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The Woodwright's Guide: Working Wood with Wedge & Edge By Roy Underhill
What supports a drop leaf when it’s up? Sliding supports can pull out, and wings and gate legs can open on metal hinges. But when you want a single rail and leg to swing out, you use the knuckle joint — a wooden hinge with intermeshed wooden barrels turning on a metal pin. This joint shares characteristics of the rule joint, in that each cylindrical knuckle fits in a concave socket on the adjoining piece. Like dovetails, however, knuckle joints always join the ends of boards, not the sides.
You can cut the knuckle joint in the middle of a single board — just remember that you will lose some length to the joint. Both ends must be true and square. Set the gauge to the thickness of the wood and mark all around the ends of both pieces. This defines a long square space, within which we’ll shape the cylinder of the knuckles.
If you want a knuckle joint that swings 180 degrees, the cylinder diameter can equal the thickness of the wood. Oddly, a lesser, 90-degree swing requires a smaller cylinder so there will be wood for a stop when the rail is straight. Usually there is some other piece in the table that acts as the stop, so the full cylinder is most common.
The center of this cylinder is the intersection of diagonal lines connecting the corners of the squares on the edges. Confirm this center with your gauge set to half the thickness of the wood, gauging in from three sides to define the same point. Set a small compass at the centers and inscribe the ends of the cylinders on both pieces.
Before we shape the cylinders, we need to saw the line defining the stops. These stops are miters, bevels that meet when the joint reaches a right angle. The line of these mitered faces is the same as the diagonals we drew to find the centers of the cylinders. Take the try square and bring the intersection of the diagonal up and across the faces. These are the first lines to saw, shallow cross-grain cuts reaching to the intersection of the cylinders and the stops.
Now you can shape the cylinders and the stops. As with the secret miter dovetail, rough in the shapes with cross-grain paring and follow with a lowangle shoulder plane. If you want to make a concave sanding block to finish the rounding, take an auger the same size as the wood is thick (or the next larger one) and bore a long hole through a piece of scrap. Saw this in half and you have it.
For the knuckles, divide the width of the rail into five equal parts using a diagonal ruler. Don’t bring these four diagonal marks over to the line; just use them to set the marking gauge for strokes delineating the knuckles. Mark both pieces all around both sides before changing the setting, and be sure to run the fence of the gauge along the same edge. Mark Xs on alternating segments to show the waste on both pieces.
Set one of the ends upright in the vise and confirm the waste pieces. Saw on the waste side of the lines, the edge of the kerf running down the center of the gauged lines. Saw only as deep as the cross-grain cuts defining the shoulders. Remove the waste with chisels as in dovetailing. An in-cannel gouge can round the hollow of the end sockets. On the intermediate sockets, undercut with scooping strokes of the chisel.
When the two fit together and work freely in your hands, clamp them together and drill the pin hole squarely through from both ends, meeting in the middle. Rub some tallow on the surfaces before you drive home the pin.
There is an old belief that whistling while cutting wood for a wagon wheel will cause the finished wheel to squeak. You can extend this whistling prohibition to include the work time on all moving constructs, including rocking chairs and knuckle joints. It’s a useful belief, because repeating it is the only polite way yet found to get habitual whistlers in the shop to give it a rest.
"The Woodwright's Guide: Working Wood with Wedge & Edge" By Roy Underhill
© 2012 The University of North Carolina Press