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Back Braces Bending The Wrong Way
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Author:  chrispyking [ Mon Jul 27, 2015 9:26 am ]
Post subject:  Back Braces Bending The Wrong Way
Hi Everyone,

I'm looking for a little advice on how I can rectify a little problem I have come across. A few days ago I glued the braces(sikta) to the back(Tasmanian Blackwood) of a dread project I am working on. Everything went really well and the glue joints turned out perfectly. See the attached photo.

Attachment:
after gluing.JPG


Fast forward a few days and I couldn't believe my eyes when I picked up the back again. The back appears to have bent the spruce braces backwards ever so slightly. See the photos. [headinwall] . Clearly the desired radius on the back is not being achieved.... Also note I have only finished shaping one of the braces.

Attachment:
Bent braces.JPG
Attachment:
Bent braces2.JPG


Unfortunately I don't have the most advanced workshop and do not have any means of controlling humidity levels but I never thought it would have an impact like this. I live in Perth Western Australia which I always thought was hot and dry and not susceptible to large humidity swings.

My question is is this recoverable and more importantly how can I prevent this happening again? I have clamped the largest brace to a template which has the correct radius however I am unsure the brace will return to its original shape.

Any help will be greatly appreciated :)
Author:  alan stassforth [ Mon Jul 27, 2015 9:47 am ]
Post subject:  Re: Back Braces Bending The Wrong Way
Hi.
I had 2 tops do that on me,
and folks here said humidity.
Makes sense.
I now glue up things at 40 percent humidity now,
and haven't had the problem.
The top swells sideways when humid,
and the braces don't.
Also,
when humidity is low,
the top shrinks side to side,
and the braces don't.
Author:  Haans [ Mon Jul 27, 2015 9:51 am ]
Post subject:  Re: Back Braces Bending The Wrong Way
You have basically answered your own question. Humidity is the problem, and the only way to prevent it is to keep your shop at a constant humidity. Ideally, you should run around 45% and when bracing, drop it to 35-40% and then back up to 45%.
Whether your back will return to it's normal curve is questionable, better to make new braces. Also never leave a top or back flat on a surface. You need to expose both sides to the same environment.
Author:  meddlingfool [ Mon Jul 27, 2015 10:35 am ]
Post subject:  Re: Back Braces Bending The Wrong Way
Sorry my friend, but you gots yourself a problem. Until you can at least identify your relative humidity, ain't no glueing gonna go down safely.

Not that I wish to drive traffic from this site but...check out the ANZLF (Australia New Zealand Luthiers Forum). You may find some neighbors who can help you out.

In any case, it is imperative to at the very least identify the RH, even if you can't control it. It will tell you when you can and cannot do certain jobs, like bracing. It's a bit of a let down, I know, but without that you are potentially wasting your efforts and more importantly potentially wasting wood which is anathema to our cause...

Every stick is useful
Every stick is great
If a stick gets wasted
god gets quite irate;)
Author:  ScooberJake [ Mon Jul 27, 2015 1:27 pm ]
Post subject:  Re: Back Braces Bending The Wrong Way
meddlingfool wrote:
Sorry my friend, but you gots yourself a problem. Until you can at least identify your relative humidity, ain't no glueing gonna go down safely.

Not that I wish to drive traffic from this site but...check out the ANZLF (Australia New Zealand Luthiers Forum). You may find some neighbors who can help you out.

In any case, it is imperative to at the very least identify the RH, even if you can't control it. It will tell you when you can and cannot do certain jobs, like bracing. It's a bit of a let down, I know, but without that you are potentially wasting your efforts and more importantly potentially wasting wood which is anathema to our cause...

Every stick is useful
Every stick is great
If a stick gets wasted
god gets quite irate;)


:D One of those little tunes that will stick with me forever.

Also, I know the rule of thumb for bracing (< 40%). Are there rules for other gluing operations as well? Do all others need to occur at 45%? How widely can I stray from these numbers?
Author:  Robbie_McD [ Tue Jul 28, 2015 7:25 am ]
Post subject:  Re: Back Braces Bending The Wrong Way
I had exactly the same problem a while back..
After I got my Abbeon and committed to doing bracing at max 35% the issues went away forever.
Now I only do bracing and box assembly during the winter months when I can accurately control the RH.
IIRC I had to chisel off the braces on the back that went backwards, re-acclimatize and start over....
Author:  Mike OMelia [ Tue Jul 28, 2015 7:45 am ]
Post subject:  Re: Back Braces Bending The Wrong Way
The good news is that back braces are easy to remove. If bringing back to normal humidity does not work, remove them by heating with a flat iron. Insulate hardwood with a slightly damp towel. You will destroy (most likely) the braces, but that's no real issue unless you are short on Sitka. My shop (garage) has a portable AC, keeps things bearable. But I always bring the stuff indoors. My dining room table is covered in guitars! I would not use a heat gun here.
Author:  chrispyking [ Tue Jul 28, 2015 9:57 am ]
Post subject:  Re: Back Braces Bending The Wrong Way
Thanks for all the responses, all really helpful. Here is a little update on progress(or lack there of)

I stopped at the local hardware store and bought a cheapish thermometer/humidity sensor to attempt to gauge what the RH would be in my garage and also inside the house. Its actually pretty handy and has a remote sensor too so I am able to monitor both simultaneously.The garage is hovering around 75% and inside at about 65%. I am going to keep an eye on it particularly once the weather improves and make a plan of action from there.

Another surprise for me that was the back has returned to its original shape today !!! I'm assuming that this must be because it is prob a similar RH to when I glued the braces on. Unfortunately I know it is something I will still need to fix so the guitar doesn't look concave or even crack when assembled. Now its time to decide wether to chisel them off or Iron them off ;)

I am still amazed that the couple mm's of back has the strength to bend the braces...
Author:  Haans [ Tue Jul 28, 2015 10:12 am ]
Post subject:  Re: Back Braces Bending The Wrong Way
Nothing will replace a humidifier, dehumidifier and good humidity gage. Those are dime a dozen gages and could vary up to 20-30%. It is not a couple of mm of back, it is 300-400 sq. inches X 2.
Author:  Alan Carruth [ Tue Jul 28, 2015 11:07 am ]
Post subject:  Re: Back Braces Bending The Wrong Way
A few years ago I built a guitar from IRW and European spruce, with some very fancy trim. Just after I finished it I put it into a gallery show in the new headquarters of the local craft league, along with another guitar. When I dropped them off in mid-December I talked with the folks about humidity, and told them that if they started to get uncomfortable from dryness the guitars would be too. A couple of weeks later I went back for the opening of the show. I have seldom seen guitars that were more distressed: the tops looked like corduroy. There were a couple of other guitars in the show that looked equally unhappy. I asked about the humidity, and, of course, they had no clue, but did say that it had been very uncomfortable.... Argh! I gave them some money for a humidifier on the spot, and they got one the next day. Sadly, the following weekend was the Martin Luther King holiday, and nobody came in to fill the humidifier, so the fancy guitar cracked. When I went in to collect it, I showed them the damage that had been done to some other wooden objects in their permanent collection. I ended up having to re-top that guitar. When I got it home the back was about 1/4" concave across the lower bout, instead of having 1/4" or more of dome. It's a wonder that didn't crack as well.
Author:  John Arnold [ Tue Jul 28, 2015 8:47 pm ]
Post subject:  Re: Back Braces Bending The Wrong Way
Quote:
Do all others need to occur at 45%? How widely can I stray from these numbers?

No. The problems arise on cross-grain joints. For example, there is no issue with gluing a fingerboard on the neck.......since the grain direction is the same.

It's nice to have control of RH, but it is not essential. All you need to do is control the moisture content of the plates. The OP's problem is that the bracing was glued when the plates were not dry enough. You can dry the wood temporarily by warming it up. All you need is to have it dry when the braces are glued on.

On a side note, the bracing does not need to be as dry, because wood moves very little along the grain.
Author:  Mike OMelia [ Tue Jul 28, 2015 11:58 pm ]
Post subject:  Re: Back Braces Bending The Wrong Way
You live in a high humidity place. So do I. You really should just drop some $$ on anything that will control that humidity. You said it wasn't much better indoors. Do you have AC? A dehumidifier is essentially an AC unit without much cooling potential. I'd get a portable AC given where u live if it was in the budget. Btw, a natural byproduct of AC is condensation which portables do a poor job of ridding. I found a "condensate pump" at Lowes that collects the water, then pumps it out a small hose through the window port. A cheaper solution is a true window AC if you can have that where u live. I can't.
Author:  Haans [ Wed Jul 29, 2015 6:20 am ]
Post subject:  Re: Back Braces Bending The Wrong Way
Our "whole house" A/C takes too much humidity out of the air. I need to run a humidifier upstairs in the summer too...
Author:  jfmckenna [ Wed Jul 29, 2015 7:25 am ]
Post subject:  Re: Back Braces Bending The Wrong Way
A cheepish hydrometer is not good enough unless you calibrate it with something more accurate. I like the Psychro-Dyne hygrometer but a good sling hygrometer or even a high quality hydrometer would work. Relatively speaking a cheap one would be ok to measure as long as you know that when it says 75% for example it's way to high and it should say 60% then you are ok and in reality that's really 45%. Another use for it is that you can at least tell the difference between two rooms like your house and your shop. You can at least see that they are different or the same but still it's measured RH is useless per individual room.

I keep my shop at 45% year round and don't make any adjustments for any type of glue up and have not had a problem. One year I went on vacation for about three weeks over the winter. The friend we had did a wonderful job feeding the cats and the fish and watering the plants but forgot to add water to the humidifier in the shop. Oil heat and cold winters is a recipe for disaster. Silly me thought it was ok to leave a guitar out on a wall hook and the poor thing cracked right down the back.
Author:  John Arnold [ Wed Jul 29, 2015 8:43 am ]
Post subject:  Re: Back Braces Bending The Wrong Way
All you really need is an airtight box with a couple of light bulbs in it to warm it up a bit. A common name is 'hot box'.
The one Ted Davis had was about 2 feet wide, 3 feet tall, and about a foot deep. It was made of 3/8" plywood. It hung on the wall and had doors on the front. Raise the temperature to 100 degrees or so, and leave the plates in there for 2 or 3 days. A thermometer inside will help with monitoring it.
Author:  truckjohn [ Sun Aug 16, 2015 10:53 pm ]
Post subject:  Re: Back Braces Bending The Wrong Way
Yes... The best, easiest thing for a hobby builder to do is to brace indoors in a humidity controlled space in the "Winter" time when it's dry and you are running the heat. Next is to intentionally control humidity in your work area.... Garages and sheds aren't conducive to this...

If you brace up and it's more dry than "Typical" - you will end up with more dome in it when it's more humid....

If you brace up and it's more wet than "Typical" - you will end up with concave/sunk in when it's more humid...

Cut those braces off, dry things out good, and re-brace... If you don't - it will suck back in once you finish it.

Sorry.
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