Lapped Finger Tenons

On this page, you'll find the tutorial and process for creating lapped finger tenons to make wood joints.

wood - 174.2x126 x18mm excluding endmill diameter

CNC machine

As I examined the shape of the cuts that would be used to recreate lapped finger tenons, I came up with the above image as the base shapes. It is made up of a 2d array of circles, with the 4 sides of each circle connected along tangent lines.

*Make sure that the diameter of the circles is at least the size of the diameter of the end mill you will be using to cut out the joints!

The joints are made of semicircles connected by straight lines, as seen above. Using the base shapes, I trimmed the lines that were not necessary to create the construction lines. According to the image, the piece on the left will be used to lap over the piece on the right (which will now be referred to as the pocketed (left) and unpocketed (right) pieces).

I continued to adjust the file, producing two prototypes in the process, one of which can be seen above.

When creating your model, make sure to trim properly - connect the lines directly to circle tangents so there are not any leftover pieces, and keep only the desired edges. Make sure only the correct, closed curves are selected to create 3d models and export for cutting.

To make sure the joints fit, unlike the prototype seen above, make sure to use these measurements:

• The diameter of the circle should be at least the diameter of the endmill used to cut the pieces out
• The length from the semicircle to the end of the pocketed piece (in this image, 18mm) should be equal to the width of the wood being used
• The length from the semicircle to the end of the unpocketed piece (in this image, 9mm) should be equal to the width of the wood being used minus the depth of the pocket

You can test your pieces by extruding the curves to create a 3d model, such as the ones you see above in red and blue.

Attached are the construction lines that were exported and sent to the CNC application. The 3 small pieces are cut with a pocket toolpath offset on the inside, then the 2 large pieces with a contour toolpath offset on the outside.

*When the pieces pictured were cut, in the file I used, the bottom edge of the last joint on the pocketed piece overlapped, and did not intersect, with the bottom edge. Since this pocket was cut out with an inside offset, this joint turned out rounded, affecting the fit of the joints. This issue has been resolved in the attached file.

Above, you'll find the images of the final joints that I cut out for this project, sanded and assembled! The file above can be adjusted and used to attach separate pieces with a lapped finger tenon joint.