The curved slats join the back legs at an angle, which makes positioning the rung mortises tricky. The wood tends to split during assembly if the rung mortises are drilled either perpendicular to or directly in line with the grain, so I orient the mortises accordingly. To do this, I lay the pair of legs on the table and rotate each one until the face grain radiates at a 45 degree angle from the center of the chair. (See figure 2).
With the back legs in this position, I mark the front of the chair and then mark the point where the legs touch in the middle. This creates a reference line for locating the rung mortises later and also establishes the inside edge of the slat mortises.
I cut the rectangular slat mortises in each back leg using a hollow-chisel mortiser and sliding V-block jig (photo M). Since the back legs taper slightly from bottom to top, I gradually decrease the depth of these mortises from
1 1/8" for the bottom slat to less than 1" for the top slat. This compensates not only for the leg taper but also for the graduated length of the slats, which I mentioned earlier.
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Next, I sand the curved edge of the slats using a stationary drum sander (photo N). As I work, I also sand the slats to thickness, checking them for fit in the mortises (photo O). After rounding the edges, I give the slats a once-over with the palm sander using fine-grit paper, then apply a coat of oil. Because the slat ends will be pegged but not glued into the leg mortises, I oil the entire surface. |
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To mortise the back legs for the back rungs, I insert a slat to each leg, then secure the leg in the V-block jig (photo P). The sliding jig I use for mortising the legs (also shown in photo M) consists of a V-grooved "trough" that bolts to the drill-press or mortiser table and a second length of V-block that slides in the trough. I secure the leg to the sliding block with rubber bands, then clamp it down tight with a vise-grip clamp to immobilize it for drilling or mortising.
I align the back-rung mortises with the slat mortise by sighting down the leg from the side of the drill press. Using the drill bit as a guide, I rotate the leg in the jig until the end of the slat intersects the vertical plane of the bit. I bore the mortise for the seat rung directly below the scored groove and the lower-rung mortise midway between the grooves. At this point, I assemble the slats, rungs, and back legs, using no glue on the slats (photo Q).
To mortise the front legs for the front rungs, I again orient the face grain at 45 degrees (so it faces the center of the chair), then lay out a centerline on each leg. (See figure 2). I mark the inside surface of the legs, then drill the mortises on the centerline just below the scored grooves.
The chairs are trapezoidal in shape (the fronts being wider than the backs) so the side rungs must be off-square with respect to the front and back rungs. To mortise for these, I first assemble the front legs and rungs (having assembled the back earlier). I secure one leg in the sliding jig, supporting the rest of the sub-assembly on a cradle beveled to the required outward angle (photo R).
Each front leg, in turn, receives an inward-angled mortise. I drill these side-rung mortises directly on the scored line, so that the front- and back-rung mortises, drilled directly below the lines, will overlap them. (See figure 1.) This overlapping allows the intersecting rung tenons to be glued to each other as well as to the leg, which reinforces the joints.
To allow for seasonal swelling and shrinking in the chair, I also orient the grain of the rungs. I turn the face grain toward the top and bottom, so the edge grain faces inside and outside. This ensures that cross-grain swelling and shrinking in the rungs and legs will coincide rather than conflict.
I cut tenons using a lathe-mounted tenon cutter (photo S). I used to hand-turn the tenons using a box wrench as a sizing tool, but I've found this specialized cutter to be much faster and more accurate. I cut the tenons approximately 1 1/8" long, drilling the mortises slightly deeper to make room for any excess glue.
In greenwood chairmaking, the tenon-mortise fit is critical. It must be tighter than a slip fit but not so tight that it crushes the wood or splits the post. This is where the difference in moisture content comes into play. I dry the rungs in a microwave oven before cutting the tenons. The microwave dries wood from the center out, unlike conventional kiln-drying, which dries from the outside in, sometimes causing case-hardening or honeycombing of the wood.
I've never measured the actual difference in diameter between mortise and tenon, although I've read that the tenon ought to be about .003" larger for greenwood joinery. Once I've driven the rung tenons home using a shot-filled mallet and block of softwood, they're impossible to remove (photo T).
Traditionally, greenwood chairmakers do not use glue, but I find that it helps reinforce the rung joints and also slows seasonal wood movement. On the slats, rockers, and arms, I use pegs but no glue. After joining the front and back of the chair with the side rungs, I check the legs for flatness on a level surface and attempt to rack the chair flat if necessary.
