When to Install a Butterfly Key?

When to Install a Butterfly Key?

I have a large piece of live edge cherry that I want to make into a coffee table.  It has been cut and stored for more than 10 years. There are a few cracks opening up on the end, and I was going to use a butterfly key joint to reinforce it. I am going to ask an Amish friend to run the slab through his thickness sander because of the hills and valleys that resulted from cutting it on a portable sawmill. It is about 2-1/2 in. thick, and it might need an eighth inch or more taken off to get it smooth. Would you put it through the thickness sander first and then do the joint, or do the joint first? Does it matter? – Duane Bailey

Rob Johnstone: I would surface the slab first, then install the butterfly key. In truth, it likely is not critical to do it in that order, but my preference is to surface, then machine. Good luck!

Tim Inman: Functionally, I don’t think it really matters whether you put the butterfly in before or after surfacing. What does matter is that this wood is showing you it is full of internal stresses and is moving! Why? Wood that thick takes forever to dry and equalize all the way to the core. If this is an important project, you might consider taking out a small core drilling near the center of the wood which, if measured quickly, can tell you the equilibrium moisture content (EMC) at the center of the piece. You must be ready to test that core immediately, though! I would use an electronic moisture meter to do the test. There are other ways, but not for here. Once you know the EMC, the core can then be replaced and bonded with epoxy almost invisibly. A little art from your brush and box of pigments, and nobody (but you) would be the wiser — except you will be much wiser, and you will know what you are working with.

If the EMC is “normal” in the center as well as the outside, then I would trust the butterfly to be strong enough to hold. If not, then nothing short of steel will ever hold. Even then, the steel will hold, but the wood will just crack someplace near the butterfly and away you go again. So, put the fix in whenever you feel led, but don’t feel confident in your repair unless you know EMC is the same inside and out.

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