Caved-in plates

[Jerry Hoffmann]

In the span of one week I had two ukuleles--that were built and sold about two months ago--come back to me, one with a caved-in back, the other with a caved-in top and back. Both of these instruments had the normal arch I put in my plates, and the bracing is on the stiff side for the backs. One is a concert size resonator with a thicker than usual back, the other is a tenor with a thinner back. One is koa, with a lacquer finish, and quarter sawn spruce bracing; the other is mahogany, with a varnish finish and the same bracing (but possibly different stock). What could have caused this? Are the luthier Gods mad at me? One symptom I noticed was that the lacquer finish was sucked into the topography of the wood figure even though it was allowed to dry three weeks before it was sanded and buffed. Is this problem reversible?

[Jim Bancroft]

Don't Know!
But, if the wood is thick enough, .040" to .060", and properly braced, I wouldn't see a problem under normal usage.

Jim

[Jerry Hoffmann]

The concert reso back is .085 and the tenor back is about .065.

[Jim Bancroft]

Doesn't sound like you are doing anything wrong. You are using nylon uke strings?
I'm building some soprano ukes and they are thinner than yours, and I have no problems.

Normally an instrument caving in is caused from abuse, not normal use.

Jim

[Jerry Hoffmann]

Here's what it looks like. The strings are nylon. I've built about 60 ukuleles and have never had this problem. The wood should be good; I know the source for the mahogany, and it's more than 60 years old. The koa and spruce were purchased form a dealer and sat on the shelf for a year before use.
I wonder if it suddenly "popped" like a sheet of metal when it inverted

[Bob Webster]

Hi, Jerry. There's a good chance that the finish issue and the caving might not be related. My guess on the caving is that something wasn't equilibrated to the right humidity at glue up and I'm betting it was the braces. The clue is that the bracing was pretty substantial. If the bracing was over-dry when it was glued up in a radiussed dish and then allowed to come to the right humidity, it will then swell and give you that kind of bow. Just a thought. Are you using vacuum clamping for your braces?

If you were using lighter bracing, then it would be a bit easier to apply that same line of thinking to humidity of the back relative to the braces; i.e. the inner surface of the back was too dry at glue up and swelled afterward. Again, this could happen when using vacuum clamping especially if you're not using some sort of breather cloth on the other side of the back (the exterior side). I made a "spruce sombrero" that way once.

Cheers,
Bob

Cheers,
Bob

[Jerry Hoffmann]

Hi Bob, thanks for your input. The bracing wasn't vacuum clamped. I usually saw up a quantity of bracing stock to keep on hand, and the batch used for these instruments was close to being used up, so the brace stock, as well as the plate stock, had plenty of time to adjust to shop humidity. There shouldn't have been any longitudinal growing or shrinking of the braces so I guess any stress from moisture changes had to be in the plate.

[Bob Webster]

Okay..., so it sounds like the outside of the back was wetter than the inside at glue up OR it got that way during the process. Is the radius dish that you use always kept in the same area as you store the wood and do the gluing?

Reason I ask is because, in my situation, I do alot of the messy stuff in my garage under uncontrolled temp/humidity. I keep a 2X2' sheet of 1/4" MDF on my bench as a working surface that I can renew as it gets unusable. I found that if I placed a well equilibrated top set down on that surface for very long, like to cut a rosette channel, for instance, the back side of the top would pick up moisture and swell on that side. I now keep a sheet of MDF in the controlled room and it gets used in the garage for the safe backer sheet. Solved my warping problems. Same thing might be going on with your gluing form?

Bob

[William F.Eden]

The only time I ever had a problem like that is when I tryed scalping the bracing on the Back.

[FRANK]

I have seen this on wood that was not properly cured since it is such a small instrument try baking the lumber in the ovan at 120 for 6 hours slowly reduce the heat take out and keep it at room temp for a week and then build I have sped up projects that I thought would take a year before the lumber was ready

[Jerry Hoffmann]

Thanks, Paul. I'll certainly be more sure about my wood in the future, but I'm still mystified. Both the top and back of the tenor have this problem, and the top is the same sitka I used on a parlor guitar that I've played for more than a year now, and it's fine. I've decided to pop in a humidifier and tie it up in a plastic bag for a while to see what happens. It's winter and humidity is low so I'm going to assume the problem is weather related. If the humidifier dries up fast, then at least I'll know the wood is probably too dry

[Bob Webster]

Hey, Jerry. If humidity is the culprit, it's the outside of the back that is dry relative to the inside. I wouldn't say anything at this point except that I'm concerned that a humidifier inside the guitar would make the back swell so much that something is going to break.

Bob

[Laurent Brondel]

Maybe you did nothing wrong and this exceptionally cold and dry winter did it. Where did the instruments come from? The caved in plates and finish issue are typical of intense dryness. Humidifying too quickly may create problems though.