How to make a cross-lap joint

The cross lap — half of the wood removed from each part — is the simplest joint there is, and the best one to learn first: two pieces crossing each other, each cut away by half, holding together on their own once assembled. It is the joint of a small table's rails, of a crate's dividers, of a lamp's fins.

How to do it, in four moves

1. Create the two parts. From the “New solid” panel pick two rectangles. Every part is a flat sheet, as thick as the material you set in the “Material” panel. 2. Cross them. Rotate the second part (<kbd>R</kbd>) and move it until the two really cross. They must not merely touch: they must overlap, that is, occupy the same space for a stretch. If they stay apart, the joint cannot be worked out. 3. Select both. Click the first, then ⌘/Ctrl+click the second. “Create joint” appears in the right panel. 4. Choose “Half and half (cross lap)” and create it. The two slots appear at once in the 3D view.

Where the cut goes

The slot is taken half from each part. The “Share of A” slider moves that split: at 50% the two parts share the intersection equally, which is almost always what you want. Moving it to 30% takes less from the first and more from the second — useful when one part must stay stronger than the other, say a load-bearing upright against a light divider.

Why it sometimes refuses

“The two parts do not overlap.” They are close but not touching, or touching only at the surface. Move them until you can see them passing through each other in the 3D view.

“Parts are parallel: the half-lap joint needs parts at an angle to each other.” Two sheets on the same plane cannot cross at half wood: there is no crossing. To join two coplanar parts you need the tabs and slots joint, which alternates solid and empty along the shared edge.

Why it will not go in, or wobbles

If the joint is too tight, the cause is almost always one of these two — and they are different things:

• The thickness in the program is not the real thickness of your sheet. Plywood sold as 3 mm often measures 2.8: put a caliper on it and type what you measure.
• The clearance is zero. Two parts cut to the exact size never go together. Start at 0.2 mm: it goes in by hand with some effort. At 0.1 mm you need a rubber mallet; from 0.3 mm up it wobbles and wants glue.

If it is too loose even with low clearance, you are losing material to the kerf, the groove the laser burns: read the kerf guide.

Try it now

Open the app and press “Example” in the top bar: two rails are already crossed and joined. Change the sizes, move a part, and watch the slots redo themselves.