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Hey folks...

It's been my understanding that the RH while jointing panels is not so important. The logic being that both pieces will expand and contract lengthwise at the same rate, so no bother.

Here's the gist of my query. Currently my wood storage area is at around 50% RH. I build at 40%. Panels that were perfectly flat after jointing and glueing in the storage room twist ever so slightly when taken to the 40% room.

It's nothing that bracing won't easily take care of, but it's one of those little niggly things I like to torment myself with.

Do you worry about RH while jointing?

Just bring the set you intend to join into to the shop about a week before you join. But typically the movement you're talking about should flatten out once the entire set acclimates. Regardless, I wouldn't get too worried about a little twist, "little" being the operative word.

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Aaron Craig

"The logic being that both pieces will expand and contract lengthwise at the same rate, so no bother."

I think this is a safe assumption when the pieces are vertical grain, but if the wood is plain sawn, as some back sets are these days, I think there may be some stresses induced by humidity changes. For those sets gluing at the recommended humidity levels might minimize those stresses (IMO).
When gluing center seams of tops and backs I hang the panels so the air circulates on both sides of the panel which allows both sides of the joint to dry evenly.

Ed: Just a few thoughts. Have glued some panels without concern for RH that have not turned into future problems ,but have had others that don't work so well. They will not lay flat and they can be flipped back and forth as if there is internal stress in the glued up panel. This has lead me to think that shrinkage across the width can vary along the length during RH changes. With this in mind I don't glue up panels until they have settled at the RH that all other gluing will be accomplished.
Tom

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A person who has never made a mistake has never made anything!!!

That's kind of where I'm starting to lean. Why not just put it in the RH room for a week before jointing, eh?

I,too, didn't bother about the RH when joining panels, and still don't for the tops, but have had one back that, when in low humidity, is perfect, but in high humidity, a sort-of hollow develops in the back near the tail block......

Disappears completely when dry times arrive aagain, but it worries me a tad....so, now only dry backs.

Wood has virtually no movement along the grain.

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John

Which is what I had thought as well. But it begs the question, why does wood perfectly flat at 50% Rh stay permanently cupped or twisted at 40% RH after joining?

I suspect it's the run out. At any rate, I would always glue them at 40%. But, maybe store them at 50%.

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Pat

as John pointed out wood doesn't move along the grain but only moves across the grain. Why a plate moves has to do more with the top layer of cells. These absorb and loose moisture more efficiently than the cells below. The deeper the cell the slower the moisture will absorb or shed.
If you think of it as layers this may help. Now when you look at the wood structure you have hard cell and soft cell. The softer cells will expand and contract. So this is the reason that wood won't lengthen the hard cell won't move but will allow the soft cell to push and pull against it.
On perfect quartered wood this happens at a more steady and uniform rate , you will see it cup on the dry side and bow up on the wet. Once flipped it may rather quickly balance. Now to add some more confusion when you see the wood that would be more rift or flat , the soft cell can now twist the plate by the way the hard cells are and you can now see that the board would be able to move in different configurations.
Some woods are more stable than others, air and kiln dried wood will also act different.

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John Hall
blues creek guitars
Authorized CF Martin Repair
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