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The Woodwright's Guide: Working Wood with Wedge & Edge By Roy Underhill
When the oval table gets large enough to merit drop leaves, we return to circles. Circular rule joints on drop leaves look good even when the leaves are down, and they keep the leaves fully supported when up. Like their namesake joints on folding rules, one element of the rule joint is concave and the other convex — both elements sharing a common center. With a hinge also set at this common center, the drop leaf can fold down with its concave edge constantly cupping close to the convex edge on the main top.
Rule joints use special hinges made with one leaf longer than the other and with the countersunk holes on the face opposite the barrel. Their thickness and individual geometry determine the center of rotation, so have these hinges in hand and see that they are all well matched before cutting any wood. Working from them, and the thickness of the tabletop, you can make scratch stock scrapers that will guarantee a close-fitting joint. I’ll describe the steps for drawing the joint first on paper to then use as a guide for filing the steel scrapers. I’ll describe this for flush hinges, but you can have them countersunk or, even better, set in sloping recesses.
Draw two parallel lines separated by the thickness of the tabletop, adding one vertical line representing the two butting edges of the top and leaf. Take the hinge and measure the distance from the face of the hinge to the center of its pin. Measure up the same distance from the lower line and draw there a faint parallel line.
Measure down from the top line to the depth of the square-butting fillet at the top of the joint, less than a third of the thickness of the top. Draw another faint parallel line at this level.
The distance between these two faint lines is the radius of your rule joint. If you have a set of hollow and round or table planes, you will want to adjust the depth of the top fillet to make this radius match. Lacking such, the scrapers can do the whole job. Set the point of your pencil compass at the intersection of the vertical line and the lower faint line. Adjust the compass to touch the upper faint line and draw the quarter circle into the leaf half of the joint, then faintly mark the intersection A on the opposite half.
Reset the compass to pivot on point A and draw the arc of the convex element. With a few more straight lines for the fillet and bottom, you have the paper patterns to glue on your scrapers for grinding and filing. The concave element on the leaf needs to be just a shade larger than the convex element, so take this into account. Since the convex scraper shapes the concave element, leave it a pencil line larger than your drawing when you file it to shape.
Scrapers can do the final shaping, but faster-cutting tools can remove most of the wood. Lay out the same lines as before on the tabletop and the leaf. When you gauge the shoulder of the fillet on the upper face of the main top, gauge this same line on the underside. You will need this lower line when mounting the hinges.
For the concave element, mark the upper and lower limits of the curve with the marking gauge. You can rough in the hollow with a gouge, or with the fillister or rabbet plane. Follow with a round plane and then the scraper mounted in a scratch stock. The scratch stock can be any handy scrap wood, but it must have shoulders to ride against the guiding surfaces of the tabletop, keeping it precisely in place as you pull it along.
Begin the convex element by sinking the top fillet shoulder with the fillister or rabbet plane. Take off the corner with a plane. Follow with a hollow plane and then use the scraper.
All the care put into shaping the wood won’t help if the hinges aren’t set right. Assemble the joint with the two elements separated by a piece of paper to assure clearance. Set a hinge across the joint with the long leaf of the hinge on the drop-leaf side. Align the axis of the hinge on the line showing the center of the circle. Poke four little marks at the corners of the barrel of each hinge. Connect these four points and take out the waste wood with a chisel.
If you are countersinking the entire hinge, take care not to cut any deeper than you need, for the error will appear every time you lower the drop leaf. Cut the inset for the hinge at a slope on the drop-leaf side. This leaves the hinge countersunk at the barrel and sloping to the surface at the end — preventing these unsightly gaps.
When the hollow for the barrel is completed, set the hinge in place to mark the screw holes. In any hinging operation, it’s wise to use just one screw on each side so you can test the operation of the joint before continuing with the rest of the screws and the other hinges.
"The Woodwright's Guide: Working Wood with Wedge & Edge" By Roy Underhill
© 2012 The University of North Carolina Press