How to inlay a bowtie (butterfly key) with a router

A router and two templates make clean, repeatable bowtie inlays. Here's the whole process, from design to flush.

What you need

• A plunge router with a guide bushing set, or a brass inlay kit
• A straight or spiral bit that fits the bushing
• Template stock: 1/4" (6 mm) acrylic or MDF
• Contrasting hardwood for the keys
• Double-sided tape or clamps, a chisel, glue and a plane or sander

Steps

1. Design the key. Pick a size and style in Bowties.cc. Use the Soft style with a corner radius at least as large as your bit's radius if you don't want to chisel corners.
2. Make the templates. Download the SVG or DXF and cut the recess template and key template from 1/4 in acrylic or MDF on a laser or CNC.
3. Lay out the recess. Centre the recess template over the crack with the key's length running across it, line up the score marks, and fix it with double-sided tape or clamps.
4. Rout the recess. With the bit and bushing the templates were made for, rout the recess in shallow passes to depth, keeping the bushing against the template edge.
5. Rout the key. Fix the key template to contrasting stock a little thicker than the recess is deep, grain running along the key's length. Rout around it and cut the key free.
6. Test the fit. The key should press in with light hand pressure. Ease a tight spot with a file or sanding block rather than forcing it.
7. Glue and flush. Glue the recess walls and floor, tap the key home, let it cure, then plane or sand it flush with the surface.

Tips

• Do a test inlay in scrap first, every time you change bits or bushings.
• Stop the crack from growing before you inlay: a slab that is still moving will push on any key.
• Fill the crack itself with epoxy or a sliver of wood if you want it closed. The key holds it, it doesn't hide it.
• Laser users can cut the keys directly: choose Laser-cut keys and set your kerf for a press fit.