Butt Case Joints
Butt joints are one of the weakest forms of case joints, but they are the easiest
and quickest to make. A butt joint is achieved whenever you join the straight and
square end grain of one board to face grain of another board. End grain is notoriously
difficult to glue or fasten, and butt-joints present the smallest amount of surface
area to be used for gluing. In order to achieve any significant strength and sturdiness,
you must reinforce the joint.
Glued Butt Joint
As its name implies, the glued butt joint is held together by the force of the glue
adhering to the end grain of one board and the face grain of another. The face grain
of most wood species providing a very good glue bonding surface. The end grain,
however, is notoriously weak and is prone to failure.
Butt Joint with Glue Block
Glue blocks are square or triangular wedges of wood that are glued to the hidden
side of the joint. These blocks can run the full length of the joint, or only be
used in strategic locations. One problem with case joint glue blocks, though, is
that their constructions usually has the glue block's grain oriented perpendicular
to the case boards' grain, resulting in a cross-grain construction. This can
cause problems when wood movement is involved.
Butt Joint with Fasteners
Nailed Butt Joint and Screwed Butt Joint
Nails or screws can be used to fasten a butt joint. Inserting the nails or screws
at alternating angles strengthens the joint and prevents the boards from separating.
In softer woods that have trouble holding screws or nails in the end grain, you
can reinforce the end grain portion with a strategically placed dowel which provides
enough cross-grain to increase the holding power of the screws.
Dowelled Butt Joint
Dowels are a decorative replacement for nails or screws. Again, they can be inserted
in alternating angles, increasing their holding power. Blind dowels can be created
by using stopped holes on the face grain board, however these holes are difficult
to produce without the aid of specialized tools, since doing them by hand or even
a drill-press is prone to uneven hole depths, or mis-aligned holes.
Using a contrasting wood type for the dowel can produce an attractive decorative
element to the workpiece.
Biscuited Butt Joint
Biscuits serve two purposes in case joints: They provide positive registration between
boards during assembly, and they provide a mechanical brace against shifting in
the finished product. Biscuits are essentially thin oval wafers that are inserted
and glued into a matching pair of slots that are cut into the two pieces to be joined.
When the biscuits are used at the top of an upright board, offset the location of
the slots slightly by 1/4 to 1/8 of the upright's thickness towards the hidden
side of the vertical. This is because if the joint should fail, the damage done
to the wood will be in a hidden location, and more easily repaired invisibly. If
the biscuit were placed in the center, there is an equal risk that the damage would
be exposed.
When the biscuits are used to secure a horizontal shelf to an upright member, the
biscuits should be offset slightly by 1/4 to 1/8th of the shelf's thickness
towards the bottom of the shelf. When the shelf is loaded with weight, the biscuit
will be applying force towards the top of the shelf. By shifting the biscuit's
location downwards, you thicken the amount of wood that is resisting that force.