How do I clamp a glue-up so it comes out square?

A glue-up is the one stage in woodworking with a deadline. Everything that can go wrong has to be anticipated before the glue is open, because there is no time to solve it afterwards.

What you'll need

• Clamps, more than you think
• Scrap cauls or offcuts to protect the work
• A tape measure and a damp cloth

Step by step

1. Dry run the entire assembly.Every clamp positioned, every part in place, nothing glued. It exposes the joint that will not close, the clamp that will not reach and the part that is the wrong way round.
2. Lay out everything within reach first.Clamps opened to roughly the right width, cauls cut, cloth damp, glue open. Once the glue is spread you have a few minutes.
3. Use cauls under the clamp heads.Scrap offcuts spread the pressure and stop the metal jaws denting the timber. On a panel, cauls also hold the surface flat across the width.
4. Alternate the clamps above and below.All clamps on one face bows the panel towards them. Alternating sides balances the force and keeps the assembly flat.
5. Check square by measuring diagonals.On a rectangular assembly, measure corner to corner both ways. Equal diagonals means square. This is far more reliable than a square held against one corner.
6. Pull it square with a diagonal clamp if needed.A clamp across the longer diagonal, tightened gently, brings the assembly back. Recheck both diagonals after every adjustment.
7. Tighten only until glue squeezes out evenly.A continuous small bead all along the joint means enough pressure. Cranking further starves the joint and can bow parts.
8. Deal with the squeeze-out now or later, not in between.Either wipe it immediately with a damp cloth, or let it go rubbery and pare it off with a chisel. Half-wiped glue spreads into the grain and shows up under finish.
9. Check it sits flat before you walk away.On a known flat surface. A twist set into a frame at this point cannot be corrected afterwards.

Tips

• Equal diagonals means square. It is faster and more accurate than a try square on one corner.
• Alternate clamps above and below to stop a panel bowing.
• You always need more clamps than you own. Borrow before starting, not mid glue-up.

Common mistakes

• Skipping the dry run — Every problem you would have found dry — a joint that will not close, a clamp that will not reach — arrives instead with a clock running and glue setting.
• Tightening the clamps as hard as possible — Excess pressure squeezes glue out of the joint and bows components. Enough is a continuous small bead of squeeze-out along the joint.

If it doesn't work

Questions people ask