There are many cases where joints such as the mortise and tenon, used for table and chair legs, are still necessary in spite of modern resin adhesives. However, the strength of traditional woodworking joints is enhanced by a resin bond.
Mortise and tenon joint
This is used for framed-up constructions including door and cupboard frames, chairs and tables. It gives the strongest connection and the best-looking finish.
The setting out and the tools required have been described. You also need to know the correct size for the joint. A tenon should be one third the thickness of the wood and for a rigid joint it should have a shoulder all round it. If a mortise and tenon is needed at the end of a piece of wood, such as when joining the top rails of a table to the legs, then to avoid the mortise being an open-ended slot the tenon is made only two thirds the width of the rail and a stub called a haunch is made for one third of the width. An extra 25mm of timber can be allowed on the leg length to avoid it splitting. This is cut off later when the glue sets.
Splitting is a problem with the dowel joint, which is used as an alternative to the tenon. The dowel joint can never-be as strong as the mortise and tenon joint because the size of the dowels is limited. There is no point in increasing the diameter of a dowel as this will seriously weaken the rail. A rail 25mm thick would support a dowel or tenon about 8mm thick and leave about 8mm each side to support the dowel or to form a shoulder to the tenon. If the strength of the joint needed to be increased for any reason, the tenon could be nearly doubled in thickness to 15mm, still leaving a shoulder of 5mm at each side which would provide stability to the joint. If a dowel was increased to this size there would only be 5mm at each side of the hole to support the dowel and this could prove insufficient. This is one of the reasons for failure of the dowel joints between chair legs and rails.
The traditional joint for shelves which are fitted into cupboards and bookcases is the housing joint. This is a groove cut into the side member and it must not be deeper than one third the thickness of the side. In solid timber the depth need only be a quarter of the thickness. In order to prevent the joint showing, the housing can be stopped about 25mm from the front edge.
Housing joints are not really suitable for chipboard because the boards are not very thick and the edges of the groove are prone to breaking away. A chipboard shelf, even though it is not suitable for heavy weights, needs better support, and this can be provided by a thin strip, or batten, of timber glued and pinned inside the cupboard.
Simple butt-jointed constructions, which are made by cutting the ends of boards exactly square both in width and thickness, are suitable for solid timber which can be glued and nailed or screwed at the joint. Chipboard would have to be fixed by means of a corner block.
Butt-jointing of narrow boards to make up a wide board such as a table top is done by planing the edges perfectly straight and square in thickness and then gluing the edges before cramping the boards together. They may later be given additional support by screwing battens to the underside. Again, this type of jointing process is not suitable for chipboard construction.
Halved and lapped joint
Another strong alternative to the mortise and tenon joint is the halved and lapped joint. Usually just called a halving joint, it needs no special jig to set it out. It can be marked out using a marking gauge and cut with a fine-toothed saw, preferably a tenon saw. As its name suggests it is made by simply cutting away half the timber of each piece of wood so that the two pieces will fit together flush with each other. The joint is made in the same way even if the two pieces of wood are of different thickness. In this case the gauge is set to half the thickness of the thinner piece and both timbers are marked from the face side. The
face side is left intact on the thinner piece and the back is removed. The opposite is done to the thicker piece where the joint is cut on the face side to a depth of half the thinner piece. The joint can then be assembled with the two face sides being flush with each other.
The joint is both glued and screwed. Where possible the screws are inserted from the back of the joint so that they do not show when the project is completed.
Apart from corners being halved, the joint can also be used where timbers cross each other. Such places are the diagonal cross-rails at the bottom of table legs and any form of diagonal bracing.
Reinforcing butt joints
Although they are used as a substitute for a mortise and tenon, dowels should really only be used to strengthen ordinary butt-joints. A minimum of two dowels must be used in each joint to avoid any tendency to twist. A groove or saw cut should be made along the side of the dowel to allow the air trapped at the bottom of the hole to escape; excess glue will also be released this way. The holes in each piece of timber should be slightly countersunk to make it easier to remove excess glue cleanly. Chamfering the ends of the dowels will enable them to enter the holes more easily.
This is usually made in mouldings and is secured by small nails or pins. Where extra strength is needed the joint can be reinforced by veneer keys, glued and driven into saw cuts across the point of the mitre. This is sometimes used for picture-frames.
Another simple carpentry joint is notching, which is cutting a small recess or housing in one piece of timber so that another piece can be secured in it. This is a method often used for securing the shelf-bearers where
racks or shelving are being made.
Where an angled timber meets a horizontal timber, a bird’s-mouth joint is used. In this joint, a cut is made to fit the vertical face of the horizontal piece and another cut made to match the top horizontal face. The result is a right-angled v-shaped joint of a type much used in roofing work.
Another carpentry joint, used in rough woodwork instead of the mortise and tenon, is the bridle joint. In this construction, which is set out like a mortise and tenon, instead of cutting a hole for the tenon, the sides of the mortise are cut away leaving the centre solid. The end of the mating piece of timber then has what would have been the tenon cut out leaving the two sides as a forked joint which will fit over the centre web.
At a corner the bridle joint looks like a mortise and tenon which has been cut too near the end of the wood leaving one end of the mortise open to make a slot. The joint is as strong as a mortise and tenon when it is made in the middle of the rail, but when used as a corner joint it lacks stability. The recess in the middle of a rail holds it firm.