Mortise-And-Tenon joints are an extremely old construction technique that has stood the test of time and is still being used today. Examples of this ancient joint is found in Egyptian furniture thousands of years old. It can produce joints that are extremely strong, and the technique can be scaled up or down in size with great success.
The joint has two basic components: a mortise which is essentially a hole cut into one board, and a tenon which is a tongue that is shaped onto the end of another board which will fit into the mortise. The cheeks are the sides of the tongue or mortise, and the shoulder is the portion of the tongue-board that rests against the mouth of the mortise board. The length of the tenon is the distance from the end grain to the shoulder. The width of the tenon is the distance from tenon edge to tenon edge. The thickness of the tenon is the distance from tenon face to tenon face. The mortise depth is the distance from the mouth of the mortise to the bottom of the mortise. The width of the mortise is sized to receive the tenon's thickness. The mortise length is sized to accomodate the tenon's width.
A very important consideration for the design of mortise and tenon joints is wood movement. By its very nature, mortise-and-tenon joints involve cross-grain joinery, which introduces the risk of joint failure due to seasonal wood movement. To accomodate for this, various factors should be considered when designing the joint to optimize the joint strength.
If the tenon is to be full-length, it should be wedged from the exposed side. If it is not to be visible, it should extend approximately half-way into the stile, or three-quarters if the stile is narrow.
The optimal tenon width is the full width of the rail, however this leads to problems with wood movement for wide rails. Multiple tenons, equally spaced, produce a design that retains the tenon's strength yet resists against wood movement by distributing the stress.
If cutting the mortise and tenon by hand, the mortise walls and the tenon should all be of equal size (one-third the stock thickness). This is to prevent accidental splitting of the boards during the construction of the mortise or tenon. If cutting the mortise and tenon by machine, very little stress is applied and therefore a tenon thickness of one-half the stock's thickness can be used. which gives equal strength to the tenon and the mortise walls once the joint is glued.
This is an extremely common joint that is formed by cutting the mortise completely through the stile and sizing the tenon to match, flush with the far side of the stile.
The end of the exposed tenon can be further secured by adding a very narrow slot, either diagonal or straight across the width of the tenon, then inserting a solid wedge into the slot once the joint has been assembled. This can greatly strengthen the joint's holding power, but care should be taken to not split the joint apart by using an overly thick wedge. See wedged mortise-and-tenon joints below
The blind mortise-and-tenon joint gives the outward appearance of a butt joint, however has all the strength and advantages of a mortise-and-tenon joint. The mortise does not extend completely through the stile, and therefore the tenon is not visible once the joint has been assembled. This is the most commonly-seen version of the mortise-and-tenon joint in use today.
A haunch is a short tongue that protrudes from the rail's shoulder, between the rail's edge and the tongue's edge. When a mortise and tenon joint is constructed with a stile that has a groove through which a tenon is cut, such as in frame-and-panel construction, the normal technique for forming a mortise and tenon would leave a void at the end of the slot, and this void would be visible on such assemblies as panel doors. In order to compensate for the slot, a haunched tenon is constructed so the haunch fills the groove at the tenon's edge. Depending on the final use, the rail can have a haunch on only one side, or both sides, as required.
Mortise-and-Tenon joints can be further strengthened by the addition of a wedge. A thin kerf slot is cut into the end of the tenon, then after the tenon is inserted into the mortise, a wedge is inserted into the slot to secure the joint. Wedged joints such as these may not even require any glue, especially if the mortise is tapered to be wider at the wedge end, so that the joint can not be pulled apart by brute force.
The slot can be cut across the tenon's width, along the tenon's width, or even diagonally, to provide a variety of detail, however to produce the strongest joint, the finished joint should force the tenon apart in the direction towards where there is the most wood so that the wood around the mortise won't split under the tension. The slot should be terminated by a hole, slightly wider than the slot's kerf, to help prevent the wood from splitting further into the rail. A good technique to use is to drill the oversized hole first, then cutting the slot just as far as the hole.
A fox-wedged tenon is a variation where a wedge is used on a blind tenon. This variation is deceptively simple, yet frustratingly difficult to execute perfectly because by its very nature, once you have begun assembly you can never disassemble it, even if the assembly goes wrong! The idea is to create a stopped tapered mortise, wider as it gets deeper yet which is only one-half to two-thirds the stile's width. The tenon is cut square, slightly shorter than the mortise is deep. The trick is to determine how wide the bottom of the mortise actually is, and to calculate the appropriate wedge thickness so that the tenon ends press against the sides, yet allow the wedge to be inserted completely. The assembly process involves placing the wedges loosely in the slots, carefully inserting the tenon into the mortise, then forcing the tenon into the joint which will force the wedges into the slots. Once this process is begun, the tenon starts to widen, and the joint is impossible to separate successfully, but if done successfully, you are left with a very strong mechanically bound joint that will last you for many years without any visible fastener, and optionally without any glue either.
As an alternative to wedges, a pinned or pegged mortise-and-tenon joint is extremely strong. After glue-up, drill one or more evenly spaced holes from face-to-face through the stile, close enough to the rail to pass through the tenon, about half-way down its length. Then glue and insert dowels or pegs. These pegs can be made of the same wood to hide the reinforcement, or they can be of a decorative contrasting wood. After the glue has set, the pegs can be trimmed flush with the face.
Loose tenon joints are constructed by mortising both the side of the stile and the end of the rail, and then inserting an appropriately sized tenon during glue-up. Even though the tenon is not integral to either piece, it still creates plenty of long grain to long grain glue surface to create a very strong joint.
Mortise-And-Tenon With Stuck Molding Joints
When the stiles and rails have a decorative profile, resembling a molding, cut directly onto the wood's edge, an option to produce a flush joint edge is to carefully chisel away the profile from the tenon area but leave a mitered section at the ends. Then form a similar miter on the tenon's shoulder near the ends. When the two pieces fit together, the gluing surfaces will be flush and a neatly mitered joint will be remain.
Producing a curve on the inside corners of a frame joint involves more than simply using wider stock for the rail, and hollowing out the edges to produce the required curve. Doing so would leave tiny slivers of wood that would be prone to breakage, and would also waste an enormous amount of wood to construct.
A far better option is to produce two matching pieces that each make up half of the curve. The grain on each piece would be in the direction of the rail or stile to which it will be attached, and the joint between the two will be a simple miter. A simple way to accomplish this is to take a square piece of wood, the size of the curved section, cut it diagonally and then flipping one half over and then re-gluing them together. Once the glue has set, cut the board diagonally from corner-to-corner again, but between the other corners this time. This will give you two blanks that can be placed into the inside corners of the mortise-and-tenon joint and shaped at will. An even better suggestion is to shape the two (or more) corners simultaneously before attaching them so that you can ensure the curves are identical.
For rails that are more than ten times their thickness, multiple tenons should be used. This means that a standard 3/4-inch-thick board move than 7 1/2-inches thick should be treated this way. The proportion between the tenon and the spaces between is the key, and not the actual number of tenons. It is recommended that the space be divided into thirds, two-thirds being tenon and one-third being space, equally distrubuted along the end of the rail. If the rail is of a material that is prone to cupping or warping, you can leave a short haunch between the tenons and notch the mortise to match.
For light-duty frames which are anchored and do not need to support any weight, the tenon can be extremely short, such as just short enough to fit into a panel groove. This joint provides long grain to long grain, but it is very minimal.