Dovetail joints are widely used for box and drawer carcases and upright frames in furniture construction. They provide a strong corner joint which is resistant to pulling stress, indicated by their widespread use between the front of a drawer and its two sides. There are many variations of dovetail joints; in general, the wider the pieces to be joined the more tails and pins required. You can use dovetail joints to join timber of different widths; always form the tails in the thinner piece and the pins in the thicker piece. The length of the tails must be equal to the thickness of the piece they are to fit into, except when the joint is stopped (which will be covered later in the Course). The angle on the tails and pins should be about 80 degrees.
Mark out the tails first, ensuring they are all the same angle by using a sliding bevel or dovetail template and spacing them evenly across the width of the timber. Measure the thickness of the thicker piece of timber and mark this distance in from th end of the other piece. Add 2mm to this depth line for waste, which can be planed off afterwards, and lightly score round all four sides with a sharp cutting knife. The order and spacing of the recesses and tails and the half-recesses at both ends is all important. For example, on a three-tailed joint the order of tails and recesses should be half-recess, tail, full recess, tail, full recess, tail, half-recess.
Decide the number of tails you require and divide this number into the width of the timber; for example if you want three tails and the timber is 90mm wide, the figure you will get is 30. Divide this by five to find the basic unit of measurement, that is 6. Double the basic unit to find the length of the full recesses (12) and treble it to find the length of the tails (18); the half-recesses are always one unit wide (6). Therefore, in this example, the figures are 6, 18, 12, 18, 12, 18 and 6, making a total of 90mm.
Mark out each of these measurements along the base line and use a sliding bevel or dovetail template to mark an 80 degree angle from each point in alternate directions, working across the timber from left to right. After marking out the first half-recess, measure across its narrow end; this dimension should not be less than about 4mm. If it is between 3 and 4mm, reduce the width of all the tails by 1mm on each side (thus increasing the width of the recesses); if it is less than 3mm, reduce the number of tails until the minimum dimension is reached. A scale drawing of the joint will help.
Make a dovetail template from a thin sheet of aluminium or stainless steel marked out to the dimensions. Bend over each end to 90 degrees in opposite directions. One of the upstands is held against the end grain of the timber on which the tails are to be cut and the tails are marked out along the angled sides.
Mark these lines squarely across the end grain and repeat the markings exactly on the reverse side.
Cut out the tails with a dovetail or tenon saw, keeping to the waste side of the line; remove the waste with a coping saw, leaving the last 2mm which is removed with a bevel-edged chisel — always chisel only halfway through the timber from both sides. Save time when making several joints by nailing the pieces together through the waste timber, clamping the pieces in a vice and cutting all the tails at once.
Use the cut piece as the template for the timber in which the pins will be cut. Hold the two at right-angles, with the cut tails across the end grain of the other piece, and score accurately round the tails with a sharp marking knife. Mark the waste wood with a pencil. Mark on the base line in the same way as for the first piece and mark squarely along the grain from the corners of the marked pins down to the base line. Cut out the pins and chop out the waste as for the tails. Don’t clean up the joint before assembly because this may make it loose; offer the parts together and check for fit. If the joint seems too tight, you will have to ease it; if it is too loose, pack it with wafer-thin offcuts. Apply woodworking adhesive and tap the pieces together, driving any packing pieces firmly into place.
Check with a try square for accuracy and cramp until dry. Clean up the joint with block plane and abrasive block.