When Would I Use a Haunched Tenon?
I'm looking forward to using my new mortiser, and am curious about haunched mortise-and-tenon joints. When should I incorporate a haunch in a mortise-and-tenon joint?
—Jim Morelli, Boston
Simply put, Jim, a haunch—that extra bit of material atop a tenon—gives a tenoned rail more resistance to twisting. It also adds a bit of extra gluing surface, which never hurts. Any large project assembly, including paneled frames and doors, benefits from haunched tenons, as do frequently stressed joints, such as chair and table legs and rails.
To appreciate the contributions of a haunch, it helps to compare a haunched mortise-and-tenon joint to similar joints. For example, a typical mortise-and-tenon joint proves plenty strong for many applications. But if you make the tenon extra wide for twist resistance, there may be precious little material at the end of the stile to prevent breakout [Drawing 1]. A haunch preserves twist-busting tenon width, and sufficient breakout-deterring material at the end of the stile.
For light frames, such as small cabinet doors, the trusty stub-tenon-and-groove joint [ Drawing 2] proves sufficiently strong. It's also easy to make, and fortunately, you can use it to make bigger doors, too. Simply incorporate a haunched mortise-and-tenon at each frame corner. To do that, cut the panel-holding grooves as always, but leave the rail tenons extra long. Then cut mortises in the stiles to accommodate the tenons, and leave haunches to fill the gap at the end of each stile [Photo below].
When planning a haunched tenon, remember the "rule of thirds" proportioning shown above. Your joints will be strong for years to come.