How do I join boards edge to edge into a panel?

Edge jointing is the operation that turns narrow boards into a table top, a door or a carcass side. It is also the joint people most often over-engineer, because the intuition that a bigger joint must be stronger does not apply — a long-grain glue line in a modern PVA is genuinely stronger than the surrounding wood, and every reinforcement you add is there for alignment, not strength.

What you'll need

• A jointer plane, a router with a straight bit and fence, or a table saw with a good blade
• Sash clamps or pipe clamps, at least three
• Straight battens for cauls
• PVA wood glue
• Winding sticks or a straightedge (optional)

Step by step

1. Arrange the boards before you touch an edge.Lay them out, shuffle for grain and colour, and mark a triangle across the whole panel so the order and orientation cannot be lost. Every subsequent step depends on the boards staying in that order.
2. Make the mating edges straight and square.By hand plane, on a jointer, or with a router and a straight fence. The edge does not have to be perfectly square if you use the match-plane trick below, but it must be straight along its length.
3. Use the folding trick if you cannot get the edge square.Plane the two mating boards together, folded face to face in the vice. Any angle error is then equal and opposite, and the boards lie flat when opened out — an old technique that removes the need for a perfectly square edge.
4. Test the fit dry, with no clamps.Hold the boards together on a flat surface and look along the joint against a light. No light should show anywhere. A joint that needs a clamp to close is a joint that will be under permanent stress.
5. Leave a slight spring in the middle if anything.A joint very slightly hollow in its length — a fraction of a millimetre over a metre — closes at the ends first and puts the ends under compression, which is exactly where a panel is most likely to open later.
6. Glue both edges and rub the joint together.An even film on both edges. Rubbing the boards together briefly squeezes the glue into a thin film and pulls the joint closed by itself. You should see a fine continuous bead of squeeze-out.
7. Clamp from alternate sides.Clamps only underneath bow the panel upwards; clamps only on top bow it down. Alternate above and below, and use moderate pressure — enough for continuous squeeze-out, not enough to starve the joint.
8. Fit cauls across the panel at each end.Battens above and below, clamped, keep the joint faces level with each other while the glue sets. Wax them or put tape on them so they do not glue themselves to the panel.
9. Check flatness before the glue grabs.Sight across the panel with winding sticks or a straightedge. You have a few minutes to release a clamp and correct a twist; after that it is permanent.
10. Leave it a day before flattening.The wood near each glue line is swollen with water. Level it now and the joints sink as they dry, leaving a ridged surface.

Tips

• Biscuits or dowels in an edge joint add alignment, not strength. They are worth it on a wide panel of many boards, where holding everything flush by hand is difficult, and unnecessary on two or three boards with good cauls.
• Rubbed joints — glue, rub together, and leave with no clamps at all — genuinely work on short, well-fitted edges and are the traditional method.
• Glue is at its strongest as a thin, continuous film. Starved joints and thick joints are both weak; a fine even bead of squeeze-out along the whole length is the visual sign of the right amount.

Common mistakes

• Clamping a bad joint closed — The wood is then permanently stressed and the joint fails first, usually as a split beside the glue line rather than in it. Fit the edges properly instead.
• Clamping all from one side — The panel bows away from the clamps and sets that way. Alternate the clamps above and below the panel.
• Over-tightening the clamps — Excessive pressure squeezes almost all the glue out and starves the joint, and can bow the boards. Moderate, even pressure with continuous squeeze-out is the target.

If it doesn't work

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