How do I use a biscuit joiner?

A biscuit is a compressed beech oval that sits in a curved slot in each piece. When water-based glue reaches it, it swells and locks tight. It is fundamentally an alignment device: it stops two boards sliding out of line while you clamp them, which on a long edge joint or a panel of cabinet parts is worth a great deal of time.

Safety

What you'll need

• A biscuit joiner
• Biscuits in the size you need
• PVA wood glue
• Clamps
• A pencil

Step by step

1. Dry assemble the parts in their final position and clamp them.Everything is marked in place. Take the parts apart only after marking, and keep the orientation marks on them.
2. Mark a single tick across the joint line.One pencil stroke that crosses from one piece to the other. You are matching the machine's centre line to the tick, so no measurement is needed and no error can accumulate.
3. Space the slots 150 to 200 mm apart and keep them off the ends.Start at least 50 mm from an end so the slot does not break out, and space evenly along the joint. On a cabinet part, a biscuit near each end and one or two between is plenty.
4. Set the depth for the biscuit size you are using.The machine has detents for the common sizes. Use the biggest biscuit the material will take — a bigger biscuit means more glue area and more alignment.
5. Set the height from the same face on both pieces.This is the rule that decides whether the joint comes out flush. Reference the faceplate against the good face of both parts, or lay both flat on the bench and use the base, but never mix the two.
6. Plunge with the machine held firmly against the work.Steady pressure, all the way to the stop, then let it retract. Do not twist the machine, which widens the slot and loses the alignment you are buying.
7. Dry fit the whole assembly with the biscuits in.Check it goes together and the faces are flush before glue. A biscuit that is too proud in its slot means the slot has not gone deep enough.
8. Glue the slots as well as the biscuits and the joint faces.Glue in the slot is what makes the biscuit swell and lock. A dab in each slot, glue along the edge, biscuits in, assemble and clamp.
9. Clamp and check flush before the glue grabs.Biscuits allow a small amount of slide along the joint but almost none across it, so any misalignment left now is permanent.

Tips

• Biscuits swell for a while after they meet glue. Do not sand or plane a biscuited joint flush for at least a day, or the swelling can telegraph as a bump under the finish once it dries back.
• In a mitre, biscuits go in at an angle and cross the joint neatly, but you need to check the slot does not break through the outside face of a thin mitre.
• Keep biscuits in a sealed box. They absorb moisture from the air, swell, and then will not go into a correctly cut slot.

Common mistakes

• Referencing off the fence on one part and the base on the other — The two slots then sit at different heights and the joint can never be flush. Pick one reference surface for the whole assembly.
• Marking with measurements rather than a tick across the joint — Every measurement carries an error and the errors do not cancel. A tick that crosses both parts cannot be out.
• Cutting slots too near the end of a board — The slot is longer than the biscuit and can break out through the end grain, which shows on the finished piece.

If it doesn't work

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