This, however, may have to be modified to make the divisions come out right on the width of the wood. A bevel is then set to the angle of 80°. which is the most suitable for dovetails, and lines are marked from the end of the wood through each of the divisions on the center line 0 0 to the inner line 1 1 at the root of the dovetails, the bevel being reversed to mark the slope in opposite ways. A line of dovetails thus marked is shown in Fig. 8. The next step is to make a number of sawcuts through these sloping lines, stopping each cut when the saw reaches the inner line 1 1. The sockets may now either be cleared out before the other part on which the pins have to be cut is marked, or the marking may be done through the saw cuts.
If the latter method is followed the piece to be marked is screwed end upwards in the bench vice, and the piece which has been sawed is laid with its end in correct position on top of it, Fig. 9. The marking is then done by the end of a dovetail saw inserted in each cut in turn and drawn over the end grain of the upright piece sufficiently for the teeth to leave a mark, which is sawed to after the top piece has been removed. In starting the saw afterwards, however, it must be kept slightly to one side of the original mark to prevent the pins from being a loose fit. By the other method the sockets are first cleared out, and the marking done with a scriber, Fig. 10, the pieces in either case being adjusted in the same way in their correct relation to each other while the marking s done.
By these methods, of course, no fitting is supposed to be afterwards required, the only parts that are touched with a chisel being the end grain at the roots of the pins and dovetails. These are cut slightly concave from each face of the wood to insure a close fit at the external faces. In small dovetails these portions are removed by a chisel and mallet in the same way as mortises are cut out with a bow or keyhole-saw, unless a handsaw is available. Sometimes they are bored out, but in all cases a chisel is used to finish to the line.
The part should be tried together before gluing. The glue should be applied to both parts, using a slip of wood if the sockets are too narrow to insert a brush. The parts should be an easy driving fit, and a block of wood is laid across the surface to receive the mallet or hammer blows, which would bruise the surface, and be rather too local is delivered direct.
Lap dovetails are more troublesome to cut because the saw cannot go through where the lap occurs. The marking out from this of the lapped piece is very simple, the end of the piece merely having to be adjusted to a line which leaves the thickness of the lap beyond. This line is gauged from the inside face, with the gauge set to the length of the dovetails of the first piece. The scribing or saw marking is then done as before. Fig. 10 shows the parts in position for marking a lap dovetail. In cutting the spaces out, however, and forming the lap, most of the work has to be done with a chisel.
The secret dovetail, Fig. 4, may be regarded as a double lap with the laps mitred. A thickness of about one-fourth of the total is gauged from the exterior faces of both parts on the ends, and also on the sides if the mitre is to be carried across them. Then a similar distance back is gauged on the inner faces, representing the amount the lap extends beyond the dove-
tails each way. In this case it is better to mark and cut the pins first because if the other procedure is followed, it is difficult to mark through the narrow entrances to the sockets, for the outside being closed by the lap, the marking has to be done from the back, as shown in Fig. 11. In these cases the rebate at the front should be cut before the dovetails are proceeded with.
The dovetails in this kind of joint are often continued along the complete width of the stuff, and only the front faces are mitred, leaving the joint edgewise like Fig. 6. This rather simplifies the work, but it is not, strictly speaking, a secret dovetail, as that in Fig. 5 is, in which the mitre extends over the edges as well as where the faces meet.
Another variety of dovetail sometimes required and rather troublesome to mark out, is that shown in Fig. 12, in which the dovetailed parts meet at right angles, but slope, or are splayed depthwise. In such cases it must be borne in mind that the dovetails must be parallel with the edges of the wood, and can only be marked from the sloping ends by having bevels set to two different angles, which must first be discovered by marking two oppositely inclined lines at 10°, with the edges. The pins may be gauged from the edges or a bevel set to the angle may be used ; but they cannot be squared from the ends, as in other work.
The remaining Figs., 13 to 16, show examples of dovetail joints in other circumstances than the box-form constructions of the preceding figures. Some of these are common in carpentry and pattern making. Fig. 13 is a single dovetail uniting at right angles the ends of the two pieces that are nearly square in section, and consequently not suitable for jointing by a series of inteilocking projections and recesses, as in the previous cases. In a case like Fig. 13, it is a matter of considerable importance as to which piece the pin is formed on, for it is obvious that the joint would stand a great deal more internal pressure against the dovetail than at right angles to it, where it would only have to force parallel surfaces apart. In most cases, therefore, it would be better arranged in the position shown in Fig. 13, where there would be greater risk of the vertical member being strained outwards than of the horizontal one being forced upwards. A dovetail joint like Fig. 13 is often employed in preference to a mortise and tenons, in which the surfaces of the joint are parallel in both directions.
Fig. 14, A is a dovetail joint for uniting timbers end to end without the attachment of battens, or exterior means of securing them. An alternative to it is to cut the socket or recess in both portions, and insert a separate piece or key to hold them together. Fig. 14, B, is a dovetailed half-lap joint where the end of one member has to meet an intermediate portion of another. It might also, instead of going completely across, be stopped at some distance short, being then practically the same as Fig. 14, A, except for the difference in the direction of the grain.
Fig. 14, C, is a dovetail joint sometimes employed for uniting pieces as shown, the dovetail usually being on end grain and the rebate across grain. It is an alter-tive to a plain rebated joint, but involves more work, with only a very slight increase in efficiency, for in either case the parts are usually held by screws or nails. To insure a tight fit and easy insertion, a slight amount of taper is often given lengthwise to the dovetail, and the groove it slides into, so that the former will enter slack, and be driven tight when the two parts are in their correct position with each other. Both Figs. 14 B and C are often made with only one side dovetailed and the other parallel with the surface of the piece that enters. The main object of this is to avoid the extra work of dovetailing both sides.
Fig. 15 is a half-lap dovetail joint differing from Fig. 14, B, only in the direction in which the dovetai is cut, Fig. 14, B, being designed to resist pulling back-
wards in the plane of the joint, and Fig. 15 to keep the broad surfaces together and flush on the exterior. Fig. 14. B. is employed more frequently than Fig. 15. Fig. 16 is a dovetailed mortise and tenon in which the ends of two rails are united in a mortise in a post. A half dovetail is cut on the end of each rail, so that they will fit together with the rails in line with each other. The mortise is cut sufficiently wide to permit these ends to be inserted from opposite sides, and then the dovetail is closed laterally and kept so by folding wedges which fill the extra width of the mortise, thus making it impossible to withdraw the ends without first loosening and removing the wedges. A single end may be wedged similarly by sloping one of the sides of the mortises to fit the dovetail. This is a very useful joint for temporary work.
Fig. 17 is a dovetail key used for strengthening and keeping very wide pieces of wood from curving. Its edges are dovetailed similarly to Fig. 14, C, to keep it in place, and it is tapered lengthwise, so that it can be driven to a tight fit and further tightened if necessary subsequently to compensate for shrinkage. Sometimes the key stands above the surface of the other piece to give as much stiffness as possible; but it is, if necessary, planed flush.