How do I draw finger (box) joints by hand without the gadget, and how do I tune the fit?

The idea

A finger joint is a row of equal rectangles notched out of the end of a board. Both mating boards can come from the same layout, shifted by one finger. Fit is then tuned in the toolpath, not by redrawing.

Drawing it (method from a published machine maker's tutorial)

1. Measure the real material thickness and the real end mill diameter with digital calipers. A tape measure is not accurate enough for this joint.
2. Use Draw Rectangle to draw the finished size of the box side.
3. Draw a small rectangle as wide as one finger and as tall as the material thickness. Pick a finger width that divides evenly into the board width (the tutorial uses five 1 inch fingers on a 10 inch board).
4. Select the small rectangle, then the large one, and use Align Left and Align Top.
5. Use Array Copy on the small rectangle. Columns equal the number of fingers, gap equal to the finger width.
6. Open Create Fillets, set the radius to half the end mill diameter, choose Dog-Bone Fillet and click each inside corner.
7. Clean up overlapping lines with Interactive Vector Trim so one closed outline remains.

Cutting and tuning

1. Use the 2D Profile Toolpath, outside the vector, with the measured bit diameter entered. The tutorial cuts 0.020 inch deeper than the material thickness.
2. Cut a test pair in the same material.
3. Adjust Allowance Offset. The official documentation defines it this way: a negative number cuts the shape smaller, a positive number leaves it larger. On fingers, positive means tighter and negative means looser.
4. Documented starting values: users of the box gadget on a public CNC forum start at 0.003 inch of clearance and found that too tight for pine plywood sheathing, moving to 0.006 inch. A tooling maker's article suggests about 0.1 mm. Treat 0.003 to 0.006 inch as the range to test, since material, bit and machine all change it.

Pro tips

• A tooling maker's guideline: make fingers at least twice the end mill diameter wide.
• Turn on a separate last pass so the final cut is light and the size is more accurate. See the entry on Separate Last Pass.
• If you would rather not draw, see the entries on the Box Creator gadget.