
I think I will set up a sling psychrometer as Hesh suggested to be sure of my readings, and try some of these other techniques (plates into a bag after glue-up)
Note in this case, the back had a nice dome on assembly (with the sides firmly clamped in the mold) and it lost its dome thereafter....not concave just flattened out...with the sides still in the mold....and with constant Rh

Just a quick counterpoint -- I have never had problems with my plates flattening or losing their radius. Once I left an unattached top and back for months at a time while I worked on a different client's guitar. I use a 25' radius and 15' radius. As long as the RH of the environment is stable, the plates should be stable. If not, then you have issues present that should be addressed. In other words, why is your bracing not yielding a stable structure. If anything, this should be figured out before you assemble the guitar as that will just mask the problem.

One thing I do recommend is buying acrylic radius templates - several luthier suppliers have them. It is incredibly easy to sand more or less radius into a brace during the sanding process -- the radius gauge will allow you to ensure that your brace radius is true. Lastly, I recommend using well-seasoned brace wood and ensuring that your radius dishes are indeed accurate. Otherwise, this should be a fairly straightforward process. My guess is that RH is to blame.

As far as maintaining the arch of the back, I have settled on this method. I like this method as it help resists the back flattening from the headblock rotation under force of the strings. In other words, I hope it will combat neck resets.

Toonces Eh. I think I've seen that face before but I don't want to argue about it.

Thats a nice looking approach to the back bracing. Are the braces lapped into the centre strip/brace?

Best of the Season,
Danny

HI Danny,
The centerstrip is 1/8" x 1/2" and is unbroken for its entire length. This really helps with maintaining the back arch and was my first step on the road to my current bracing pattern. It still wasn't rigid enough for my tastes and so I added a modified floating brace (inspired by Jeff Traugott's back bracing). The floating brace is capped to ensure that the ladder braces do not lose their strength. The back radius is perfectly maintained with this system and if anything, my sound has improved.

The only tricky part of making an instrument this way is getting the slots cut in the ladder braces (both for the floating brace and for the back centerstrip). I radius my braces first and then put them all together in a jig where I make a cross cut with a router -- this allows me to get perfect alignment so the ladder braces fit snuggly over the centerstrip and floating brace. It is definitely more work but I do feel this system will strong assist in maintaing a stable headstock (and thus, a stable neck geometry).

Best Regards,
Simon

It almost seems a shame to cover that with a top, Simon!



Alex

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"Indecision is the key to flexibility" .... Bumper sticker

Mr. Thecat,
I like the taller center strip (brace), but why not carry it into the heel block (morticed, with a fillet)? My crystal ball sees a dimple between the heel block, and the center brace.

Hi Robbie,

I once had the same issue, but I want to mention first that I don't see how it's possible for a back to maintain, say, a 15' radius if the bracing is contoured to 15'. The back wants to flatten out and the arched bracing is counteracted to one degree or another, depending on how stiff the wood for the back is.

More to the point: when I used to encounter the same problem you're describing, I was sanding the contour of the sides in a dish while held in a form, but not using the form when gluing the back on.

I build with fairly thin plates -- Rosewoods in the range of 0.075" to 0.085" (depending on density) and less dense woods like Walnut and Mahogany are still built under 0.090". I guess it is possible if you all are using very thick back plates for the braces to lose a bit of their radius; however, my 15' radius is perfectly maintained after the braces go on. With all things lutherie, the process and dimensions all play an important role. And yes, you should absolutely be gluing the braces to the plates in a radius dish.

Eric, regarding the bracing -- the structure is incredibly stable as is. I already spend a great deal of time with the back bracing in its current form and I prefer to keep the process as simple as possible. I personally don't see any tangible benefit for carrying the centerstrip into the headlock and tailblock. It certainly wouldn't hurt the structure but I already feel I am pushing overkill with what I've got already. FYI, the are no dimples - otherwise, 99% of the guitars in the world would manifest that problem as there are very few guitars with the centerstrip inlet into the blocks. The key thing for avoiding bumps/dimples is ensuring that the radius and geometry of the rims is perfectly matched to your plates -- if you do that correctly, then you can get by with a far less rigid structure than I use and have absolutely no problems -- bear in mind, the vast majority of the guitars on the planet are built with methods that produce a much weaker structure.

As an aside, whenever I have looked at following an unusual path concerning bracing or structure, I have always gone back to the Martin template and compared my designs to the Martin paradigm. The standard bracing pattern and body structure works very well. If your designs increase the strength of the structure then you will not have issues. The problem begins when you try out something radically different and don't have a way to compare -- in those cases, you just have to build it and then carefully examine the structure under string tension. And so, when evaluating my back bracing (deeper headblock, double sides, floating braces, etc.), it should become apparent that the structure will perform its role at least as efficiently as your typical instrument. Hope that helps.

Robby I want to reiterate a point. It doesn't seem possible to me that your relative humidity is constant. I do not rely on hygrometers and therefore must rely on where I feel the relative humidity is at the particular time. This works extremely well and I have sent guitars all over the country as well as other countries. It seems to me that you're going about it the opposite way that I would think is a good way to do it. I particularly would want the relative humidity low when I do the bracing. from then on I would raise the humidity or put the instrument in an area where the humidity is higher. If your instrument is anywhere near your humidifier then it could possibly be getting higher or even wet from the droplets. But I think you get my drift here. There's no possible way that the back can go from domed to flat unless the room relative humidity goes down.


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Ken

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Ken

No, if he glued the braces down, then immediately wiped everything down with a damp cloth, the wood would have changed dimension before the glue had set.
then when it dried out it would have changed back, constrained by the bracing.

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Old growth, shmold growth!

I have a sling psychrometer you can borrow if you like, I'm in the SW.

I think it was about 70 bucks at acklands.

It's inevitable that one needs to clean-up the squeeze out. I always equally dampen both sides of the wood. Wood wants to cup, or concave to the dry side.

Bingo!
I believe that was the evil of my ways.
By aggressively cleaning up the fish glue squeeze-out with a damp cloth, the wood "would have changed dimension before the glue had set"
Very much a very local RH problem.
When I re-did back bracing on my current project that had previously exhibited this behavior to the max, with no damp cloth wipe down, and damping the other side a bit just before glue-up, using wax paper on the radius dish instead of newspaper, all is fine.
Thanks everyone for their comments, much appreciated!

I get very minimal squeeze out. The squeezout I do get is just little balls that ooze out.

i wait until they're just beginning to harden and lightly scrape them off.

_________________
Old growth, shmold growth!

BTW the problem in this case was further exacerbated by the fish glue.

Fishglue has a longer open time.

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Old growth, shmold growth!

I have been watching this thread for a bit now and find some similarities in the one I am working on as well as an issue I had a few years back.

Could it have something to do with the natural way a top or back wants to bend? The spruce top I have has been sitting in my shop for a few months (no braces). The back and top were pressed between a couple of pieces of cardboard in a box under my bench. I just recently pulled them out. They were both very flat when I pulled them out but but now my top has a slight back bow. My shop maintains between 40 and 45% humidity so it is pretty steady. I noticed the top starting to bow about 2 or 3 weeks now and it appears to have stabled out. It is only bowed about 1/2" so it doesn't concern me too much.

Could it be that your top (and mine) could have the rosette applied to the wrong side? I didn't think that would be and issue but seeing my top and what you are explaining could be part of the reason they want to bow the opposite direction? We know that a piece of wood will want to cup in the direction the tree grew so is it possible that there is a right way and a wrong way to put a top and back on?

Also, one of my first brace repair was a guitar that had loose braces on the top. It was an unbound back so it was easiest to just remove the back and redo the braces that way. It was a number of years ago and I was only using Titebond and did pretty much what you described wiping it down with a damp rag. After I let it dry clamped in my go-bar deck over night. When I pulled it out it looked great but after a day the top was nearly flat. I managed to redo it and finished the job but I decided then to only use HHG and have never had the problem since.

I have never used fish glue but I know it has a long open time compared to hide glue. Could the combination of the natural direction of the wood and the combination of the moisture of the glue and wiping it down be the cause?

I am not sure if this is any help but thought I would put it out there since I see how my top is bending and thought yours may be the same.

Bob

Thanks Fillipo.
The live back braces are Gore-size - a beefy 10mm deep and 18mm high, tapering down to 3mm.
RobRus, don't have issues with the tops, just the backs.
Per your comments, as the plates are bookmatched, if the issue was with the woods, one would bow one way, and one the other (as i have seen in some spruce plates)
In this case, they bowed evenly, telling me it is surely an RH problem, and most probably the result of over-zealous scrubbing of the excess glue with dampness.
Lesson learned.