Cardboard Ball Chair

Step 8 Finishing Ring - Size, design and construction

Start the ring the same ways as the half-sphere: Stapled first layer, 1/2 inch templated cardboard pieces overlapped every row until the final height is reached.

Depending on the final depth of the chair you may want to go beyond the initial half-sphere. I have door openings around 29 1/2 to 30 inches (depending on trim), not counting the door itself which can be taken off the hinges. I built a ring portion onto the half-sphere, but had to stop at 5 inches to allow the finished chair to fit through my doorway.

With a half-sphere 24 inches high and 2 inch half-round pieces (for the rounded edge) I had a 3 inch ring left to build. To make the inside of the chair feel as encompassing as possible I decided to make the height of the perpendicular sides (90 degrees off the 3 inch side) 6 inches higher.

In retrospect I should've made the height of the ring the same on all sides, or made the high sides even higher. A same-height ring would've been easier to finish and the feeling of wraparound would've been the same. A ring with a much higher high-side, such as 12 or 15 inches, would've made for a very cozy and enclosing space inside the chair, but could've made the chair impossible to get through my doorways. I had only 1/4 inch to spare when getting through the final doorway. Making those sides higher could've been bad.

The high-low design I chose required a smooth sine-wave shaped edge. This was more work than I expected. To cut the edge I designed a full-size quarter-circle pattern in InkScape and printed it on multiple sheets of paper taped together. I taped the pattern to my model, traced the line, moved it to the next quarter section, did it again, and again to get the smooth line to cut the edge.

Cutting the unfinished edge of the ring was difficult to do smoothly. The biggest challenge was the need to have it be at a right angle to the surface of the sphere. The cardboard layers are parallel to the surface of the turntable. If left uncut the half-circle disks wouldn't lay correctly on that surface.

If a only a half-sphere chair is wanted, no cut would be needed to apply the half-disks. The flat surface of the half-sphere (where it sat on the turntable) is already at a right angle. This surface would also be good for gluing and not need a paper-mache layer. However, a chair such as this wouldn't have as enclosing a feel. It would be more like a solid papsan. Then again, a half-sphere chair could be larger and still fit through doorways.  Decisions, decisions...

For days I contemplated making this cut using a power tool such as a router or RotoZip. The 4 inch depth of cut was a problem though. I wanted to make a jig but in the end I decided to save time by freehanding the cut with a hand saw, eyeballing the right-angle. I was left with a very jagged cut. I wrapped the cut surface with 2 layers of newspaper and paper-mache to create a flat paper surface for gluing.

Since writing this instructable I've figured out a way to use the inner cutter arm to cut a flat (non sine-wave) surface. Clamp a fairly rigid blade to the flat surface of the inner cutter arm, with the tip past the end a small amount, about a half inch. The blade is pushed into the cardboard on the cut line when setting up. Pull the inner cutter arm so that the clamped blade cuts on the line. Another piece of wood could be mounted to the bottom of the arm to keep the height exact.

Multiple cuts would need to be made. The blade is advanced further and the cut made again. The blade used would have to be at least 8 inches to allow for the 4 inch final depth of cut and to provide an ample clamping surface. An inexpensive kitchen knife from a thrift store with the handle cut off could work well for this.