These are employed chiefly for uniting wide and comparatively thin pieces of wood at right angles, as the sides and ends of box-like constructions. But the dovetail can be employed in many other circumstances and it is not by any means the only form of joint, or even the one most frequently adopted, for jointing under the conditions mentioned. In particular classes of work, however, the dovetail is invariably employed. In cabinet making dovetail joints are constantly used; in joinery and in pattern-making, less frequently; and in heavy carpentry scarcely at all. One reason for this is that in cabinet work appearance is a consideration of much importance. Another is that joints must be made as secure as possible in themselves, so that they can be held by glue alone. In a rougher class of work equal strength can be attained with less labor by the freer use of screws, nails, bolts, and other means of attachment between parts more simply fitted.
Well fitted dovetail joints are so strong that for most purposes there is no advantage in using anything besides glue to hold them together; but occasionally in carpentry and pattern work they are nailed as well. When they break it is not usually by direct pulling apart in either direction, but as the result of racking strains across the corners of the frame. If these are sufficiently severe the dovetails shear away in line with their grain.
Dovetails may be divided into three classes - common, Figs. 1 and 2; lap, Fig. 3; and secret, Fig. 4. The first two are the strongest and most easily made, the others only being preferred in some cases for the sake of appearance. The common kind may again be divided into two varieties - those in which pins and dovetails are of equal width, Fig. 2, and those in which the pins are narrower - generally about one-fourth the
width of the dovetails, Fig. 1, or in some cases as narrow as it is possible to make them, their thinnest part being only the width of a saw-cut. Of these varieties,
the equally divided type, Fig. 2, is the stronger, because each piece, of wood is cut away to the same amount, and the parts interlock at equal intervals. The appearance of this joint, however, is not considered so good as that in Fig. 1, and consequently the form shown in Fig. 2, which is known as the "cistern dovetail, " because it is employed for cisterns and similar large, heavy boxes, is scarcely ever adopted in cabinet work.
Lap dovetails, Fig. 3, are used chiefly for drawer fronts, the front generally being about 1/4 in. thicker than the sides of the drawer, and this extra 1/4 in. is used as a lap to conceal the dovetails, which can only be seen from the side when the drawer is open.
Secret dovetails, Fig. 4, are entirely concealed when the parts are together, the exterior appearance being either that of a plain mitre joint, the inner detail of one-half of which is shown in Fig. 6, or a mitre with a step or shoulder, Fig. 6, which represents the end dovetails in one of the pieces. Another form, in which the lap is not mitred, is shown in Fie. 7.
Dovetails, when done by hand, are first marked out and cut on one piece, and then transferred from that by direct marking to the piece that has to lock with them. This is more reliable than separate markings out on each piece would be, and it also lenders very careful division on the first piece unnecessary. The pieces to be united must first be planed to thickness
Fig. 6 and width, and squared to length. This, of course,, must be accurately done. Then a gauge is set to the thickness of the stuff, and lines are gauged from each end completely around each piece - that is, if the pieces to be dovetailed are 1 in. thick, lines are gauged 1 in. from the end of each piece across faces and both edges. Usually the pieces to be jointed are of similar thickness; but if they are not the parts must be gauged accordingly. It is now a matter of choice whether the pins or sockets shall be marked first. The projections on the left-hand parts of Figs. 1, 2 and 4 are called the " pins, " the spaces which these fit into the " sockets." Many saw the pins first, but for work done in quantity it is better generally to take the sockets first, because a number of pieces can then be clamped together and cut without separate marking for each. It is not really essential that dovetails should be marked out other than by the eye; but careful men will divide and mark them with a bevel.
If a box or frame which has to be dovetailed together is longer in one direction than in the other, the longer pieces, or sides, are selected for the sockets and generally a half-dovetail is allowed at each edge of these pieces, Fig. 8. To mark them out properly a line, 0 0. Fig. 8, is gauged midway between the end of the wood and the root of the dovetails, which latter is already indicated by the lines previously gauged, and marked 1 1 in Fig. 8. The divisions are made on the line 0 0. If we take now about a third of the thickness of the material we shall have suitable width for the sockets on the line 0 0, and if we make the dovetails or intervening portions of wood four times that amount we shall have a good proportion for each.