With a special jig, a good bit of time and a LOT of cardboard you can make the very comfortable, very groovy ball chair. Since the bulk of the material is scrap cardboard this also makes a nice recycling project.
You can upholster the final chair like I did or just paint it and fill it with pillows. The plans for this instructable are for a 4' diameter chair that will (just barely) fit through a 30" doorway. You could scale it up or down easily depending on the size of person it's for and how much "surround" you want.
The chair stands on the base by gravity alone and the unattached design allows for easy readjustment of the ball. I like sitting in it more upright, and my daughter likes it kicked farther back.
I made this for my daughter's 13th birthday and it was a big hit. She's an avid reader and it makes a perfect cozy zone to open a book in. When she saw it she called me a "mad genius" which is high praise from a 13 year old. One of her first questions was "how will I get it to college?" I think she likes it and plans to have it around for a long time. It was a big project, but well worth the effort.
Step 1: Sphere Cutting Jig
The most important part of this project is the jig. This large but relatively simple jig makes a perfectly shaped, hollow, half-sphere . The horizontal turntable and outer cutting arm cut the outside. The inner cutting arm cuts the inside surface, the hollow area of the sphere. Be aware that this jig needs to sit on a sturdy table or sawhorses. The pieces it makes are heavy so pick a place for it carefully.
The turntable doesn't need to be perfectly circular since it's the rotation, not the shape, that cuts the circle. In fact, an octagon shape would be preferable to a circle as the corners on the octagon would make for easier turning during a cut. An octagon shape would also be easier to layout and cut.
I used a few pieces of leftover 3/4 inch melamine for the turntable and base and standard 2 x 4 lumber for the swing arm and mounts. Some 3/4 inch plywood would work fine for the turntable and base. Don't use 1/2 inch or thinner material due to the weight it'll need to support. To make a round turntable use a router mounted on a long strip of material such as fiber board, screwed loosely to the radius point. Measure carefully for the center, mount the radius board and cut a perfect circle.
The swing arm arrangement is connected to its mounting points with 1/2 inch lag screws. These fasteners can be loosened to allow the arm to swing freely, or tightened to hold it in place.
Both cutters are simple utility knife blades. I used blades with holes so they could be mounted to their bases with small screws. These blades are inexpensive and easily replaced as they dull.
The inner cutter arm is made with a 2 inch by 3/4 inch piece of wood with a slot cut in one end for the swivel. Use an extra piece of stock on the other end, held in place with 2 screws. The inner cutter blade is trapped between the two pieces of wood and clamped in place by tightening the screws. The distance from the swivel mount hole to the blade clamp will determine the inner radius of the sphere. To get a 4 inch thick wall use a 24 inch outer radius and a 20 inch inner.
I am 6' tall and this inner diameter is comfortable enough when sitting in the chair with a papasan cushion. I would've preferred to make the ball slightly larger, but getting the finished chair through doorways was the limiting factor. A larger radius is more comfortable. It's less likely to feel like your head is being tipped forward.
A point I can't stress enough is take careful measurements. Measure your doorways very carefully or you will make a cool chair that will live forever in your shop.
See the instructions on step 8 for additional tips and considerations on how big the chair could/should be.
The swivel mount for the inner cutting arm is made with a 3/8 or 1/2 inch thumb screw with a 1/4 inch hole drilled in the center of the flat portion. The swivel mounts to the turntable using nylon-insert nuts and washers. These nuts allow for a precise adjustment without slipping. The swivel must spin freely, but not wobble. Use a thumb screw long enough to go through a pivot hole in the base. In this way the swivel mount also functions as the turntable axle.
The width of the base doesn't need to be exactly as noted in the plans (72") but beware that having too small a gap between the edge of the turntable and horizontal mounts will make for frustrating cutting before the layers of cardboard get higher than the mounts. Too close and your glued cardboard will keep bumping into the mounts and have to be cut away to spin the turntable.
Step 2: Get Some Cardboard
By get some cardboard I mean get lots. Then get some more. This chair uses a huge amount of material and creates lots of scrap.
I started the project using 1/8 inch cardboard. This thickness is VERY easy to source and you'll be doing stores a favor by taking it off their hands. However, building up a 29 inch chair will take a lifetime. Do yourself a favor and take the time to find 1/2 inch cardboard used for watermelon or pumpkin bins.
Some stores are reluctant to part with these bins as they can be used with a shallow plastic tray insert to merchandise smaller produce. I had to check back frequently to my neighborhood market to get the bins as they emptied, but before they are crushed and sent to the recyclers. This generally means going to the stores early in the morning when the stocking is done. Be prepared to help the produce guys or gals move some melons to free up your bins.
I used about 8 of these 4' x 4' bins for my chair. Sourcing this many can take awhile so I would have to periodically use up what I had then replenish my supply.
A few parts of the chair benefit from thinner cardboard. The very first layer needs to be stapled or nailed to the jig and this is easier done with thin cardboard. As the top of the half-sphere is being completed the cuts will get less perpendicular to the curved surface and require a very shallow cut. Using thinner cardboard on these shallow cuts will take more time, but will be easier with a short blade.
Step 3: Setup and Cut First Layer
As noted in the previous step, this layer should be made from thinner cardboard such as 1/8 inch. If you mark the turntable with circles on the inner and outer cut lines it will make setting up the first layer easier.
Place large pieces of cardboard on the jig overlapping the cut lines by at least 1". Leaving less will cause a rough or jagged cut. Stapling the first layer is best as the staples will easily tear out of the layer when the completed half-sphere is removed from the jig.
Note: Do NOT glue this layer to the table. You want it to hold securely to the turntable, but be easy to remove. Small nails or brads will probably work too. Don't scrimp on fasteners to attach the layer to the jig. The finished half-sphere will be heavy (approx. 50 lbs.) and having it shift or become detached would be very bad. Once you glue the second layer to the first you are committed and cannot secure the first layer any more.
Although the picture shows many small pieces being used, fewer large pieces works better. Large pieces are easier to fasten and are less likely to shift.
When the first layer is secure make your cuts. The outer cut is made by loosening the lag screws holding the swing arm and moving it down until it pierces the cardboard. Lock the arm in place by re-tightening the lags. Rotate the table and the blade should cut the cardboard easily and cleanly. The inner cut is made by grasping the inner cutter arm and pulling the blade all the way around the turntable circumference.
You should be left with a clean ring of cardboard with a 4" cross-section width. See the second picture for an idea of how the cut ring should look.
Step 4: Prepare Ring Piece Template, Cut Stock
At first I used different shaped pieces of cardboard but that was inefficient. To save time I made a keystone-shaped "brick" pattern out of a piece of fiber board and used that for cutting consistent cardboard pieces from the 1/2 inch stock. The time it takes to do this is well worth it.
I have a well-appointed shop so I was able to quickly prepare my pieces by cutting long 6 inch strips on a table saw. I then made the angle cuts with a radial arm saw locked to 10 degrees. These cuts could also be made with a hand held radial saw or a handsaw. Even though the 1/2 inch cardboard cuts easily with a hand saw, I'd use that as a last resort as there will be many yards of cardboard to cut.
The pattern was made when the ring was approximately 2 inches high. It is a fairly accurate 1/16 section of the circle. For the next few inches of rise the cardboard pieces fit together well with little or no gaps. However, as the layers rose and the diameter of the rings got smaller the pieces didn't fit together as well. The final piece to finish each ring was usually a partial piece that had to be hand-cut to fit the gap. This didn't turn out to be a problem because small gaps won't weaken the body of the sphere, larger gaps can be filled with scrap and a paper-mache layer will be applied to the half-sphere when the cutting is complete.
Because the angles and width of the pieces will change as the project progresses it's safer to only prepare a few rings' worth of pieces at a time. I found myself adjusting the size of my ring pieces every few rows and it saved me from having pre-cut pieces that were too narrow to have enough overlap for a clean cut.
Step 5: Position, Glue and Cut Layers
The first of many layers!
For glue I used an interior-grade Elmer's Glue-All. This was a nice inexpensive glue ($13 a gallon) that setup quickly and never shifted once set. I used just under a gallon for the entire project.
I stretched the glue a bit by thinning it with water. This wasn't done to save money but to make the glue easier to work. By thinning the glue it grabbed better when positioning the pieces. A stencil brush made applying the glue easy. The best I can describe the change in consistency is the glue is initially the viscosity of room temperature honey, but is better thinned to something like a bottled syrup.
To set a piece apply a moderate layer of glue on two pieces on the previous layer. Apply to adjacent halves. Overlap the pieces as in brickwork. When placing the pieces the glue will grab better if the piece is moved around a tiny bit before final positioning. This action spreads the glue evenly.
I put weights on each layer after gluing. This is shown in some of the pictures. This may not have been necessary. I'm sure I got a slightly better bond, especially when some pieces didn't lay perfectly flat, but the thinned glue and small movement to spread the glue seemed to hold the pieces very securely. Not using weights would've saved a surprising amount of cumulative time and effort. A heavy 4 foot by 4 foot board could work as well as the multiple weights and would take less effort.
Once a full layer is setup and the pieces are secure perform a cut as on the first layer. Loosen and move the swing arm until its cutter pierces through the securely glued cardboard piece. Lock the swing arm in position and rotate the turntable through a full rotation, cutting off the overhanging scrap.
As stated in the previous step, make sure to leave at least 1/2 inch of cardboard overhanging the inner and outer cut lines for the smoothest cut. As the chair body builds the width of the pieces will have to increase slightly. As the rings get smaller remember to make a new template, or just eyeball a larger width to give the required overlap and angles.
Be sure to save a good amount of scrap to use for filler pieces and as stock for the rounded edge (step 9). The rounded edge uses 4 inch diameter cardboard disks cut in half. Smaller pieces of scrap can be thrown away without wasting too much material.
Step 6: Build Chair Body, Layer by Layer
Repeat the previous step many times. For a 48 inch outer diameter the height of the first big piece, the half-sphere, is 24 inches. By using 1/2 inch stock this is approximately 48 separate layers, although the first and last half dozen layers will come out better when using thin stock.
Step 7: Finish Half-sphere
When the half-sphere is nearing completion the inner cutter arm becomes more difficult to use due to the constricted space and the shallow cutting angles. The rising body essentially swallows the cutter arm. At this point switching to thinner cardboard such as 1/8 or 1/4 inch will build the height more slowly, but will give smoother cuts without switching to longer blades.
At a point approximately 5 inches from the top the inside cuts will no longer be possible. At this point remove the short clamping section from the inner cutter arm and allow it to fall inside the form. For the remaining layers I used single pieces of good quality, straight 1/4 inch cardboard with the shrinking inner circle approximated. Cut out that piece and center the hole when gluing.
When the top is reached, you're done with the first big portion. This is a good time to have a beer if you haven't already. It's a major milestone, but approximately the halfway point. Mix up some wheat paste and use newspaper to paper-mache a layer or two to cover the cut edges of the layers.
After the paper layers dry completely use a thin metal tool such as a wide chisel or pry bar to work the cardboard half-sphere off the turntable. The staples used to hold the first layer down will tear out of the cardboard, but cause little damage. The torn areas will eventually be covered anyway.
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.
Step 9: Finish Rounded Edge
To finish the exposed edge of the chair I used 4 inch cardboard disks cut with a hole saw. Each disk was cut in half and wrapped with paper-mache'd newspaper to make a gluing surface. The flat edge of each half-disk was then glued to the paper-covered edge of the ring piece.
Since the circumference of the inside cut is smaller (125 inches vs. 150 inches) there will be a gap between the outside edge of each piece. Small pieces of 1/8 inch cardboard used as shims fill those gaps.
In retrospect I wish I'd made a jig or saw guide for cutting the disks in half. I eyeballed each cut but didn't realize that even a slight variation in height is glaring when the pieces are arranged side by side.
To level out the pieces I made a thick batch of wheat paste to use like putty. I'd assumed it would fill in the gaps. The paste was thick but shrank and cracked as it dried. It did level out the gaps a bit but not as well as I'd have liked.
My wife made a concoction called paper-mache mush for another project that would've worked much better. It's wheat paste that has shredded and saturated newspaper mixed into it. Mix wiht a hand mixer. Add some white glue and this gray oatmeal like mush makes a great filler material. However, this shouldn't be necessary if the disks are cut carefully.
An upholstered chair is less likely to show these imperfections. A painted chair more so.
Step 10: Remove Outer Ring From Jig and Glue to Half-sphere
When the paper layers on the ring are completely dry, remove it from the jig as you did the half-sphere but be careful to not let the piece flex too much. It won't have as much structural integrity as the half-sphere.
Test fit the ring portion on the half-sphere without glue. Hopefully the ring fits the half-sphere very closely. Use long thin nails such as paneling nails to help align the pieces for the test fit. These can be reused in the next step.
Apply glue liberally to the mating surfaces. Join the pieces and reinsert the aligning nails, using the previous points. Lay 2 boards at right angles across the rings. Lay weights across the boards while the glue dries.
When the glue is dry and the pieces are securely joined tie the ring and half-sphere together with multiple layers of newspaper and paper-mache. This will help hide the seam and should make the chair stronger. Use the same alternating diagonal layering technique described in a previous step. I used a brown paper painting tape for this step. While it is stronger than newsprint, the latter conforms to curves better and rarely wrinkles.
In preparing for the project I stockpiled a supply of brown paper bags from FreeCycle, but did not use very many. I found the newspaper to be easier to work on curved surfaces, which is every surface of this chair.
Step 11: Modify Sphere-cutting Jig for Base
The base of the chair is built similar to the body, but smaller, solid and has a concave surface the same radius of the chair. The chair sits in this concave well.
When sitting in the chair I wanted to be able to have my feet touching the ground. Any higher and your legs would dangle and pressure would be on the backs of your knees. I found a seat height of 16 inches worked well. With a 4 inch cushion and a 4 inch thick chair body this meant the concave surface had to start at 8 inches.
The concave surface is cut with a swinging cutter bar similar to the one for the half-sphere, but mounts to a swivel above the stock. The concave section needs to have the same radius as the outer surface of the half-sphere. The inner cutter used for the first section is 20 inches so it can't be reused for the base. The concave cutting arm for the base needs to be 24 inches.
The swivel point for the arm needs to be 32 inches above the turntable (24 + 8) and precisely centered. The best tool for this is a plumb bob. The wood arms holding the crossbar are 36 inch pieces of 2 x 4 with some simple bracing. Find the center of the crossbar member and drill a small hole to pass the plumb bob string through. Reposition the vertical 2 x 4s until the plumb bob aligns perfectly on the center of the turntable. This is easiest to setup when no stock is on the turntable and the center point clearly visible.
Redrill this hole for the swivel when the jig is setup and secured.
Step 12: Building the Base
The base of the chair is 3 foot in diameter. This is easily achieved by moving the swing arm cutter bar from 24 inches to 18. Remove the inner cutter bar completely from the jig since this piece is solid. Start the first layer as before using thin cardboard stapled to the base.
Use better pieces of cardboard for the outer edge but fill the inner area with scrap if you need to conserve material.
Build up to the 8 inch mark in preparation for making the concave cuts.
Step 13: Make Concave Cut in Base for Chair Body
As described in the 2 previous steps the concave surface of the base holds the ball. At the 8 inch mark begin making these cuts in addition to the outer cuts.
Compared to the cuts up to this point these are difficult to make cleanly. The first layers cut with the concave arm are very shallow. When using thicker material the blade won't go all the way through the cardboard and will leave exposed corrugated layers. Using 1/8 inch cardboard is preferable to the thicker material but will take more time to finish.
I used 1/2 inch cardboard to build height as quick as possible. The concave surface was so jagged and irregular it needed to be cleaned up. I attached a small router to the cutter arm with hose clamps. A wide, flat bit helped to clean up the surface, and the inevitable paper-mache layers covered the rest of the imperfections. A router or RotoZip instead of a blade may be a better solution.
Finish the completed base with 2 or 3 layers of paper-mache and you're done with construction.
Next stop, finishing.
Step 14: Finish Chair!
To upholster the chair I used a stretchy material I had lying around for a long time. I lucked out that I had a lot of it, I like the color and it is stretchy.
Instead of upholstery the chair could be painted and filled with cushions or pillows. This would be easier and cheaper. I've left the base unfinished on purpose. The new owner likes the look of it. A $20 papasan chair cushion from CraigsList is a perfect cushion for the inside.
The outer cover was made with 2 large half-circles sewn on the long curved edge. That piece was turned inside out, stretched onto the ball of the chair and held down with homemade tack strip. Tack strip is simply cardboard with nails embedded into it. I had both and wanted longer nails for a better hold so I made my own.
To make the entire inside surface cushy I used 1/2 inch foam covered with the same material. This thin foam layer makes a big difference. It's more comfortable for me to sit with my head above the edge of the cushion. This layer is much nicer to rest my head on than the cardboard surface.
I made my own 1/2 inch piping to give a nice appearance to the edge where the chair body and foam pieces meet. Piping is easily made by sewing long strips of material around piping cord. Use a zipper foot to keep the material tight against the cord. Tack the piping down the same as the outer covering.
Each wedge-shaped piece of foam has its material glued to it using a spray upholstery adhesive. It comes out of the can in a fan shaped stringy spray like silly string. It was perfect for securing the material to the foam. Because the rounded edge of my chair is not a straight line each foam insert was different. I cut each piece to fit its position and numbered the location and foam pieces. Make sure you don't number the side that gets covered by fabric!
I'm currently using T-pins to hold the foam pieces in the ball. These don't hold great and I may switch to sticky-backed velcro at a later time. They're holding for now though.
It was hard work, but I had a lot of fun on this project. If anyone else makes a version of it I'd love to see it.