How do I dial in a cross lap (half lap) joint so the parts slip together by hand?

What decides the fit

In a cross lap, each part has a notch as wide as the other part is thick. The fit depends on three measured things: real material thickness, real bit diameter, and how your machine cuts that material. The reliable method, published by a CNC machine maker, is a small test board with a ladder of clearances.

Test board workflow

1. Measure material thickness and the bit's cutting diameter with digital calipers. A resharpened bit is smaller than its label.
2. Set up a 12 by 12 inch test job in the same material.
3. Draw Rectangle: a 2 by 5 inch test part.
4. Draw the notch: width equal to the material thickness, height slightly more than half the part width. Use Align Bottom to sit it on the part edge.
5. Draw a mortise rectangle too if your project has through slots: slightly longer than the part width, height equal to material thickness.
6. Use Interactive Trim to open the notch into the part outline. Copy the part to make its mate.
7. Make two more pairs. In the second, make the notch and the mortise 0.003 inch wider than the material. In the third, add another 0.003 inch.
8. Mark each pair with one, two or three shallow dots using small circles and a Drilling toolpath.
9. Cut the outlines with a 2D Profile Toolpath with tabs and ramps, and tick Do Separate Last Pass with an allowance of 0.015 inch so the final pass is light and accurate. Cut the internal mortises with a Pocket toolpath (or an inside profile).
10. If the measured bit differs from the database, click Edit and type the real Diameter (D).

Judging the result

• Parts should slip together with hand pressure and not wiggle.
• Edges should finish flush. If one part stands proud, deepen the notch.
• Slide a part through the mortise as well.
• Use the winning pair's numbers for the real project, and repeat the test whenever the material or bit changes.

Pro tips

• Inside corners of the notch still carry the bit radius. Add dog-bone or T-bone fillets if the mating edge is square.
• Plywood varies sheet to sheet. Test from the actual sheet.
• See the entry on press-fit and slot-together parts for thickness variation.