Cutting a Butterfly Key Joint

In addition to producing an exceptionally strong joint, butterfly keys are one of the more visually appealing joints you can make.

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Step 1: Selecting the right material

Select the right wood for the butterfly key. Apart from selecting a wood with similar density dimensional stability as the wood with the crack, a good idea is to select something that would provide a good color contrast. For strength, the key needs to be cut so that the grain of the wood runs the lengthwise on the key.

Step 2: Marking the Butterfly Key

Divide the length of the key in two and mark the center as a reference line. Mark the key out, from the top of the centerline, at approximately a 75 degree angle. Carefully mark the end for the side cut line, finally marking the in from the bottom side to the bottom of the centreline.

Step 3: Cutting the Butterfly Key

Cut the key, using a blade that is easy to direct. Ideally, start the cut by scoring the end grain of the stock along the cut line. Then begin to angle the cut downward along the cut line on the top surface.

Step 4: Cutting the Mortise

Set the key face-up on the spot of the crack picked out for the mortise and trace around it. Cut the shape of the mortise with either a chisel or a plunge router, making sure to stay within the outline.

Step 5: Finishing the Butterfly Key Joint

Check that the key fits about halfway into its mortise, without much pressure and with no large gaps showing. Coat the side and bottom of the key with a film of glue, tap it into place - use a block of wood to protect the top of the key, and set it to dry. Plane edge of the key and level with a scraper, if necessary.