Expert Answers: Avoiding Splits with Wedged Tenons

Q. I was making a three-legged stool with round through-tenons attaching the legs to the seat. When I drove a wedge into one of the slotted tenons, the seat split. I was able to repair it, but I don’t want to risk more damage. How do I get a tight wedged joint without splitting the seat again?

Carl Benson
Casper, Wyoming

A. It’s a good bet that the split in your seat was caused by orienting a leg tenon slot parallel to the seat’s grain, rather than perpendicular to the grain. Consider the difference between driving a chisel with the grain in a board, and driving it into the wood at a right angle to the grain. It’s a similar principle here; wedging slots in through tenons should always be oriented perpendicular to the grain, as shown in the drawing. 

There are a few other details that deserve close attention when installing through-tenons in seats, benches, and stools. For starters, make your tenons 1⁄8" – 1⁄4" longer than the seat thickness. Drill a 3⁄16"-dia. hole through the tenon where the slot will end—usually 1⁄8" – 1⁄4" from the tenon’s shoulder. Then cut the slot (I use the band saw) from the tenon end to the drilled hole. The hole helps to dissipate the wedging action and keeps the leg from splitting. 

Take care to size your wedges properly. I make all the wedges for a seat from a single strip of wood as wide as the tenon’s diameter and long enough for all the wedges you need plus about 6". Resaw the piece so its thickness is about twice the width of the slot (typically a heavy 1⁄8"). Taper both faces on both ends of the strip using your disk sander, going from a near-knife edge to full thickness at a point along the strip equal to the length of the tenon. This will yield two wedges. Cut them off and repeat until you have enough wedges for all your joints. Apply glue to the tenons and slip the legs in place with the slot perpendicular to the grain. Then apply glue to the wedges and tap them home. A wooden mallet is less likely to split wedges than a metal hammer. Drive the wedge until moderate hammer blows no longer advance it. When the glue dries, cut off the protruding tenon and wedge with a flush-cut saw and sand everything smooth.

Ken Burton
woodworking teacher and author

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