How do I cut a sliding dovetail (like a sliding box lid) with a dovetail bit?

What makes it tricky

A dovetail bit is wider at the bottom than at the top. The software's profile toolpath positions a tool by one diameter, so you have to tell it how far to shift the cut so the angled wall lands where you want it. Both parts (the slot and the sliding piece) are cut with the same bit at the same depth, on opposite sides of the same line, with a small clearance added to one of them.

A documented method (shared on a public CNC forum for a pencil box lid)

The maker used a 14 degree dovetail bit and document variables so the math updates itself:

1. Create a variable for the lid thickness and one holding the tangent of the bit angle (the tangent of 14 degrees).
2. Copy one set of vectors to both the lid and the box body. The lid is cut on one side of the line and the body on the other. The lid is machined from its bottom face.
3. Lid toolpath: cut depth equal to the lid thickness, and an Allowance Offset of minus one times lid thickness times the tangent, divided by 2.
4. Box body toolpath: same depth, same formula, with a further 0.004 inch subtracted. That extra 0.004 inch is the sliding clearance.
5. Cut the lid first. Test it on the box while the box is still clamped to the table. If it is tight, change the clearance value and recut the box before unclamping.

Practical rules that follow from the bit shape (shop reasoning, not stated in the official documentation)

• The dovetail bit must cut its full depth in one pass. A shallower first pass would put the wide part of the bit too high and destroy the angled wall. Set the tool's pass depth at least as deep as the cut.
• Because of that, clear most of the waste first with a straight end mill in a pocket or profile that stays inside the narrowest part of the dovetail, then let the dovetail bit take only the angled walls.
• The bit cannot plunge into solid wood inside a closed slot. Start it off the edge of the part or in an area already cleared.

Pro tips

• The 0.004 inch figure is one maker's result in his wood. Humidity will change a sliding fit, so test on scrap and leave a little more room for wide lids.
• For keyhole hanging slots, which use the same undercut idea, see the entry on the Keyhole Toolpath gadget.
• For adding unusual cutters to the tool database, see the entry on bits the software has no type for.