Butterfly Basics

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This article is from Issue 53 of Woodcraft Magazine.

Stop cracks in their tracks in just 6 steps.

Wide slab tabletops are beautiful but, even when properly seasoned, big boards often include checks (splits along the ends). Gluing and clamping a crack might seem like a fast and easy fix, but any success will likely be short-lived as the internal forces that caused the crack will probably open it again or create another crack. Ripping the plank into thinner sections, jointing the edges, and reassembling the top will relieve the stress but, in addition to the extra work, milling creates unsightly interruptions in your grain lines.

When I want to stop the crack and save the plank, I use butterfly keys. They are nothing more than custom-shaped splines that span the width of the opening. Despite their delicate-sounding name, they provide a sturdy means of limiting further separation.

Historically, keys were made from matching wood in order to conceal patches. These days, they are usually cut from a contrasting species, thereby turning a flaw into a decorative detail.

Creating custom keys doesn’t require any special tools or jigs. To see how to save your next slab, follow the steps shown here.

Note: I’m not including a pattern because I think you need to let the piece determine the key’s size and shape. A key should be large enough to limit further wood movement, but not so large as to become visually overpowering. (Keys typically range from 1⁄4ʺ to 3⁄4ʺ thick, depending on the slab’s thickness.) For long cracks, two or three keys might be better than one.

A Perfect Fit from a Router Bit

If you need to make multiple butterflies, or if you prefer a power-tool approach, you can make perfectly-fitting keys with a plunge router and inlay set (09I16, $48.99) and either a shop-made or store-bought template (146903, $26.29).

To make the key, use the template guide and bit to rout the key stock. To excavate the mortise, position the template on your work, install the bushing on the guide to offset the bit, and then rout the mortise.

Note that the 1⁄8" bit is fragile. For large keys (and deep mortises), you might want to stick with my technique.

After creating a paper pattern to suit the target crack, trace the shape onto a straight-grained piece of wood. Then bandsaw the key to shape, and clean up the edges with a sharp chisel. 

Orient the key perpendicular to the crack, and lay out the mortise with a sharp-tipped pencil or marking knife. Apply a firm grip, or use a clamp to secure the key as you trace around it. (When making multiple keys, number each to ensure a match with its particular mortise.) 

Excavate the bulk of the mortise with Forstner and brad-point bits. Make the mortise about 1⁄8" shallower than the key thickness. Wrap tape around the bit to serve as a depth gauge.

Using a sharp chisel, carefully chop up to your layout lines. (When test-fitting the key, be careful not to force it. Wiggling the key in and out may compromise the final fit.)

Chamfer the bottom edges of the key to ease its insertion and to create small pockets for excess glue at the inside bottom corners of the mortise.

Apply a thin layer of glue to the inside of the mortise, and then tap the key home. After the glue has dried, plane across the patch until it’s almost flush with the top, and then finish up with a cabinet scraper and random-orbit sander.


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