Domed Back Problem

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Jerry Bassett

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Recently I tried building a tenor ukulele with a domed back. Braces were sanded to 15ft radius and glued to back using a 15ft radius dish. However after drying back did not maintain radius and was actually flat. Braces were 1/4" by 3/8" and back was sanded to .08in. What needs to be done to get domed back?
 
To follow up on Chuck's advice, it is likely the humidity dropped in your shop after you glued up. A hygrometer/thermometer combo is what I have, and I bought 3 of them. Gotta keep track of it and get control over it.
 
We are in the heating season and the humidity has been in the low 30s and pretty steady.
 
Jerry Bassett said:
We are in the heating season and the humidity has been in the low 30s and pretty steady.
Click to expand...

The humidity levels in your shop should stable at 42-50% to have the correct environment. The 30's are too low. The wood should be at 6% moisture content. The back struts should be quartered and follow the same rules as the back plate.
 
Jerry Bassett said:
Recently I tried building a tenor ukulele with a domed back. Braces were sanded to 15ft radius and glued to back using a 15ft radius dish. However after drying back did not maintain radius and was actually flat. Braces were 1/4" by 3/8" and back was sanded to .08in. What needs to be done to get domed back?
Click to expand...

Aloha Jerry,
Interested to know what you meant by the "However after drying" comment. After what was dried? Can you provide more context? Did you literally "dry" your instrument? Sorry...it just hit me you might be talking about the glue drying. But I'm not gonna delete this just in case you meant something else.

donovan
 
I have had that happen when I did not keep the body in the form during glueing both back and top to sides. Also maybe your back bracing was a little too light.
 
To me it sounds like the bracing went on a back that had a higher moisture content than what it had when the RH dropped to 30% or so. That will certainly flatten it out. If you increased the RH then that dome would come back in.

It's a hard lesson to learn, but having a controlled environment is absolutely everything when it comes to building instruments. The swings in RH between morning and noon will be significant enough to give you grief.
 
Allen said:
To me it sounds like the bracing went on a back that had a higher moisture content than what it had when the RH dropped to 30% or so. That will certainly flatten it out. If you increased the RH then that dome would come back in.

It's a hard lesson to learn, but having a controlled environment is absolutely everything when it comes to building instruments. The swings in RH between morning and noon will be significant enough to give you grief.
Click to expand...

Totally agree with everything you've said Allen.
 
As some of you know I put the dome in my one piece soprano backs by "heat forming"...95% of these are a success.. But! one thing I've noticed is that now and again, some only want naturally to go one way:confused:..when I take them out of the mold they instantly try to revert back to flatness as they cool..so I turn them over and put them back in for another session..then! they are usually happy and come out nice and domed just like I intended :D...This is just another factor to take into account ..I have seen old guitar bookmatched backs where one half flattens and the other half stays domed resulting in a split down the middle.
 
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