This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
2, To insure durable and flush connections between the parts joined together, due consideration must be given to the form and character of the joints, so that the strains to which they are subjected may be either resisted or compensated. The strains may be caused either by the behavior of the material itself or by extraneous force in the working portions, as doors, shutters, and window sash.
In Carpentry, a number of joints commonly used in house building are described in detail; and though many of these joints are identically the same in principle as those used in joinery, there is a vast difference in the degree of accuracy required in the workmanship. In joiners' work, the joints may be divided into two classes, namely, loose joints and glued joints; the former consists of two or more pieces maintained in place by the shape of the parts of union, such as the dovetail joint, Figs. 8, 9, and 10; in the latter, the pieces are secured by means of glue, as in the miter joint, Fig. 2, the halved corner, Fig. 3, etc.
3. Loose joints are seldom used in joinery, except in heavy machine frames, and in pieces of cabinetwork which are made in sections for convenience of handling, or in the exterior finish of a building where exposure to the weather would render the glued joint unreliable; and, in any case, the loose joint is usually rendered additionally secure by means of screws, nails, or wooden pins.
4. Glued joints require some mechanical contrivance to maintain them in position while the glue is setting, which usually requires from 12 to 48 hours, according to the quality of the glue, the condition of the atmosphere, and the tern- . perature of the room.
5. Considerable experience is required in order to use glue successfully. The wood must be well seasoned and thoroughly dry, and the joint must be well fitted. In gluing hardwoods, the surfaces to be glued should be roughened with a toothed comb, to make the glue hold better; the room must be at a proper temperature, and the material must be heated, so that the glue will flow quite freely. The glue must be spread evenly on the parts, which are then placed together as soon as possible, and maintained in place by means of hand screws, bench clamps, or wedged frames. All superfluous glue is then wiped off and the work left until the joint is thoroughly dry.
The most frequent causes of bad gluing are the use of an inferior quality of glue and the unequal spreading of it. A good practical way to test any brand of glue is to glue together a piece of pine and a piece of ash, clamp them up with a hand screw, and when they are dry insert a chisel in the joint and try to pry them apart. If the joint separates where it is glued, the adhesion is inferior, and should not be used for first-class work. The wood should split or give way rather than the material securing the joint.
6. The simplest joint made use of in joinery is the butt joint, shown in Fig. 1. Here the two pieces joined are simply butted together, in a similar manner to the butt joint in carpentry, but the adjacent surfaces are much more accurately fitted, and are held in place by means of screws or dowels - the former when the pieces joined are broad and thin, as in Fig-. 1 (a), and the latter when the timbers are heavy and thick, or when they are joined together on the thinner edges, as at Fig. 1 (b).
This joint is insecure in itself, and usually requires glue to make it in any degree reliable; or it may be kept in place by the woodwork behind, against which it is erected, as would be the case in an exterior casing for a window where the form shown at (b), Fig. 1, would be used, except that the top h would rest upon the sidesy.
The plain butt joint, shown at Fig. 1 (a), is seldom used in first-class joiners' work, as the opening caused by shrinkage is very apparent. The absence of a device to keep the entire surface flush allows the fibers to curl, and the effect of shrinkage and warping would be to cause the pieces c in Fig. 1 (a) to project slightly over the side of d and render the juncture too apparent. This may be obviated to a certain extent when the grain runs parallel to the joint, by forming the joint as at (c); a quarter-round f is worked on one of the pieces to be joined, thus forming fillets, as shown at g. The joint, Fig. 1 (a), may also be doweled, as shown at Fig. 1 (b).
7. The miter joint, shown in Fig. 2, is generally used when the boards are of the same thickness, where no great strength is necessary, and where the conditions require that the angle shall show no end grain of the wood, either inside or outside of the joint.
It is usually nailed or screwed together as shown, but depends upon glue to make it secure, or is strengthened by a slipfeather or spline, as shown dotted at a.
It is also sometimes secured by gluing in a key of thin wood, which is inserted in a saw kerf cut diagonally, as at b, or at right angles to the end, as at c. A keyed miter is most generally used for a joint seen only from the interior, as the keys disFigure the exterior appearance. This joint is also made with the broad portion of the wood on the face, as in the case of architraves and casings.
In mitered joints, the shrinkage of the wood does not change the appearance of the exterior of the joint, but causes the interior to open slightly, as indicated by the dotted lines. To obviate warping, however, a tongue is very essential.
8. The shouldered corner, shown in Fig. 3, is a joint used where the inside angle will alone be visible, and is an improvement on the butt joint shown in Fig. 1 (a), inasmuch as it provides two nailing surfaces, one at a and one at b.