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Help Needed: Glued Wood On Top Of Guitar Body Expanding/shrinking...


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Hey guys,

I have run into a serious problem with my Widow guitar project. I used 30mm thick Beech wood for the body, and a 10mm layer of Birch wood on top of it.

This summer I finished all the routing, cavities, holes, neckpockets, etc. The body was sanded and ready for primer soon.

I haven't had time to get any more work done on it the last few months but when I looked at it again here recently, I discovered to my horror that the top layer of birch has expanded / shrunk in its place. On some sides, it is sticking out 1-2 mm, while other places it goes in too far. :D

Talk about a bummer... I thought for sure the wood I bought was pre-dried and ready for use when I bought it. Appearantly, it still had humidity in it and is still "settling" (or whatever term you use).

How do I deal with this? Should I wait for the wood to "settle" to its final size and then try fixing the problems with re-sanding, filler, etc.? Can I dry it out to accelerate this process somehow? In either case - this sucks pretty hard. :D

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Well, unfortunately you learned a big lesson the hard way. That is why people let the wood sit in there shop for a few months or more to acclimate, and especially if they do not know the humidity content.

I don't know the expansion properties off the top of my head for those two woods, but I am sure there have been builds on here that have used stuff with a much different ratio than those two have. What type of glue did you use? I can't really see wood shifting like that unless the glue you used allows for the wood to creep, which is a bad thing wether the wood was dry or not. If it expanded/contracted the same all around, I would say that it was the wood, but with it moving in different directions, I would say that it was the glue allowing a little movement.

Where is you shop located, and how much does the climate swing in there? At this time I would probably let it sit a few more months and wait to see what the wood does and if it moves any more before trying to correct it.

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This is a little bit of info on expansion and contraction rates for these two species, as they relate to flat or quartersaw orientations. This information relates to wood that has been dried to 14% or less and accounts for increases and decreases in moisture content due to seasonal type changes(or possibly other factors that could raise or drop moisture content).

Beech;

radial- .00190"

tangential- .00431"

Birch;

radial- .00256"

tangential- .00338"

As an example of how this could be used. Say your Birch top is quartersawn(radial relation across the widest point on your lower bout), and the Beech body is flatsawn(tangential relation to the lower bout width). For the sake of a calculation we will say the woods all drop 2% in moisture content(say 10% drops to 8%), and your widest point on the bout is 13".

Birch top shrinks;

13" x .00256"= .0333" x 2(2% drop in moisture)= .0666"( about 1.7MM shrinkage)

Beech Body shrinks;

13" x .00431"= .056" x 2= .112" (about 2.8MM shrinkage)

Note; the longitudinal shrinkage is a very small amount and will be a tiny fraction of the radial or tangential. The amount of shrinkage and expansion at different widths will vary, and areas that are oriented with the longtitudinal will be slighter. You will also not likely have perfectly quartered or flat orientation so the numbers will vary with averages of these factors. So you will not see a perfectly even expansion or contraction all around the body. I picked a random percentage of moisture for the example. It is possible the wood was at different moisture contents when you glued them, this could make the relative change between the bits of wood even greater.

Just some food for thought,

Rich

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