This section is from the book "Amateur Work Magazine Vol6". Also available from Amazon: Amateur Work.
A tenon is a tongue or projection of reduced size formed at the end of a piece of wood, and a mortise is a corresponding slot or recess cut to receive it in the side of another piece. The end of the tenoned piece thus fits into the mortised piece, preventing lateral movement of the parts in relation to each other, permitting them to meet in the same plane, and forming a joint which can easily be held together by glue, pins, or other means.
Fig. 1. These joints are very common in carpentry and joinery, and in a less degree in pattern-work. They are suitable for framed articles where the members are long but compact in section. For broad surfaces they are never used, and for long-edge joints only when end grain meets side grain in which mortices can be cut; but, even then, they are rarely employed.
There are many varieties of mortise-and-tenon joint, but the differences are chiefly in the shape and proportion of the tenons in relation to the member they are formed on. There are first two important differences in the length of tenons. In one, the tenon, and consequently the mortise, cuts through the mortised piece, the end of the tenon usually being flush with the external face. In the other, the tenon only penetrates to a short distance, the mortise being correspondingly
Fig. 2. shallow. Both these forms are very commonly employed, the first being a through, or ordinary, tenon; the latter a stub, or stump, tenon. The through tenon makes a stronger joint, but there are a number of considerations which make the stub tenon quite as popular, even in work where strength is the first consideration.
The main function of the stub tenon in heavy work is usually not to hold the parts together, but to prevent their getting out of place laterally, the holding together being accomplished in some other way. In other cases, though not very often, a through tenon tion extends completely to the shoulder. The advantage of the haunch is that it prevents warping or twisting of the members in relation to each other, as completely as if the tenon extended the full width. Fig.
Fig. 3. would be so long that a shorter one would hold equally well, or quite as well as necessity demanded. Another consideration is the weakening effect of a deep mortise, which diminishes the strength of the piece it is cut in. In its simplest form an ordinary tenon is proportioned as in Fig. 1. It is made the full width of the
Fig. 4. piece it is formed on, and of one-third the thickness. When the width exceeds five or six times the thickness, it is advisable either to reduce the width of the tenon, or to divide it into two parts. The former plan often
Fig. 5. has to be adopted in cases like Fig. 2, where the mortise occurs at the extremity of the member, and it is desired to avoid cutting through the end grain. The tenon, then, even if well proportioned in itself, is reduced in width as shown, generally with a short stump called a haunch, A, but sometimes the cut-away por-
3 shows how the haunch may be tapered to nothing at the top, so that it will be invisible when the parts are together. This slightly reduces its efficiency. In Fig.
4 a pair of tenons are shown with a haunch between.
Fig. 7. In this case the piece is so wide that a single tenon running completely across would be no stronger, and would be more likely to cause trouble through warping and shrinkage than when its middle part is removed as shown. The mortised member, on the other hand, is decidedly stronger with the mortise divided than it would be with one long mortise for a single tenon. The opposite course might be adopted of having a single reduced tenon in the midle with haunches on each side,-and circumstances sometimes necessitates this; but such a joint would not be so secure as one with a tenon at each end.
A stub tenon is shown in Fig. 5 at A. Its length depends on the proportions of the parts and on other circumstances; but it is seldom made to penetrate far.
At B, Fig. 5, a double stub tenon is shown. These are employed to prevent twisting when the thickness of the parts is considerable. Being divided in this way, the mortised part is kept a great deal stronger than it would be if an equally effective single tenon was employed. A point to be remembered in cases of this kind is that the longer way of the mortise should always run with the grain of the wood. Double tenons, therefore, are often necessary, where, if only proportions had to be considered, a single tenon at right angles to the grain would be simpler.
Fig. 9. Fig. 6 A is a case where the tenon is not haunched, as in Fig. 2. It is not so often employed as the latter, and, of course, cannot be wedged, but must be screwed or pinned. It is more suitable for heavy work than for light; but in many cases a half-lap joint would be employed in preference to it. Fig. 6, B, shows a stub tenon for parts meeting at an angle. A through tenon in such cases is seldom employed. There are numerous-other slight variations of the form of stub shown. Very often the tenoned timber is notched into the surface of the other as well as tenoned.