All large pieces that are free to warp and twist, such as doors, shutters, sashes, etc., and interior panel work of every kind, should be "framed" together by making a frame of boards running parallel with the outside edges of the work. The space enclosed by the "frame" is usually filled with panels made of wood or glass. Wood panels may be set flush with the frame or sunk, as preferred. When the piece is very large - exceeding 2x3 feet - the frame should be divided into two or more panels by cross pieces, framed into the outer pieces. As the boards or planks used in forming the frame are usually not more than 6 inches wide, and often not more than 2 or 3 inches, the size of the frame is but little affected by shrinkage, while the shrinkage that would naturally occur in so wide a piece is taken up by the panels, which should be so arranged that the effects of shrinkage or swelling will not be apparent.
Framed work is also less likely to warp than any other arrangement of pieces, owing to the grain in the different parts of the frame running in different directions. The greater the number of panels, also, the less is the liability to warp.
The pieces forming the frame should always be joined by mortising and tenoning, which is similar to the same operation in carpentry, except that it has to be done with greater care and neatness.
The tenons should have a thickness from ¼ to 1/3 that of the frame, and the breadth should be about 2/3 of the breadth of the piece, but no single tenon should be more than 4 inches wide, as a broad tenon may shrink considerably and get loose, besides necessitating a wide mortise, which might weaken the frame too much. Hence when the wood is wide - generally if over 7 inches wide - a double tenon should be used, as at B, Fig. 209. This drawing also shows a haunch on the tenon, which is seldom used, however, except in the very best work.
Fig. 210 .
When the mortise comes at the end of a stile or rail the tenon is usually cut as at A, and the piece containing the mortise should be left long, as shown by the dotted lines, and not cut off until the glue is hard.
The tenon should be secured in the mortise by wedging and gluing, as shown by the dotted lines, the mortise being cut a little large to accommodate the wedges. In cheap doors the tenons are often secured by pins which show on the face, but this method of fastening is inferior to the wedged joint.
160. - When plain surfaces of boarding of considerable extent are required for wainscoting, dadoes, etc., they should be built up of narrow boards - 3 to 4 inches wide - carefully jointed, doweled and glued together edge to edge, and keyed on the back by tapering pieces of kiln-dried hard wood let into a wide dovetail groove, as shown in Fig. 210. These keys keep the surface of the boards in the same plane and allow the work to shrink and expand with changes in the humidity of the room. They should be driven tight but not glued.
In fitting the boards together they should be placed so that the direction of the annual rings in each piece is reversed.
Very often the edges of the boards are grooved and a hard wood strip or "spline" let in the whole length to strengthen the joint, but dowels are superior for the reason that a groove leaves a thin tongue of wood on each side of the board, which is liable to curl and thus cause the joint to open, as in Fig. 211. When dowels are used no such defect is likely to occur, as the edges of the boards are not cut except at intervals where the dowels are placed, and then only by holes just large enough to receive them.
When it is desired that a dado shall appear like one wide board of hard wood, a backing of pine should be glued up, as described above, and the face then covered with a thin veneer of the finished wood. In the very best work two thicknesses of veneering are used, as described in Section 178.
Occasionally doors are designed to appear as if made of a single plank of wood - generally of some expensive hard wood. To obtain this effect and at the same time have a door that will not warp, the door must be framed and filled with flush panels and then veneered with two thicknesses of very fine veneer, as described in Section 165. Scribing. - This is the operation of bringing the edge of a piece of wood, usually a long strip, to fit close to an irregular surface, as in fitting the edge of a board to a plastered wall that is not a true plane or to rough stonework. It is done by placing the board so that it will be parallel to its intended position and as near to the irregular surface as convenient, and then setting a carpenter's compass so as to cut off just enough to give the proper width to the board; one point is drawn along the irregular surface while the other is made to scratch a line on the face of the board, as shown by the dotted line, Fig. 212. This line will, of course, be exactly parallel to the profile of the surface, and when the board is cut it will fit exactly in position.