Wedged tenons are stronger with sloped mortises
Dustin Jackson, Pocatello, ID
Use a morrtise with sloped walls, because the resulting joint has a mechanical advantage over one made with a straightwalled mortise. By wedging the tenon, pushing its sides out and against the sloped walls of the mortise, you essentially create a dovetail, which locks the tenon into the mortise. However, if the mortise walls are left straight, then the wedge simply creates an extremely tight-fighting tenon.
When it comes to sloping the walls and making the tenon, I don’t fuss with angles in degrees. Rather, I make the outside opening of the mortise 1⁄8 in. longer than the inside opening. The walls on the inside just slope from the longer opening to the shorter one. I then use a handsaw to cut a kerf 1⁄8 in. from each edge of the tenon. The wedges should be as wide as the tenon and a hair…