No. 42. A haunched mortise-and-tenon is made by cutting away part of the tenon so that that part of it will be much shorter than the rest. The haunch gives the tenon great lateral strength and saves cutting so large a mortise hole. It is used in panel construction, as where the rails are joined to the stiles of doors.
Fig. 267-42 Haunched mortise and tenon
First plow the groove in all the members. This should be of the same width as the thickness of the tenons, which is ordinarily one-third of the thickness of the frame. The groove is approximately as deep as it is wide. Lay out and cut the tenon the width of the entire piece, minus, of course, the depth of the groove. The mortise should not come too near the end, or the portion of wood outside it will shear out. Hence the tenon is narrowed on the outside enough to insure strength in the mortised piece. The rule is that the tenon should be one-half the width of the rail, minus the groove. But enough of the tenon is left full width to fill up the groove at the outer end of the mortised piece. This is called the haunch. The width of the mortise is equal to the width of the groove, its length to the width of the tenon. Before assembling the panel frame, put soap or tallow on the corners of the panel to prevent its being glued to the frame.
No. 43. Table or taper haunching. Sometimes, as in table construction, for the sake of stiffening the rail, or in places where it is desirable that the haunch does not show, the haunch is beveled from the tenon to the edge of the rail.
Fig. 267-43 Table haunching