Biscuit joint

A biscuit joint uses a thin, oval wafer of wood, glued into matching half-moon slots in the two pieces. A plate joiner (biscuit joiner) cuts each slot with a small circular cutter that plunges into the edge or face, so the slots line up as long as the machine is registered against the same face on both pieces.

Sizes and depths

Lamello's three common sizes are #0 (47 × 15 × 4 mm), #10 (53 × 19 × 4 mm), #20 (56 × 23 × 4 mm), all cut with the same 4 mm-wide cutter at depths of 8 mm, 10 mm, 12.3 mm. A smaller H9 biscuit uses its own thinner cutter. Lamello points out that the cutting depth stays the same for both pieces of a joint. Every size, with inches, is on the biscuit size chart.

What the slot allows

Lamello lists sideways sliding tolerance among the system's features: the parts can be shifted a little along the slot while the glue is wet. It also notes that panels can be slid together from an angle, not only straight on as with dowels, and that thin stock of 8 to 10 mm can be joined at many angles.

Where it fits

Lamello presents biscuits as a joining system for panels. Where a joint meets end grain, the USDA Wood Handbook says a plain glued end-grain butt joint can't be made strong enough for ordinary service, and that such joints need interlocking parts glued edge grain to edge grain; its examples are dowels, mortise and tenons and rabbets, and it doesn't discuss biscuits. This site gives no strength figures for biscuits: the published tests we found each used a different rig, so their numbers can't be compared.