Router setup for carving chair seats
Disclaimer: Please use all applicable safety precautions while following these guidelines. I’m assuming a basic understanding of shop procedures and router use, as well as the robust application of common sense and care in what is an inherently hazardous environment- a woodworking shop. I cannot possibly predict all the ways in which an injury could result.
Timing: Once the setup is prepared, a repeatable seat carving ready for sanding can be completed in 45 minutes to an hour. Of course, accuracy, including that of the blank itself, is key to a good result.
The setup: A length of ¾” black iron pipe is rigidly mounted, centered at a height of 14¼” above the bench, between two supports about 32” apart. The pipe is parallel to the edge of the bench and set back 5½” from that edge. The pipe is fastened to prevent movement or rotation, lightly sanded for smoothness, and a coat of paste wax is applied. A carriage securely holding a router, attached at 3 points for rigidity, rides on the pipe with the freedom to swing in an arc and move along the pipe as needed. Two hose clamps with scraps of ⅛” rubber under them, also on the pipe, are used as stops to control carriage location and movement. The router carries a ¾” diameter, ½” shank mortising bit that sweeps an arc of 12¾” as the carriage swings on the pipe, with closest approach to the bench at 1½”. Router speed is set to avoid scorch marks. There are two pivot holes in the bench, one directly under the center of the pipe above (at 5½” setback from the edge) and one directly behind the first, at 8½” setback. The holes are sized to be a close fit to the short piece of ¾” copper tubing used as the pivot.
A sled, made to accommodate 18” x 18” seat blanks is itself 18” square, ½” thick, with added wedges to increase the effective thickness to ¾” at the rear edge. This will cause the carving to be deeper at the rear of the seat. The sled attaches to the seat blank with 4 screws at the corners and has 3 holes that accommodate the pivot tubing. While the exact location of the holes can vary with the desired configuration of the seat, the distance between centers of holes A and B should be twice the distance between the two bench holes. I settled on a 6” dimension between the sled holes. Hole C in the sled is on the front-to-rear center line and 3½” forward of A and B and used, in conjunction with the rearmost hole in the bench, for the final cleanup passes.
It is important that all aspects of the setup prevent unwanted movement or looseness, which would degrade the results. This includes the sled, which should be perfectly flat and not rock on the bench.
The procedure: To start, center the carriage directly over the bench hole and lock it in place with the hose clamps. Swing the carriage out over the floor and prop with a stick while positioning the sled, using hole A and the forward bench hole for locating purposes. Clamp the sled and blank parallel to the bench edge, using at least one deep-throated clamp to be sure there is full bench contact. Move the right-side hose clamp all the way to the right, out of the way, and slide the carriage to the right so it hangs freely beyond the edge of the blank. Start the router and, using a firm grip, make repeated passes, advancing the bit incrementally with each pass. All cuts should be from the edge toward the center, and the bit moved away from the cut on the return swing, in both cases to avoid “catches” which are dangerous and will mar the work. Many small bites produce better results than fewer larger bites.
Continue in this fashion until reaching the left-side hose clamp. Swing the carriage toward you and place a small block under it to keep the bit off of the blank while you re-position the right-side hose clamp to once again secure the router in the centered position.
The curved corner calls for the blank to be rotated counterclockwise, again in small increments, and clamped in place for each cut. Once the blank is clamped, start the router and make the cut, then shut off the router before pulling it back, to prevent catching on the cut edge. Block the router up as in the previous paragraph while rotating the blank for the next cut. Do not bog the router with a too-ambitious bite. Continue this sequence to a point just beyond a 90° arc, as observed by the edge of the blank in relation to the bench edge. With the router blocked out of the way, clamp the blank with its rear edge parallel to the edge of the bench and move the left-side hose clamp leftward along the pipe exactly 6” (if using the other dimensions I have cited) and retighten. Proceed to cut across the rear of the blank using the same procedure as on the first side, stopping at the left-side hose clamp.
Relocate the carriage to the centered position and secure by returning the left-side hose clamp to its original location, then once again prop the carriage out over the floor with a stick. Move the pivot to hole B in the sled and cut the second corner using the same steps as in the first. Continue slightly past the 90° point.
Clamp the blank parallel to the bench, slide the left-side hose clamp out of the way to the left, and proceed to cut the second side of the blank using the same method as for the other straight cuts. Remember to avoid catches when returning the bit for the next pass.
The final step, with the router again propped out of the way, is to move the pivot to hole C on the sled and the rearmost hole on the bench. The carriage must be re-secured with the two hose clamps in the centered position. Beginning with the front of the seat facing right, use the corner-cutting technique previously detailed to clean up the area around the inner end of the center ridge. The sled will be rotated through an arc of about 160°, or about 80° to either side of center. The actual removal of material will occur only in the inner couple of inches of the swing of the cradle, however, it is still important to re- clamp the blank between each pass.
The finish: If you have followed these steps accurately, there will be only minor sanding required, about 10 minutes, preferably with a DA-type sander. I start with 80 grit on a 5” pad, then go to 120 on a 6” pad. We have test driven this seat profile at our dinner table and found it to be more than satisfactory.