In this Instructable, we will create a self-supporting stool that requires no screws, nails, or other joiner pieces to be assembled. This stool is built as a three-dimensional puzzle.
Step 1: Prep Calculations and Material Gathering
As a method to visualize the complex cuts of the stool’s legs and seat, I created a 3D model of the stool in Solidworks. The height and angle between legs was adjusted to give a vertically slender aesthetic.
It turned out that the quality of the stool’s vertical form was difficult to evaluate entirely in CAD. Legs at 45 degrees to each other resulted in a vertically unappealing form. This angle was later adjusted to 33.4-degrees to achieve the desired silhouette. Further, drawings generated in CAD were not useful in manufacturing, due to the extreme precision required of the cuts, and the difficulty of establishing drawing reference points for the complex cuts on the tubes.
Step 2: Preparing the Wood
A large piece of mahogany was used as the material for the wood seat. Making use of the wood around a large crack, the wood was cut and jointed into pieces of roughly 15 x 5 x 3 inches. These pieces were glued together to form a rough 15 x 15 x 3 inch block, which was further cut, jointed, and planed to form a 14 x 14 x 2.75 inch block.
Step 3: Shaping the Seat
The block of mahogany was sanded into a rough circle of 13 inches in diameter. This was then turned on a wood lathe in order to form a bowled surface. The edges of the bowl were removed in favor of a smooth transition between the bowled surface and the edge of the seat. Seat was sanded up to 220 grit.
Sanding was used as a major material-removal process since this particular wood produced deep tears when cut with sharpened lathe tools.
Step 4: Drilling the Legs
Cuts were made with drills, slowly increasing drill size in increments of 1/4-inch diameter until the desired hole size was reached.
A two-vice set-up with V-blocks and automatic feed was consistently successful in the set-up of an angled drill. The angle was at 56.6-degrees from the normal to the mill bed, with the center of the drill at a distance of 16.75 inches from the edge of the 35-inch tube. A 2-inch diameter tube was drilled with a 1.5-inch wide hole in this fashion.
A 1.5-inch diam. tube was then inserted into the hole of the 2-inch diam. tube, making sure that the length of tubing between tube edge and intersection was equal for both tubes. Tubes rested on V-blocks lined with 120-grit sandpaper, serving to increase friction between the tubes and the V-blocks. Pieces of duron were added beneath the blocks supporting the 1.5-inch diam. tube, in order to minimize rocking. Perpendicular alignment of the two interconnected tubes to the mill bed was done by matching a ruler that touched the edge of the mill bed on one side, and both ends of the tubes on the other. The difference in tube diameters was taken into account by eyeball measurement. Strap-clamps held the configuration securely in place. The center of the drill was placed 1.4826 inches from the intersection point of the two tubes.
Drilling left big burs that needed to be removed with careful ling, to maintain the shape of the holes.
Step 5: Drilling Holes in Seat Bottom
Flat-ended holes for the legs to fit into were drilled with forstner bits. A practice piece made of glued two-by-fours was used to gain confidence in the drilling process.
The assembled legs were centered on the bottom of the wooden seat, and the contour of each leg was clearly marked. Secondary markings 3/4 inch toward the direction of the center of the seat were made for each leg. These secondary markings were used to indicate the edges of the holes to be drilled.
The first hole to be drilled was the 2-inch diam. hole. A digital angle gauge was used to determine the angle from the 2-inch diam. leg to the back of the wooden seat. The bed of the drill press was tilted to this desired angle, and the edge of a forstner bit of 1.5-inch diam. was aligned with the secondary marking for the 2-inch diam. leg, before drilling. A hole of 2.25-inch depth from initial contact with wood was drilled. From there, the 2-inch wide hole was led by hand and the 2-inch diam. tube was inserted to the bottom of the hole.
The 1.5 inch diam. tube was then inserted into the corresponding hole on the 2-inch diam. tube, and the point at which the 1.5 inch diam. tube hit the seat was marked as the next hole edge for drilling. Previous markings on the seat were used simply as points of reference for accuracy of the hole orientations. A 1-5/8 diameter hole was drilled to a depth of 2.25 inches. The slightly larger hole size made it easier for the tube to fit into place.
The process was repeated for the 1-inch diam. tube, with a 1-1/8 diameter hole and 2.25-inch depth of cut.
Step 6: Polishing and Finishing
With the stool assembled, a final hole of 3/16 inch diameter was drilled into the side of the seat closest to the 1-inch leg, through and into the metal leg. A bronze pin is inserted into these holes in order to secure the legs from falling when the stool is picked up.
The ends of the legs touching the ground were cut and sanded at angles to be ush with the ground. Bronze tubes and pin were sanded to 400-grit and then buffed with a tripoli compound.
Wooden seat was furnished with shellac. An initial attempt at finishing with linseed oil amplified small marks on the wood, and was discarded in favor of shellac.