Necessity of a Model
The seat blank is a critical piece of this rocking chair. All of the spindles and the legs attach directly to the seat. I made six bow-back chairs back in the mid-nineties and didn’t model the first seat blank. Unfortunately, I didn’t nail the design on the first attempt and chair number one is not as comfortable as the other five. I decided to take the time to model a seat blank for this rocking chair.
Gluing Up the Blank
I selected construction-grade 2×6 pine for the blank and alternated the heartwood in hopes that the blank would remain stable.
I was surprised that, despite this precaution, the blank released a tremendous amount of stress after I shaped it and it curled up 7/16”. This will impact the splay angle of the legs, but I’ll continue to use the blank for subsequent steps. Update: The seat continues to curl up. The total curve in the cross section is now 3/4”. I will no longer sculp the seat when using construction-grade pine for a chair model. It’s still worth it to use the pine in order to figure out angles for the various spindles and the leg holes. The only disadvantage of not shaping the seat is that the model will be 3/4” higher than the final product.
Marking Out Curves and Leg Holes
The curve for the front and the back of the seat blank is not a perfect radius. The back curve seems to have a flatter section in the center.
My technique for marking out these curves is to use a flexible stick. However, the curve for the back was too tight for my curve marking stick. No problem, I ripped a scrap piece of Formica and used that instead. I determined where the curve would terminate on the side of the blank. I chose 8” from the back and made marks on the blank here and at the center of the back. This is a four-handed operation. My lovely wife works well with me as we determine a pleasing curve and then she traces it out. Once you’ve determined a good curve, measure where the curve stick intersects the curve termination mark and then mark a symmetrical mark on the opposite side of the stick.
This will help achieve a symmetrical curve. I clamped a piece of wood at the back of the blank to provide a place to oppose the curve stick and flatten out the curve in that section. I used a similar procedure on the front curve without the flattening step.
Drilling Leg Holes with a Compound Angle Drilling Jig
The leg holes must be precisely located since they intersect the rockers. The back legs are located to create a 5 degree splay angle between the front and the back legs and also rest in the curve formed when shaping the seat blank. I’m trying a 15 degree splay angle to achieve the desired width for stability on these shorter than normal legs (they’re shorter because they terminate in the rocker versus resting directly on the floor.) The front legs are angled 7 degrees forward. The back legs are angled 30 degrees back.
I mark the leg hole locations with a compass. This provides a better means of aiming the drill press to the correct location than a center point or even a cross marking. Tip: it’s much easier to position the seat blank accurately on the jig if the blank is square when drilling the holes.
I use a home-built compound angle drilling jig to drill these 1” diameter holes on my drill press. Old-school Windsor chair builders might do this by eye with a bit brace…not me! I previously used a jig fixed at 10 degrees. However, I needed a 7 degree splay on my bar stools this past summer and decided to build an adjustable jig that work for any future projects. I rip a piece of MDF the correct width to obtain the angle I need and then label it for future use. This piece is inserted in a slot, opposite the hinged side, and raises the table to the correct angle. Gravity keeps everything in place.
I clamp the jig to the drill press and the seat blank to the jig while drilling each leg hole. There’s too much torque, when using a 1” Forstner bit, to not take this precaution.
Note: When drilling the left rear leg, the 30 degree angle may put the seat blank too close to the drill press handles. I removed them and used a screw driver inserted in the holes to plunge the bit.
Shaping the Seat with an Arbortech Turboplane
Note: It may make more sense to shape the seat first and then drill the leg holes. It is possible that, in the process of grinding the blank, the seat could splinter at the leg hole. It did not splinter on my pine blank. However, I may run a test piece of cherry before committing to this order of shaping with the 8/4 cherry seat blank.
I used to use a King Arthur chainsaw shaping disk mounted to my angle grinder. I would then clean up those marks with a carbide grinder as pictured.
I recently purchased an Arbortech Turboplane grinding disk. This disk cuts with more of a planing action, rather than grinding. It worked quite well with the pine blank. However, knots seemed to draw it off course more than the carbide disk.
The final surface was much cleaner than previous disks produced. I was able to easily clean up the Arbortech’s marks with a 50-grit sanding disk.
This disk seems safer than the Lancelot as the cutting depth is limited by the projection of the three cutters through the disk profile. Once the seat is taken down to the final depth, it is much easier to plane a flat section with this disk. I’m impressed with its quality of cut, safety, and ease of use.
I’ll turn the four legs and build the prototype rockers to fit them in. This will provide a stable base from which to fit the spindles and the back crest.
-- Mark, Florida