The classic, and a most successful, way of joining end-grain to side-grain, the mortise and tenon joint (Figure 2.25) can have hundreds of different variations. The mating parts can be rectangular (traditional mortise and tenon) or round (dowels). A well-made joint offers mechanical restraint in all directions except direct withdrawal of the tenon from the mortise and is therefore able to offer good resistance to compression, shear and racking. The basic joint can be improved in several ways. Providing a shoulder to the tenon gives additional bearing surfaces on the outside of the mortise to share resistance to compression. Pegging the shoulder increases resistance to racking and greatly improves performance in tension (Figure 2.25a). Gluing will also improve performance in some respects particularly by adding side-grain to side-grain shear resistance that will oppose rotation. A mortise and tenon joint functions like a cantilever beam, so increasing the height of the beam (normally the tenoned member) will reduce the axial stress on the joint for the same load on the beam (Figure 2.25b). For further explanation of this phenomenon see Hoadley (1980). Increasing the depth of insertion of the tenon will similarly provide an increased area for gluing. However, it will also increase the dimensional conflict caused by the opposing grain directions of the components. Arranging the grain direction so that the longitudinal grain of the tenon is matched to the radial grain of the mortise and the longitudinal grain of the mortise is matched to the radial grain of the tenon will give the best dimensional stability. Dividing the joint into multiple tenons will provide greater bonding surface and minimize dimensional restraint (Figure 2.25c-e). In some cases, such as carcase pinning, multiple tenons are preferable to a single wide joint (Figure
Figure 2.25 Examples of framing joints: mortise and tenons: (a) mortise and tenon pegged close to the shoulder of the joint; (b) increasing the height of the tenoned member will reduce the axial stress on the joint; (c) double tenon; (d) twin tenons; (e) twin double tenons; (f) carcase pinning using multiple tenons; (g) carcase pinning used on a drawer rail; (h) through-wedged mortise and tenon; (i) fox-wedged mortise and tenon
2.25f,g). Tenon depth more than compensates for the loss of width and the increase in tenon height and the number of side-grain to side-grain surfaces greatly increases the strength of the joint.
Wedged mortise and tenon joints are primarily intended to provide lateral pressure on glued surfaces and dovetail-style mechanical interlock (Figure 2.25h,i). They may also provide an effective stress relief slot that helps to preserve the glue line during shrinkage of the tenon caused by compression set arising from moisture cycling.
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