Monday, February 26, 2007
A Better Joint
The round mortise and tenon is one of the easiest joints to make, it is also one of the worst when it comes to gluing. Glue only works to bond wood with strength when the long fibers meet long fibers (as opposed to the endgrain). The round mortise is almost all endgrain making it a poor glue joint. As seasonal movement causes the tenon to shrink and swell, the glue bond is stressed and in time will fail. The through tenons that we use in chairs are wedged, to flair the tenon and create a tighter fit. One day last winter, I looked at a wedged tenon in one of my chairs around the house and noted a gap along one side of the wedge. It reminded me of something I read in Bruce Hoadley's book Understanding Wood. He noted that by splitting the tenon, we supply a plane of failure, which relieves the stress on the glue around the tenon. When the tenon shrinks, instead of breaking the important glue bond around the tenon, a harmless crack appears next to the wedge. So now, whenever I glue in a wedge, I am careful to put glue only on one side of the wedge (plenty to hold the wedge in place). The unglued side of the wedge is free to open and close as the humidity changes, a far superior result to the joint failing!
So, save some glue and make a better joint.