Miter Joint. Fig. 282 shows a plain miter joint. Its sole recommendation lies in the fact that it exposes no end grain, for, from a mechanical point of view, it is weak and faulty, - weak because difficult to fasten, and faulty because, as the two pieces forming the joint shrink, each will become narrower on the lines A, A, and produce the change of form shown by the dotted lines B and B'. As a result of this change either the angle C between the two pieces must become smaller, or the joint must open, forming a wide crack on the inside, which is represented by the triangle BDB'.

Miter joints between two pieces of different thickness are made in the form illustrated by Fig. 283. Occasionally this is used when the pieces are of the same thickness, Fig. 284; for while it has the advantages of the plain miter joint, it is stronger and less affected by shrinkage.

230. Glue, and brads or nails, the usual fastenings for miter joints, may be supplemented by a fillet inserted as shown by A, Fig. 285, or by small pieces inserted in saw cuts which are made across the angle of the joint, as shown by A, Fig. 286.

Fig. 285

Fig. 286

231. Dovetail Joints have already been discussed (171-176). They can be made much stronger than any of the other angle joints herein considered. The plain dovetail, Fig. 199, is sometimes objectionable because it exposes end grain, but the checkered appearance of a well-made joint almost counterbalances this objection. In the lap-dovetail joint, however, Fig. 201, the end grain disappears from one face, and in the blind dovetail, Fig. 203, from both faces. The blind dovetail is certainly all that could be desired as far as strength and appearance are concerned, but it is difficult to make.

Fig.287

232. Mortise-and-Tenon Joints in joinery are different from those employed in carpentry only in the proportions of their parts and the accuracy with which they are fitted. When the thickness B, Fig. 287, of the pieces joined is the same, the thickness A of a simple tenon may vary from one-third to one-half that of the piece on which it is formed, practice tending toward the larger figure; and its breadth C ought not to exceed seven times its thickness. For the thickness given, shows a tenon of the greatest breadth allowable. The breadth is thus limited because the sides of the mortise derive their support from the solid material at its ends, and they become too weak for good service when the limit named is exceeded. Again, the tenon, if too broad, will not stand the pressure of wedging, but is likely to become distorted, thus putting additional strain on the mortise and frequently causing it to split. See Fig. 288.

Fig. 288

Fig. 287

233. When the piece on which the tenon is to be formed is very broad, a single tenon, if employed, leaves wide shoulders, AB, Fig. 289. These are open to objection because of the tendency of the tenon piece to warp so that its surface at D will not agree with the surface of the piece it joins at C. Under such circumstances a double tenon, Fig. 290, may be used. This will give the support that is needed, and will not violate the principle laid down in 232. Double tenons, how-ever, while they obviate one difficulty introduce another. The tenons are unyielding, and, if the piece is very wide, its shrinkage is likely to produce a crack between them, as denoted by the dotted lines A, Fig. 290.

Fig.289

Fig.290

234. Haunching is a device by which the tenon proper is supplemented by very short tenons, or "haunches," as indicated by the dotted outline, Fig. 291. These prevent the tenon piece from warping and the danger of its splitting from shrinkage is not increased. If the piece shown by Fig. 289 were haunched, the imperfection it illustrates would be removed.

Fig. 291

Fig. 292

235. Four tenons may be used in a single joint when the pieces to be united are very thick and wide, Fig. 292. By their use the parts are made small enough to prevent shrinkage from producing a bad joint.

236. In forming a joint at the extremity of the mortise piece, a single tenon, if employed, must be cut away at one side, as shown by Fig. 293. Such a joint may be haunched, Fig. 294, or if the pieces are sufficiently wide, two tenons may be used, Fig. 213.

Fig. 293

Fig. 294.

237. Mortise-and-tenon joints in joinery are capable of all the modifications of form which they are made to assume in carpentry. They may be housed, for example, or made in any of the oblique forms.

Paneling.

238. A Panel is a board, or a combination of boards, employed to fill an opening within a frame. Thus, in Fig. 295, the pieces F constitute the frame, and the pieces A,B, C, and D are panels. The primary purpose of this arrangement is to give an extended surface of wood so constructed that the pieces of which it is made shall be well and neatly fastened, and, at the same time, the dimensions and the general appearance of the whole be unaffected by shrinking or swelling. To enhance the attractiveness of the surface, both frame and panel are frequently embellished, sometimes so richly that we lose sight of the mechanical necessity of the panel, and come to regard it as a means of decoration.

Fig.295

239. The Frame taken by itself is, in general, made up of vertical and horizontal pieces united by mortise-and-tenon joints. Vertical pieces extending the full length of any frame are called "stiles," and horizontal pieces, "rails." Each of these parts should be as narrow as is consistent with the degree of strength required. The width of a rail should never be more than twice that of the stile, which, as a rule, should not exceed four and a half inches. A consideration of Fig. 295 will show that, although the door is three or more feet wide, the only surfaces whose shrinkage can affect the width are the two 4 1/2-inch stiles. Large surfaces are covered not by increasing the size of the parts but by increasing their number.

A fillet e is often inserted to cover the end of the tenons, which would otherwise show on the edge of the door.

240. The panel may be either fastened to the back of the frame or inserted in a groove, or "plow," formed in the frame to receive it. In either case, provision must be made for shrinking and swelling. When fastened to the back, screws are usually found to make a sufficiently yielding joint. When fitted into the frame no fastening is needed beyond that derived from its position. It must fit loosely enough to draw out on shrinking, but not so loosely as to rattle.

In Fig. 295, A is a plain panel screwed to the back of the frame, and the frame about it is stop-chamfered. This is, probably, the simplest combination of frame and panel. In common with all panels fastened in this way, it is best adapted to work that is to be seen from one side only, as a closet door or the permanent lining of a room.

B shows a plain panel fastened to the back of a frame which is ornamented by a molding.

C differs from B only in being let into the frame instead of being screwed to the back. The reverse face c may be ornamented by a molding in the same manner as C, or by a chamfer.

D shows a raised panel embellished by a rabbeted molding. The reverse face d is a plain raised panel.

A panel and frame may be plain on one side and ornamented on the other; the ornamentation on one side may differ from that on the other, or the sides may be similar; and any form of embellishment that may properly be applied to board surfaces may be used in connection with this work.