The Mortise and other Joints.
In constructing roofs, floors, and other structures of timber, the various beams are framed, or fastened together, by certain processes calculated to insure strength and permanence in the framing, which ought to be understood, and their names remembered.
The Mortise and Tenon joint is used when one beam is to be attached to, and supported by, another, without resting on it, but so that the beams may be in the same plane. The mortise is a hole cut into, or through, the side of the one beam, into which hole the end of the other, cut down to fit the form of the hole, is inserted and fastened. It is obviously necessary to consider two things in determining the size and form of the mortise and tenon. First, that by the former the one beam may not be too much weakened, and yet that it should be large enough to give the tenon that fits into it, sufficient strength to enable the beam to carry the weight intended.
If the one beam is horizontal, and the other to stand perpendicularly upon it, the tenon need only be large enough to retain the upright beam in its place. The foregoing figures are the most usual forms of mortises and tenons, and will explain their use and principle.
It is obvious that two mortises never should come opposite each other on the two sides of the same beam.
When the tenon comes through the beam, it is secured from drawing by a pin or peg put through it.
The Dovetail is used to secure one beam into another, when they have to resist any strain acting so as to draw them asunder, rather than to carry any weight; it is consequently employed to frame wall-plates, or the timber laid in walls to carry the ends of beams of floors, roofs, and so on, which plates tend to bind the walls together as well as to receive the ends of the beams. The term is derived from the end of one beam being cut into a shape resembling the spreading tail of a bird, which is pinned down in a corresponding wedge-shaped recess cut in the other beam to receive it. It is clear from this construction that no force, acting in the direction of its length, could pull the first beam out of the second without breaking off the dovetail, which the tenacity of wood-fibre renders nearly impracticable in one of any size. The dovetail is extensively used in all cabinet-making, and may be seen in almost any mahogany or deal-box.
When two beams of equal thickness are required to cross one another and to lie in the same plane, they are halved together; that is, a notch is cut in each of half the thickness of the other, then the uncut part of each lies in the notch of the other respectively, and the two are pinned together.