Q When and why should I use haunched tenons?
—Greg Isles, via email
A Chris Hedges replies:
A standard tenon typically has four shoulders. A haunched tenon loses one shoulder in order to gain a shortened tenon section (haunch), as shown in the drawing. There are structural and production-related reasons for using haunched tenons. Structurally, the greater overall width of the cheek on a haunched tenon increases the glue area and enables the joint to better withstand twisting and racking forces. This extra strength isn’t as important with smaller doors, but it can be very helpful with large, heavy doors and in other situations where racking and twisting stresses are a concern.
From a production standpoint, rails with haunched tenons allow you to mill a continuous groove in the mating stile for a contained panel, rather than a stopped groove, necessary for a standard tenon. Cutting the haunch requires an extra step, but simplifies the grooving process.
