Template routing makes this inlay easy.
Butterfly keys create an iconic woodworking shape. Often, they’re incorporated into a natural “slab” tabletop to prevent a split in the wood from opening up. The wedge-shaped wings are oriented across the grain (Photo 1, below), locking the split in place. But butterfly keys can also make attractive inlay accents, even if they are faux rather than structural. You can arrange them cross-grain over a glue joint, or even along the grain, as we’ll show in the following technique, to hide a knot, pitch pocket or other natural defect. Whatever the intention, butterfly keys are easy to make using a simple inlay kit. It consists of a threaded template guide that fits in a router’s subbase and a 1/8-in.-dia. straight or spiral router bit. A separate bushing, also included with the kit, fits over the template guide’s collar to offset the bit position for milling the inlay’s mortise. Then, by removing the bushing, you can use the same setup for routing the key so it fits its mortise precisely. The technique is easy to do. Here’s how.
How to make butterfly inlay keys:
Step 1: Select a butterfly key template with an opening large enough to completely cover the defect you plan to hide. Most acrylic butterfly key template sets will include several sizes. The template opening should be at least 1/4-in. larger than the longest or widest part of the defect; the inlay will turn out smaller than the template opening.
Step 2: We’ll start by cutting the mortise. Draw crosshair reference lines across the defect to help you position the template squarely, relative to the edge of your board or glued-up panel. Now mount the template over the defect with strips of double-sided carpet tape, and press the template down firmly (see Photo 2).
Step 3: Install the router bit that comes with the inlay kit in your router’s subbase. Since the process of cutting the mortise and the inlay key will involve plunging the bit down into the wood, choose a plunge router for this technique. Screw the template guide to the router base, and press the bushing into position around the guide collar (see Photo 3).
Step 4: You can mill the mortise as deep as the router bit will allow, but a particularly deep mortise isn’t necessary. Remember, the inlay is only decorative here. A mortise depth of 1/8- to 1/4-in. is sufficient, and it will enable you to make an inlay piece thick enough to be easy to work with and reasonably durable. Measure the thickness of the acrylic butterfly key template material, and add it to the depth of the mortise you want to make. Adjust the router’s cutting depth to this measurement.
Step 5: Set the router into position over the template opening, start the machine and lower the bit to about half the final depth of cut. Feed the router around the inside of the key shape, pressing the bushing up tight against the walls of the opening as you go. Once you’ve “traced” the key shape with the first pass, move the router around the opening to mill away the rest of the waste (see Photo 4). Check your progress. If all of the waste material is routed away, repeat the process with the bit lowered to its full cutting depth to complete the mortise.
Step 6: Now you’re ready to make the inlay. First, be sure to remove the bushing from the template guide. This will shift the bit’s cutting position over by exactly 1/8-in. — or the diameter of the bit.
Step 7: Choose a piece of scrap stock for your butterfly key that’s thicker than the inlay you want to make. A piece of 3/4-in.-thick material will work fine. The inlay scrap should be large enough to provide adequate support under the acrylic template. If it’s narrow, such as the scrap shown in Photo 5, place two more scraps on either side of it that match its thickness. Secure the inlay stock to your workbench temporarily with double-sided tape or clamps. Now, tape the same template you used for mortising to your inlay workpiece. Press the template down firmly to secure it.
Step 8: The inlay piece should be slightly thicker than the mortise is deep so you can scrape or sand it flush with the surrounding wood. Reset your router’s depth of cut about 1/32- to 1/16-in. deeper than for routing the mortise.
Step 9: Cutting the key to shape is more exacting than routing its mortise: you are literally tracing the outside edges of the key as you cut it to shape. If the guide collar doesn’t follow the edges of the template opening precisely, you will change the shape of the inlay piece and open up gaps in the final fit. To prevent mishaps, start the router with the guide collar pressed tightly against the template opening. Slowly plunge the bit into the wood, and rout clockwise around the opening. Be very careful that the guide collar never loses contact with the template walls as you proceed. Make two passes: one at about half the inlay depth, then a final pass to full depth (see Photo 5). Blow or vacuum out the shavings and dust between passes. Just to be safe, rout two inlay pieces, in case one ends up fitting better in the mortise than the other one.
Step 10: The easiest way to free the inlays from the scrap is to resaw them with a band saw. Set your band saw’s fence so the blade will intersect the bottom of the inlay cuts and release the inlays when it passes beneath them. Feed the scrap on edge against the fence to cut the inlays free (see Photo 6).
Step 11: Check the fit of the inlays in the mortise and choose the one that fits best. If it doesn’t slide down into the mortise easily to begin with, you may need to remove a bit of material around its edges to improve the fit and taper it slightly. A sharp cabinet scraper works well for this task, and so does a sanding block. Scrape or sand below the top edges of the key so you don’t change the shape of its “show” face (see Photo 7).
Step 12: When the key slips down either completely into its mortise or to about 2/3rds of its thickness, it’s ready to install. Spread a thin layer of glue onto the bottom face of the key (see Photo 8). Tap the key home gently with a mallet, if necessary. Cover the inlay with a piece of scrap first, to protect it from the mallet blows and to support the fragile corners.
Step 13: Plane, scrape or sand the key flush with the surrounding wood to complete the installation (see Photo 9).