A request for advice on bracing

[Adam Savage]

Greetings folks,
Its been a while since ive posted here (life and lack of building gets in the way!), but im working through a ukulelre build that's progressing nicely (ish), but ive hit a snag and id like a bit of advice where to go next.
The issue is that the arch I put on the back (hornbeam, with 3 horizontal spruce braces) has returned to effectively flat now that i have started carving the braces. So far, i have tapered the ends and started to round off tge edges.
I wonder what might be the best way forward?

Is it likely to be a problem if i do nothing?
Should i selectively reduce the height in the central third of the braces to try and relieve tension?
Remove the braces completely and re-do? (Really not keen on doing this as there isnt much thickness in the back to play with, and time is of the essence (its a christmas gift))

I am building in the fashion of Cumpiano, no outside mold or radius dish. The braces were radiused using a template and router prior to gluing. The glue up using a flexible backing slat as a caul. The hornbeam i am using is not quite quatersawn, and is about 2mm thick. To (slightly) complicate matters, the ukulele will be a Christmas gift for my 6 yr old son, and has 2 necks (baritone and guitarlele).

Your thoughts are appreciated 
Adam

[Mario Proulx]

Chisel off the braces and re-do. Total time will be under 30 minutes...

[Adam Savage]

Thanks for the advice Mario, deep down i thought that's what I'd have to do but it's always good to hear another voice confirm.
I assume that it's a purely mechanical procedure - chisel as much off as possible then scraper to remove any glue residue (titebond)?
However, i think you overestimate my skill level if you think i could do it all in 30 mins! An hour, maybe....

Cheers,
Adam

[Alan Carruth]

Usually the arch won't change unless the humidity has since you glued on the braces. As Mario says, the better part of valor is to take them off and start over.

[Adam Savage]

Alan - thanks for your input. I can't see how the humidity would have changed much. I have been keeping the wood (both un-processed and in glued-up form) inside my centrally heated house and taking it briefly into the garage to work on before bringing it back inside. Also, the top (spruce at 2mm) has been braced with spruce from the same billet as the back bracewood, and it appears to be stable.
Perhaps it is the hornbeam?
Maybe I should bite the bullet there too, remove the reinforcement strips (3 of them as it is a 4-piece back) and flip it over if it has a tendency to curl? Accepting a slightly less 'pretty' surface and a little less thickness once glue has been removed?

Cheers,
Adam

[Barry Daniels]

No, the fact that your braced plate warped is positively evidence that you had a change in relative humidity. Nothing else would cause this. You have to monitor and adjust relative humidity to keep it stable and have any chance of keeping these thin plates of wood stable.

This is a hard lesson that EVERY new guitar builder goes though.

[Adam Savage]

Barry,
Thanks for your input too - i am sure that humidity is the answer, but i dont understand how one plate has moved and the other hasnt when the bracewood is the same and theyve been kept primarily in the same, stable (centraly heated) environment. I like to understand the 'why' as well as the 'what'. Also, im not sure what i can do to change things - my workshop will remain an unheated garage, and i keep the wood indoors as much as i can.
I live in an area (central Scotland) that generaly doesnt experience major temperature/humidity swings. Perhaps an accurate hydrometer is called for? Would you have any recomendations?
Or would a bi-strip of wood made in the workshop suffice?

Many thanks,
Adan

[Alan Carruth]

Adam Savage wrote:
" ...i dont understand how one plate has moved and the other hasnt when the bracewood is the same and theyve been kept primarily in the same, stable (centraly heated) environment."

Assuming you're talking about the top and back of the same instrument, it's most likely due to different expansion rates with humidity change for the different woods. These can vary widely, both between species and even between cuts of the same species. If that back is flat cut it probably has a higher expansion with a rise in humidity than it would if it were quartered. That's one reason we tend to use quartered wood, even when, as is usually the case with hardwoods, the cross grain stiffness is not that much different between quartered and flat cut.

Central heating is actually a major thing that contributes to indoor humidity variation. If the outside temperature is falling, the indoor humidity will drop, even if the outdoor humidity stays the same .Cold air can't hold as much moisture as warm air, so as things cool off outside the amount of moisture in the air is reduced at the same R.H. You bring in that cooler air and warm it up, and the indoor R.H. is lower. In the days before central heating people bundled up more indoors, didn't heat as much, and the indoor humidity stayed more constant. That's why we comfortable moderns have so much more trouble with instruments cracking.

There is an amusing passage in Mace's "Music's Monument" where he talks about the proper way to maintain your lute. He advocates putting it under the covers in your bed when you get up in the morning, since the humidity in the bed will be a bit higher than in the room. He does warn that he knows of several instances where lutes have been ruined by people 'tumbling on the bed' because they didn't know the lute was there.....

[Bob Gramann]

I don’t know if Amazon provides this hygrometer (https://smile.amazon.com/gp/product/B01 ... UTF8&psc=1) in the UK, but it is relatively accurate (once calibrated and it can be calibrated) and inexpensive. I found out about them from a post on another forum. I bought a few so I can give them with my guitars to my customers. I have a couple of expensive Abbeon hygrometers (which I calibrate annually) for my own use. These particular cheap, electronic ones track the good ones pretty well. I build in the 40-43% humidity range. I have a sealed workshop with a dehumidfier controlling the humidity. Once you can measure your humidity, you may wish to close off a room and put a dehumdifier in it (with an accurate humdistat) for your wood storage and building. When I was first starting, after I learned about humidity, I would only glue when the humidity was right. Usually, that meant only doing bodies in the winter.

[Chris Reed]

Barry,
Thanks for your input too - i am sure that humidity is the answer, but i dont understand how one plate has moved and the other hasnt when the bracewood is the same and theyve been kept primarily in the same, stable (centraly heated) environment. I like to understand the 'why' as well as the 'what'. Also, im not sure what i can do to change things - my workshop will remain an unheated garage, and i keep the wood indoors as much as i can.
I live in an area (central Scotland) that generaly doesnt experience major temperature/humidity swings. Perhaps an accurate hydrometer is called for? Would you have any recomendations?
Or would a bi-strip of wood made in the workshop suffice?

Many thanks,
Adan

The brace wood has barely moved - it's the main plates which have expanded or shrunk across their width.

I find a bi-strip of wood works fine for UK conditions.