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View Full Version : Cupped back and sound board/ help!



Bayard
12-20-2016, 10:51 AM
So I finally getting around to building my first Ukulele. I bought a raw material kit a few years ago and all of the pieces have weathered fine.
I constructed the back and the sound board using a go go cage type of set up. No problems there. However after removing both parts with all components glued and looking great. The back and sound board have cupped?
The cupping started after I shaved down the tone bars. They are still a little rough but close to what they should be for assembly.
The cupping is happening towards the exterior mostly on the tail side.
So I ask why?

The back and sound board are Koa, extremely ugly Koa but it is my first build so I figured why not.
I am not blaming the wood

Since I was in the process of getting these panels to thickness I resawed quite a few of panels for future use. They are very close to the thickness I desire and there all dead flat after glue up.
I do not want to repeat this cupping in the future so any help would be great. Thanks in advance
Bayard

ProfChris
12-20-2016, 12:17 PM
Almost certainly you glued the bracing on when the humidity was high. Now it's lower and the plates have shrunk widthways, cupping away from the bracing as the tension has to go somewhere.

The solution is to remove the bracing (heat usually, but depends on the glue you used), clean off all glue residue, and reglue in low humidity.

If you used Titebond original, a domestic iron on its lowest setting should soften the glue. Use on the opposite side to the bracing to get good contact, don't hold it on the wood too long in case of scorching. A spatula should slide into the joint when it's softened.

Bayard
12-20-2016, 01:56 PM
Excellent
Thank you

Beau Hannam Ukuleles
12-20-2016, 02:01 PM
Ugly koa?- It might actually be the wood. The less quarter sawn it is, the more you have to fight natural warping etc.

Also, if you glue 2 bits of wood together, you have to be sure they both have the same moisture content. If one is dry and the other wet, then warping, cracking, etc happens

Matt Clara
12-21-2016, 08:49 AM
...using a go go cage type of set up.

Like the ones in the image below?
https://thewalruswaspauldotnet.files.wordpress.com/2016/02/lada-edmund-jr.jpg

spongeuke
12-21-2016, 08:52 AM
That set-up induces twisting

Matt Clara
12-21-2016, 08:55 AM
...and reglue in low humidity.

Average humidity, perhaps? I've heard 45% is good and is what I shoot for, though it goes as high as 55% in the summer and down to 40% in the winter.

resoman
12-21-2016, 09:07 AM
Purdy warped

ProfChris
12-22-2016, 01:39 AM
Average humidity, perhaps? I've heard 45% is good and is what I shoot for, though it goes as high as 55% in the summer and down to 40% in the winter.

I meant low in terms of the humidity you expect the uke to live in. In the UK I wouldn't expect to see 40% in a domestic home, so I probably glue up around 50% (my meter is two strips of veneer glued up cross grain, and when they bend to the left of my pen mark I'm ok to glue).

If I were building for someone in New Mexico I'd have to get a lot lower.