A Lesson in Butterfly Keys
I want to reinforce a checked board with a butterfly key. What’s the best way to lay out and install them?
Gordon Echols, Lynchburg, VA
A butterfly key will not only help prevent checking, but they also add interest and beauty. Because the keys you need are structural, plan to make them 1/4 in. to 3/8 in. deep. If the checking is very serious, put keys into both sides of the board. Longer keys provide more strength, but keep their overall length under 3 in. to avoid problems caused by wood movement in the keys themselves.
I cut keys on a bandsaw, but you could use a handsaw. Either way, plan to use a chisel to clean up the point of the V at the center of the key. If you bevel the sides at a very slight angle, the key will be easier to install.
Hold the key in place with a steady, firm hand and mark its position on the board with a knife. I use a small router to hollow most of the recess, because that guarantees the key will sit on a good, flat surface. I use a 3/16-in. straight-cut bit and move the router from right to left—climb-cutting to keep the bit from pulling past my layout line. Clean up the sides and corners with a chisel.
Test-fit the key. I like to be sure that at least half its thickness will fit in the recess, and if it sticks, I gently pry it free with a knife or chisel. Trim and pare as needed for a snug fit. When you’re sure the key will fit all the way, put glue in the recess, hammer the key home, and then plane it flush.
Gary Rogowski is a contributing editor
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