View Full Version : Concave back

11-28-2016, 11:37 AM
I wonder if anyone has had this happen to them. Awhile ago I came across a quartersawn white oak board, 3.5" wide. The rays are very obvious and it has a diagonal chatoyance that's striking. I came across some luthiers who use 3 or 4 piece backs so I decided to try my hand at a tenor guitar for myself. I used a shallow radius dish to sand the braces and then used a go bar deck to glue them. I sat it aside for awhile to do some necessary work. Recently I looked at it and the back become concave. I put it back on the radius dish but it's quite stiff. I'll just use it as is if I can figure how to sand the sides in the other direction.

11-28-2016, 12:48 PM
My guess is that you glued it up with high relative humidity. Now that the heating season is upon us, your oak has shrunk across the grain and against the braces, and this is causing it to dish. I had that happen on a nice cherry top. I am more careful about RH when I glue up now.


11-28-2016, 12:57 PM
I agree with Nelson, I had a similar experience and now always monitor RH and only glue up when below 50%.( here in UK)

11-28-2016, 01:00 PM
That sounds reasonable, thanks.

Michael Smith
11-28-2016, 01:03 PM
As others have said this is a humidity issue. It would be good to get that under control in your workspace. Perfectly quaretersawn backs will be less prone. Species also has an effect.

11-29-2016, 09:06 AM
Take the braces off. Control humidity and re-brace.

You're only asking for heart ache if you continue to build with a concave plate.

11-29-2016, 10:01 AM
There are a few banjo ukes with concave resonators..Ideal for pot bellied players :D

12-03-2016, 12:41 PM
Thank you for the response. I will re-do the bracing, I hadn't thought of that, just tried to figure how to go forward with the back the way it is now.

Beau Hannam Ukuleles
12-03-2016, 01:16 PM
Take the braces off. Control humidity and re-brace. .

yep- . even better is to cut it back in half and reglue it, just in case but it may not need that