MARKING AND BORING POST AND LEG MORTISES
Laying out the seat and the mortise outlines
Lay the seat blank face-up on a work surface, mark the center of two adjacent sides and use a carpenter’s square to extend the lines to the opposite sides of the blank, bisecting the center of the seat blank. The resulting grid will help you draw a symmetrical outline for the seat. To mark the outline, either use an existing chair and copy it or create your own design using the dimensions in the chart on page 14 as a guideline. In each corner of the seat you will need to mark a mortise for a leg. The back of the seat will also require an extra mortise in each corner for the chair’s posts. Use a measuring tape to ensure that the mortises are perfectly symmetrical. You will also need to mark reference lines to help you drill the mortise so the legs and posts will splay out away from the side of the seat at the proper angle. There are no strict guidelines for what this angle should be. Again, use an existing chair as a model. Legs typically flare out by 105° to 115°; posts by 110° to 120°. Use a protractor to mark a reference line through the mortise and extend it toward the center of the seat (above). Repeat this procedure for each mortise.
Making a rake angle jig
As well as splaying out, the mortises for the legs and posts must be angled—or raked—toward the front or back of the chair. To help you drill the resulting compound angle you will need the help of a simple jig in addition to the splay angle reference lines you have already drawn on the seat. Use a sliding bevel to mark the rake angle on a piece of scrap stock (left). This angle should be between 10° and 12° from vertical for the front legs and posts, and almost twice that for the back legs. Clamp the stock in a vise and cut along the line. You will need to make a separate jig for each of the different angles you will require for the chair.
3 Drilling the post mortises
To drill the post mortises, clamp the seat blank to a work surface with a scrap panel underneath to protect the surface and reduce tearout. Next, install a spoon bit in a brace; an auger and a spade bit are suitable alternatives. Line up the drilling guide with the splay angle reference line for one of the post mortises. Then center the bit on the mortise and begin drilling, keeping the bit parallel to the slope of the rake angle jig (right). Repeat to bore the other post mortises.
Drilling the front and back leg mortises
Use the appropriate rake angle jig to help you drill the back leg mortises, then drill the front leg mortises. In this case, the same guide was used for the front leg mortise as for the posts. Line up the jig with the splay angle reference line and bore the mortise. Then cut out the seat and sculpt it (page 73).
1 Turning the legs and stretchers on the lathe
Install the square leg stock between centers on your lathe and adjust the tool rest as close as possible to the stock without touching it. Use a roughing gouge to shape the leg. making sure that you keep the bevel rubbing at all times and the tool pointing in the direction of the cut. Then turn the tenon with a parting tool. Turn off the lathe periodically and use calipers to check the diameter of the tenon (left). Then produce the strechers the same way, turning a tenon at each end.
2 Cutting kerfs for wedges in the tenons
Remove the finished leg from the lathe and wrap it in a rag leaving the tenon end exposed, and secure it in a bench vise. The rag will protect the stock from the jaws of the vise. Use a backsaw to slice a kerf in the center of the tenon to a depth of about three-quarters the length of the tenon. Repeat for the other legs and stretchers.
ASSEMBLING LEGS AND STRETCHERS “I Drilling the stretcher mortises
-L Stretchers are installed between legs to provide sufficient tension in the leg assembly. Dry assemble the legs in the seat with the kerfs in the tenons perpendicular to the grain of the seat, and position the stretchers. Then cut a piece of scrap stock slightly longer than the distance between two legs. Clamp the wood between the two legs just below the mortise level to serve as a reference for keeping the drill bit horizontal. Place a wood pad between the legs and the clamp jaw, or use clamps with non-mar – ring jaws like those shown at right. Next, install a brad-point bit in an electric drill and wrap a piece of tape around the shaft of the bit to mark the depth that the mortise should be drilled—the length of the stretcher tenon. Holding one leg near the bottom, sight along the horizontal guide and drill the mortise to the proper depth (right). Repeat for the other mortises, readjusting the height of the horizontal guide as required from stretcher to stretcher.