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.
9. The shouldered-and-mitered joint, shown in Fig. 4, is a combination of the forms shown in Figs. 2 and 3. It presents the advantages of showing no end grain as does Fig. 8, and possesses the additional strength secured in the method of joining the pieces of Fig. 3. Its use, however, is limited to light work, where no great strength is required, and is principally serviceable where a miter joint is required between pieces of unequal thickness, as in Fig. 4.
10. The tongued-and-grooved joint, Fig. 5, the dado joint, Fig. (J, and the housed joint, Fig. 7, are all modifications of the same form, where the end of one piece is let into the side of another piece about one-third of its entire thickness. These joints are very frequently used in joinery. They are secured by means of glue, nails, or screws, or may be left loose to allow the members to come and go in shrinking and swelling.
In joining the back and sides of a drawer, the forms shown in Figs. 6 and 7 are very frequently used, as the projection of the sides of the drawer, as from c to d, renders the end less likely to be pulled off by the jar of the contents when the drawer is suddenly opened; and any unequal shrinkage of the two pieces will not tend to split the material, if the joint is not secured by too many nails.
11. The dovetail joint is one of the most important in joinery. It furnishes a rigid method of securing pieces together where the fibers of the material are approximately at right angles with the joint.
Upon analyzing the conditions of strength existing in a dovetail joint, it is evident that the strength of the entire joint depends upon the combined value of the dovetails, the same as a nailed joint depends upon the combined resistance of the nails, or as a nut on a bolt depends upon the aggregate strength of the threads for its holding power.
The extent to which this is true may be realized by considering wherein lies the strength of each dovetail.
In Fig. 8, the projections e upon the piece b are liable to fail by the shearing of the flare or dovetail upon each side of the projection, the strength in this case depending upon the capabilities of the wood to resist shear across the grain; while the projections upon the piece d depend for their strength upon the resistance of the flares or dovetails to the shear of the wood parallel to the grain.
The projections upon the pieces d and b are also liable to fail by shearing through, or by breaking off where they join the main pieces of wood; consequently, the distance hh and the corresponding distance on the projections on the piece b should never be so slight as to be liable to break before the shearing strength of the wood across, or parallel with, the grain is realized.
There is still another factor entering into the strength of a dovetail joint, and that is the adhesive strength of the glue which binds the fibers of the projections upon the pieces united, giving rigidity to the joint and preventing the piece d from pulling away from b in a direction parallel to the grain of the latter.
12. Dovetail joints are of three kinds. One form, shown in Fig. 8, and known as the lap dovetail, has its dovetails on d, and its dovetail pins projecting from the body of the piece b a distance equal to the thickness of the piece d. The ends of the pins and dovetails show on opposite sides of the corner when the pieces are joined.
13. Another form, known as the half-lap dovetail, shown in Fig. 9, is a joint in which the pins c, though cut down from the end of the board e a distance equal to the thickness of the board /i, are not cut entirely through the board e, but have, as at f, g, a lap which, when the joint is closed, covers the end of the dovetails on //, and prevents the character of the joint from appearing on e at all, though the end grain of the pins c will show through the board h. This joint is well adapted for the fronts of drawers, as shown at (a), as the piece c, which shows no dovetails, forms the front of the drawer, and the ends of the dovetail pins on the side piece h are hidden when the drawer is closed. The side elevation of the joint, and the relative size of the dovetails and dovetail pins, are shown at (b). The reference letters refer to the same details in both (a) and (b).
14. The third form is known as the blind or secret dovetail, and is shown in Fig. 10. In this joint about 2-31 three-fourths of the thickness of the board is dovetailed, the remaining portion being united in a miter joint. Both the dovetails and the dovetail pins are here cut only partially-through the board, and the upper pin a' of the piece c is cut off on the miter line of the angle, while the upper half mortise b' is cut only to the miter line of the piece d, thereby concealing entirely the details of the true character of the joint, when the pieces are united.
This joint is seldom used in joinery, except for the front corners of highly finished drawers and boxes, where, for the sake of appearance, it is better for the dovetails not to show. The blind dovetail is not as strong as either of the other forms, partly because it utilizes only a portion of the available thickness of the wood, and partly on account of their concealment, it is impossible to see whether the pins and mortises fit each other accurately.
In preparing the dovetail joint, the pins r, Fig. 8 (a), are marked and cut first; the piece e is then laid across the end f, and the mortises o are carefully scribed with a knife-point on the side of the end of f. The pins must be sawed square from the end, or parallel to the edge t t in Fig. 8 (b), and the mortises must be cut square across the end of d as at i i, the greatest neatness and accuracy being required to insure good work.
15. In all joiners' work, face and edge marks are made so as to distinguish the face and edge from which all measurements must be taken and all squaring done. At the same time they serve to assist the joiner in determining the edges that are to be in contact, and prevent mistakes due to putting the pieces together in other than their proper relations.
16. The doweled joint, shown in Fig. 11, consists of a plain butt joint between the two pieces e and f, either between one end and a side, as at (a), or between the two meeting edges, as at (b). Holes are bored and dowels are glued and inserted as shown. The object of the dowels is to keep the faces flush, and render the joint stiffer if subjected to a transverse strain. The holes should be somewhat deeper than the length of the dowels, to allow for shrinkage of the material and at the same time insure a close joint. The form shown at (a) is a substitute for a mortise-and-tenon joint, but is not to be recommended.
17. The mortise-and-tenon joint as used in joinery is precisely the same in principle as the same joint in carpentry but the fitting is much more accurate. This joint, in varied forms, is used to secure the rails and stiles of doors and windows, to unite the members of heavy machine frames, and to connect the various parts of tables, chairs, and other pieces of furniture.