THE PURPOSE OF JOINERY

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Now that you know the basic types of joinery, how do you choose the right joint for a particular woodworking job? Consider that every joint must fulfill two important requirements:
• It must support the load of the other parts and any external weights or forces that might be applied to the completed project.
• It must let the wood move as it expands and contracts with changes in temperature and humidity. And if the joint is to be glued or fastened, as most are, there is a third requirement:
• It must provide a suitable gluing surface or anchor for a fastener. Use whichever joint best fulfills these requirements.
SUPPORT THE LOAD The parts of a woodworking project are elements of what engineers call a “stress system.” Each joint must withstand a certain amount of stress pushing or pulling at the members of the joint. This stress comes from many different sources. It could be external (coming from outside the structure); for example, when you sit on a chair, your weight stresses the chair joints. If you scoot the chair across the floor, the friction between the floor and the chair legs creates stress. Or the stress could be an internal factor, inherent to the structure. The tension in a woven seat, for example, stresses the joints between the rails and the legs. Even the weight of the individual chair pans, no matter how small or light they may be, is an internal stress to be reckoned with. There are four types of stress, categorized by the direction of the force relative to the joint (SEE FIGURE 1-3):
• Tension pulls the members of a joint apart.
• Compression squeezes the members together.
• Shear pushes the members in opposite directions. The lines of force are parallel, but not aligned as they are with tension and compression.
• Racking (or bending) rotates the members around one another.

Even before they’ve been glued or fastened, fitted joints resist one or more types of stress. (SEE FIGURE 1-4.) After they’re assembled, they resist all types to a greater or lesser degree. When choosing a joint, try to pick one that will withstand the anticipated stress without glue or fasteners. That way, if the glue bond or the hardware fails, the joint will stay together

Fore your information Of the four types of stress, racking is the most destructive. A racking force bends the members of a joint like levers. A lever, as you know, will move a heavy object when you apply a relatively small force- the force is multiplied by the pivoting action of the lever. For this reason, a small amount of racking will pop a joint that might otherwise withstand large amounts of tension, compression, or shear.

For most woodworking projects, however, you
must do more than pick a joint or two. You must
design an entire system of joints- this is what a
structure is. To build a structure, you must determine
not only the types of joints in it but also their size and
location relative to each other. This isn’t difficult; it
just takes some thought. There are a few simple commonsense
methods for designing a strong, durable
structure:

1-3 Four types of stress may tear
a wood joint apart – tension, compression,
shear, and raching. Of these,
racking is the most destructive.
• Use larger joints and structural members. This
distributes the load over a larger area and larger mass.
(SEE FIGURE 1-5.)
• Use smaller members, but more of them. This
too increases the area and mass that supports the
load. (SEE fiGURE 1-6.)
• Triangulate the members. Rearrange the structural
members or add new members, braces, glue
joints, or fasteners to create structural triangles.
When a triangle is fastened at all three comers, it~
very rigid. This is why engineers triangulate bridges
and roof trusses. (SEE FIGURE 1-7.)
• Orient the wood grain properly; wood is always
strongest parallel to the grain. A tenon or dovetail cut
across the grain will soon break.
• Increase the glue surface in a joint by making the
fitted surfaces more intricate. (SEE FIGURE 1-8.)
• Increase the size or the number of fasteners.
• Use both glue and fasteners.

I -4 Even before a butt joint is
glued or fastened, it will withstand
compression, but any amount of tension,
shear, or racking will pull it
apart. A monise-and-tenon joint, on
the other hand, will resist compression,
shear, and racking. Only tension
can pull it apart before it’s
secured.

1-5 The racking force applied to
both of these mortise-and-tenon
joints is equal. But on the large mortise
and tenon (bottom), the load is
distributed over a larger area and
more mass. The stress at any one
point in the joint is a good deal less
than that on the smaller mortise and
tenon (top).

l-6 You don’t have to use massive
structural members or joints to support
a large load. On this Shaker
rocker, the load is distributed over
many small, round mortise-andtenon
joints. The chair’s frame and
joinery appear very delicate, yet it
has survived constant use for almost
two centuries .

l-7 Structural triangles don’t all
have to look like roof trusses. On a
table, the upper part of the leg and
the apron form a hidden triangle
that keeps the structure rigid. On a
board-and-batten door, the nails form
triangles that keep the door square.

1-8 The adjoining members of
both the butt joint (left) and the box
joint (right) are precisely the same
size. However, the fitted fingers of
the box joint offer more gluing surface
than the fiat surfaces of the butt
joint. Consequently, the box joint is
much stronger.

1-9 Wood moves in three
different planes- longitudinal, or
parallel to the wood grain, radial, or
perpendicular to the annual rings,
and tangential, or tangent to the
annual rings. Wood is fairly stable
longitudinally – it will only shrink
or swell .1 percent of its length
when originally cut. However, it’s
unstable radially and tangentially.
Furthermore, the tangential movement
in mos t woods is about rwice
the radial movement. Radial movement
averages 4 percent (of the
original cut dimension) and tangential
movement averages 8 percent.

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