There are two main types of construction in woodwork. One is box construction in which the project is mainly composed of sheet materials which are fastened together without being attached to a framework. The other is frame construction in which lighter panels are supported on a framework. As this framework is made rigid by the various joints which are used in its construction it is better able to support doors and is less likely to sway with the movement of the doors.
Mortise and tenon joints
The basic joint for framed construction is the mortise and tenon. This is used because the shoulders of the tenon make the frame rigid and the tenon itself enables the members to be joined without seriously weakening the timber at that point. For cupboard frames the ends of the top and bottom rails are tenoned to fit into mortises in the sides or stiles, while for door frames and window frames the joint is made the opposite way round: the sides or jambs are tenoned to fit into mortises in the head and sill. The mortise and tenon is one of the most useful joints and has several forms.
Halved and lapped joints
These joints can be used as a substitute for the mortise and tenon, but when this is done the appearance should be preserved by allowing the stiles of the cupboard frame to run up the face of the top and bottom rails. They cannot be used for window and door frames, but a housing joint can be made in the head and sill so that the jamb can fit into it. This joint would have to be well nailed. Doors which are to be clad with hardboard or plywood can also be made with halving joints. It is best to fix the hardboard panel to the frame using an adhesive because the nails always seem to show through the paint however careful you are in punching them down and filling the hole.
Traditional box construction uses dovetail joints as these give a pleasing appearance at the corners of the furniture. This joint is not used much today because of the amount of veneered chipboard which is used for furniture, and dovetails cannot be made successfully in chipboard. Some machine-made dovetails are used in quality furniture but they lack the decorative appearance of hand-made joints because the dovetails and the pins which separate them are all the same size.
However, dovetails provide a joint which will resist a lateral pulling force better than most other joints, so they are especially useful when making wooden brackets. Here the dovetail is cut in the top member, and the top end of the upright member has the cut-out for the tail to fit into. If the angled member of the bracket is let into the top and upright member, then the bracket will have considerable strength.
The correct angle for the sides of the dovetail is one in eight (7 degrees) for hardwoods and one in six (9 ½ degrees) for softwoods. If the sides are made too steeply angled the pointed corners of the tail will break and if the angle is too shallow it will have little resistance and pull apart easily.
Where constructions such as chairs and tables are being made, the mortises for the rails, which are at right angles to each other, meet inside the leg. This means that the tenons must be mitred at their ends to give the maximum length to each.
Tops of solid-wood tables must be fixed by means of slotted metal plates or by wooden buttons which engage in grooves in the inside face of the top rail. This is to enable the wide wooden top to move with changes in atmospheric conditions. Solid tops which are battened on the underside must also be allowed to move. This is done by not gluing the battens, and making slots for the screws instead of tightly fitting holes. Failure to provide for the movement of natural timber tops could cause serious splitting in a warm atmosphere.
Side panels fitted into grooves in the framework must also be allowed to move and therefore they are not made a tight fit and they are not glued or pinned. Chipboard and laminated boards do not suffer this and can be securely fixed.
The legs and rails of chairs and tables, for example, must be cramped up carefully to avoid the whole article becoming twisted. Sight across the construction, viewing one rail against the same one on the opposite side, and you will be able to see whether they are in line or whether they are sloping in opposite directions. If they are not all in line, the piece of furniture will rock or wobble on its feet.
To make a wobbly chair or table stand firmly, wedge it level on a flat, level surface. Take a strip of wood about 3mm thicker than the amount the leg is short and mark a cutting line against the top of it on each leg. Never cut one leg to suit the others.
Working with man-made boards requires different techniques to conventional joinery and involves very few cut joints.
Much modern furniture is basically boxes made from veneered chipboard. Construction is often of the knock-down type so that the units can be taken apart and fitted into a cardboard box which will go in a car boot or on a roof-rack. Any permanently constructed units are generally joined by means of dowel joints. There are two main requirements for making furniture like this and they are the ability to cut a straight, square line and to drill neat, perpendicular holes. This latter requirement is easily met with a dowelling jig or a vertical drill-stand.
Various proprietary connectors are available, including a plastic chipboard plug, and a barbed bolt which is tightened with an Allen key. This type of furniture is ideal for the beginner to make, as the boards can be bought cut to length and plastic blocks can be used for the joints and for supporting shelves. The blocks are simply screwed into place using double-threaded chipboard screws, enabling you to produce the furniture quickly. Doors can be of the lay-on type so that there will be no need to plane them to fit into an opening. Hinges can be of the concealed lay-on type which need no cutouts and are just screwed into place.
Working in blockboard and pine-board follows a more traditional method, as some woodworking joints can be employed. This kind of board will accept a mortise and tenon joint as well as a notch or a rebate in the edge. The boards will also accept screws in the edge which is not a good practice when using chipboard. To some extent you can make housings into the board so that shelves can be fitted.
This can be used for making doors and for cladding other types of framing, but it must be conditioned first by damping the back of the board, using about 1 litre of water to a 2440X1220mm sheet. The wet sheets should be placed back to back for 48 hours before use. This process enables the board to stretch so that when it is fixed and dries
to the moisture content dictated by the conditions in which it is being kept, it will tighten up and will not buckle. It would do so if it were perfectly dry when it was fixed to the framework and was then taken into a more moist atmosphere. The treatment is not necessary for free-moving panels.
Sheet materials, especially thin boards like hardboard and plywood, are cut most easily if they are supported along the whole length of the cutting line on both sides of the cut. Lack of support allows the sheet to sag or bend and jam the saw.
Because there is no basic framework to hold the whole construction rigid, stability depends on the hardboard back of the unit being fixed securely to the sides and rails or shelves. In addition, any plinth or top rail will help to prevent the front of the unit swaying. This can prevent sliding doors from meeting the sides properly and it will make hinged doors swing open or closed.