Plywood is inherently weak unless you use proper joints. Craftsmen choose between four common types of joints utilising penetration with glue. Furniture makers like the dowel or biscuit joint. Cabinetmakers typically use the spline or half lap, although both disciplines are not limited to any single type of joint.
Cabinetmakers use the spline joint to connect plywood together to make freestanding islands or any jamb that is wider than 48 inches. To create a spline joint, cut a long channel or "dado" down both sides of two corresponding pieces of plywood. Then, insert a wooden spline into one piece with glue. Clamp the other piece of plywood onto the first piece using the extended spline from the first piece as the common joint.
Biscuits are small, wooden "footballs" that you insert into divots. The divots are cut into the wood with a handheld machine called a biscuit cutter. The biscuit joint can be used to join smaller pieces of plywood together when a spline is not appropriate. Biscuit holes should be cut into the wood about 4 inches apart Biscuits can then be inserted with glue. Both pieces of plywood can then be clamped.
Dowels are similar to both splines and biscuits. Dowels are also surgically installed, but penetrate deeper than splines or biscuits. They can be used to connect plywood for furniture-making. Typically, 3/8-inch dowels work best for plywood since most common plywood applications are no thicker than 3/4 inch. Craftsmen use a small, handheld tool called a dowelling jig to drill holes along the edge of the plywood. Craftsmen hammer dowels into the holes with glue, align the corresponding pieces of plywood together and secure them with clamps.
Half-lap joints can be used when the joint will not be exposed to a great deal of stress. The half-lap is created with a router by cutting a 3/8-by-1/2 -inch channel or "rebate" down the top of one piece of plywood. The corresponding channel is cut on the bottom of the corresponding piece. Glue is placed on both channels. The channels fit together and can then be clamped tight. The half-lap joint is used to join plywood together for cabinet jambs or doors.