Wooden-boat builders have long used “bird’s-mouth” joinery to construct hollow masts and booms. We landlubbers can take advantage of this strong edge-to-edge joint, shown above, when making cylindrical objects, such as columns, arched chest lids, or turned vessels.
The bird’s-mouth joint tops a miter joint for a number of reasons: First, introducing the cutaway “mouth” creates more gluing surface. Secondly, the mouth cradles the mating workpiece, making it nearly impossible for the joint to slip out of alignment during a glue-up. Finally, you cut only one side of the joint, so you reduce your machining time—and chance for error—by half.
Boatbuilders make the bird's-mouth joint on very long pieces using a tablesaw. As with any multi-faceted project, though, a tiny error in the cutting angle can become huge when compounded at each joint. You'll reduce your error rate to near nil, however, when you machine your workpieces with a bird's-mouth router bit (sometimes called a "multi-sided glue joint" bit). By fixing the cutting angle at the factory, these bits virtually guarantee your success when making 6-, 8-, 12-, or 16-sided cylinders.
So, how many sides do you need? That depends ultimately on the nature of the project. The more sides in your cylinder, the smoother the curve. If you plan to turn the cylinder blank round, more sides also mean less waste, because you can use thinner stock to construct the blank. On the other hand, if you want the cylinder to have an angular, faceted look, use fewer sides.
For a simple column, setting up to use the bit is as simple as the joint is strong. In your router table, install the proper bit for the number of sides (or staves) in your cylinder. Set the bit’s cutting height to leave a small (say, 1⁄32 "), flat bearing surface on the workpiece, as shown in the photo below.
After routing all the staves for the cylinder, apply glue to the routed edges, stand the staves on end, and clamp them together with band clamps. If you have a lot of sides to assemble, a pair of scrapwood discs that fit inside the cylinder will help keep it round.
Want to be a little more creative? You can use the joint to decorate and "break" the edges of a case or chest, as shown in the photo below Or, if you're up for a challenge, use bird's-mouth bits to create tapered cylinders or cones, like the ones illustrated shown below.
Things start to get a little tricky here, though, as the number of staves in the cone doesn’t necessarily match the number of the cutter. (For example, the 12-side cutter can be used to cut cones with 3–12 staves.) The fewer the staves, the flatter the cone—a 3-sided cone looks like a squashed pyramid, while an 11-sided cone is nearly a cylinder. You will need to do some figuring to determine the cutting angles for the staves. Detailed instructions and simple formulas for making the calculations come with the bits.Before you buy, we encourage you to view the instruction manual on Lee Valley's Web site. At $26 each, these three bits (one for 8-, one for 16-, and one for 6- and 12-sided cylinders) aren’t budget breakers, but you’ll want to make sure you get the right bit for your project. Sources:
Written by: Dave Campbell with Jeff Mertz
Photographs: Marty Baldwin
Illustration: Tim Cahill