Coming soon (June 12) in the August 2012 Issue of Popular Woodworking Magazine, is an article by Willard Anderson titled “Rule Joints by Hand and Power,” which both explains the history of rule joints and walks you through the process of cutting them. After cutting the joint, part of the secret to creating a joint that works smoothly is setting the hinge. Anderson’s instructive photo and description (below) makes it clear not only how to mortise and mount a rule joint hinge, but also explains why it matters. If you want to try out cutting the joint yourself, building Steve Shanesy’s Shaker Drop-leaf Table is both a handsome and manageable project.
The example at bottom is the instructive but “worst case” scenario. It shows the hinge mounted upside down with no mortise. Note how far down and out the drop leaf hangs.
Just above it the hinge is mounted correctly but only the barrel is mortised in. The drop leaf hangs even with the table leaf, but there’s still a large gap.
The middle example shows the hinge mounted correctly and fully mortised in. Note that the drop leaf hangs even with the top of the land and up against the table leaf.
The top two images show the joint with the hinge mortised in correctly, but mounted either forward or behind the scribed centerline. In this case, the joint can only be partly opened, rubbing either at the bottom or at the top, respectively.
The two principles at work here are that as the hinge pin is moved up into the stock, the drop leaf rests higher; and as the hinge is mortised in, the drop leaf is pulled towards the table leaf.
– Willard Anderson