When two pieces of timber have to be united at their ends, as in lengthening the beams for roofing, partitions, the masts and keels of ships, etc., the operation is called scarfing, and the joint a scarf. The methods of scarfing are very numerous; those fig ured below will serve to illustrate the principal.
The following are the principal rules for scarfing as stated by Tredgold.
The length of the scarf should be, if bolts are not used—in oak, ash, or elm, six times the depth of the beam; in fir,12 times the depth of the beam. If bolts and indents are combined, the length of the scarf should be—in oak, ash, or elm, twice the depth of the beam; in fir, four times the depth. In scarfing beams to resist transverse strains, straps driven on tight are better than bolts. The sum of the areas of the bolts should not be less than one fifth the area of the beam, when a longitudinal strain is to be borne. No joint shoald be used in which shrinkiug or expansion can tend to tear the timbers. No joint can be made so strong as the timber itself. When two pieces of timber are connected so that the joint runs parallel with the fibers of both, it is called a longitudinal joint; but when the place of the joint is at right angles to the fibers of both, an abutting joint. A very short tenon is called a stub tenon. When a second minor tenon is made projecting from the principal tenon, it is called a tusk tenon.
For lighter joiner's work, other methods of framing are used, and adapted to the work—to boards generally instead of beams; thus, for example, the mortise and tenon joint, made oblong instead of square, is used in framing doors, shutters, drawing-boards, or any other kind of extended superficial work liable to warping. An outside frame or
skeleton is made with a panel or panels in the middle, and each piece of the frame has the grain at right angles to the piece into which it is mortised, in order that they shall eventually correct the warping.
Dovetailing is extensively used for connecting boards at right or other angles, as in making boxes, drawers, etc. Common dovetailing is usually glued. Nails or pins and glue are used with the miter and other notched joints.
Boards may be united at their edges to form an extended surface, as a fiat plank par tition, etc., either by simple gluing of the shot edges, by a rebate, or by a plowed groove and a corresponding projection. The rebate is cut by means of a rebating plane; that in the figure is combined with a bead, the usual joint for wooden partitions. The groove, a sort of extended mortise, is cut by a plane with a projecting iron called the plow.
In all cases where glue is used in joints, it should be applied to both surfaces, which should be rubbed and pressed together until nearly all the glue is forced out, then kept pressed by a cramp or weights. White lead is used for outside joints.
Special departments of this subject, such as roofs, staircases, etc., will be treated under their respective heads.