I made a pair of these chairs a few years ago while I was in school, and had access to a nicely-equipped shop and, most importantly, a laser-cutter. The tennis balls you see are not glued in place; they are held only by different-sized holes in the top and bottom sheets of plywood. The holes on the bottom are smaller so the balls don't push through when you sit on it. To create the contoured effect, the sizes of the top and bottom holes vary in specific ratios. The balls can still freely rotate and some of them can come all the way out. That said, it is much easier to make without all the contouring, but decidedly less comfortable. I made prototype version in which the balls do not contour, and will include some pictures of that chair at the end of the instructable.
I have unfortunately lost some of the construction photos, so, in lieu of illustrations, have tried to make the written part as detailed as possible.
Everyone always asks me if it's comfortable. Yes. The key here is that the balls are not glued in place or screwed through somehow, so they can flex and deform. With the body distributed over fifty of them, no single one makes a pressure point; the slope on the seat pan, and the U-shaped contour on the back match butt and back as closely as possible, giving spine and tailbone some breathing room.
The fact that they are not fixed in place also means that they shrink and expand from their holes with changes in heat, humidity, and altitude, as the air trapped inside the balls changes in volume.
Step 1: Finding a Chair Frame
1. Find an old chair frame. I used a popular stacking office chair by Galaxie Furniture Company that has been in production for a lot of years. They are common at yard sales, church basements, dumpsters, etc. The dimensions are specific to this frame -- tweak according to your frame, whatever size it may be.
2. Unscrew the cushions, which are held in place by four metal straps that span the frame from side-to-side. Use a Dremel or an angle grinder to cut the straps off the frame, and then grind down the weld spots smooth. The frame should now just be two sides, only held together by the crossbars on the legs. The cushion straps acted as a brace, so the frame will now be lacking stiffness.
Step 2: Substructure
3. Cut two pieces of 3/4â MDF 16-1/4â by 14-1/2â. These will become the substructure underneath the plywood, and give the frame its stiffness back. The square cross-section tube that composes the frame of this chair is 3/4" thick, thus this thickness for the substrate. You could also use plywood, but that is substantially harder to drill through, especially holes this big.
4. The 16-1/4â dimension is side-to-side, the 14-1/2â dimension is from front-to-back. Draw a grid on the MDF that has lines every 3â from front-to-back and 3-1/4â from side-to-side.
5. Using a hole-cutter bit and a drill press, bore a 2-5/8â diameter hole at each of the twenty-five intersections on each grid on each piece of MDF. This size hole is slightly bigger than the maximum diameter of a new tennis ball (approx. 2-1/2").
6. 3-1/4â inches down from the end of the chair frame top, drill a 1/8â diameter hole from the side of the frame. Drill another hole eight inches below that one. Take care to make the hole as straight as possible, because youâre really drilling two holes â one through each side of a hollow section of tube. If the two holes are misaligned because you didnât drill straight, it will be hard to get the screws through. Repeat a total of eight times, two holes on each side of each eventual cushion.
7. Insert the MDF between the sides of the frame 1/4â shy of the ends of the frame and screw stainless steel, 1/8â diameter metal screws with washers through the holes and into the MDF. You should now have two pieces that secured with four screws each in the approximate position of the old cushions. In the first picture, the two pencil lines on the back piece where the screws should be centered. In the second picture, you can see the screws coming in from the sides more clearly. The seat piece has one screw that has to be longer, to penetrate through two of the square cross-section frame pieces.
Step 3: Plywood Finish Sheets
8. For this step, I used a LaserCamm rapid prototyping machine that uses a laser to cut sheets of flat material. This is where the pictures break down, since you can't really photograph the LaserCamm in action. This step can be done by hand, but thin plywood tends to chip and scar at cut edges â the laser leaves a clean, burnt edge and is also perfectly accurate. To avoid this, another sheet material such as masonite, could be used. Cut four pieces of 1/4â plywood or other suitable sheet material 17-3/4â across and 15â front-to-back. That leaves a quarter-inch of overhang to the front and back of each cushion, and 3/4â overhang on each side, which overlaps the steel frame.
9. Starting from the center instead of the sides, mark the same grid as you did on the MDF, 3â front-to-back and 3-1/4â from side-to-side.
10. Mark each piece with a number, one through four. From front-to-back, the balls will slant downwards. Starting with piece number one, which will be the bottom plate of the seat pan, use a hole-cutter bit to drill the following diameters of holes, each centered on the appropriate intersection on the drawn grid. The first row, side-to-side, should be 2-7/16â in diameter. The next row, again side-to-side, should be 2-3/8â in diameter. The following rows should be 2-5/16â, 2-1/4â, and 2-3/16â. If you want to save the time and trouble of all the contouring business, just make two pieces where all the holes are 2-7/16", and another two pieces where all the holes are 2-3/8".
11. Piece number two will be the top of the seat pan. From front to back, the rows should be 2-3/8â, 2-7/16â, 2-1/2â, 2-9/16â, and 2-5/8â.
12. Piece number three will be the back of the back cushion. From side-to-side, the balls make a U-shaped contour. This time, each column of holes will be the same diameter, instead of each row. Using a hole-cutter bit, drill a column of holes 2-3/16â in diameter on the outside two rows. On the two rows just inside, the holes should be 2-5/16â. The center column should be 2-7/16â.
13. Piece number four is the top plate of the back cushion. The two outside columns should be 2-5/8â in diameter. The two columns right inside those should be 2-1/2â in diameter. The center column should be 2-3/8â.
14. Sand, stain, and seal the pieces as desired.
The first picture shows piece number one, as denoted in the steps above, already screwed and glued to the substrate on the underside of the seat pan, and the balls sitting in there. The second photo shows the same, plus piece number two, the top of the seat pan, glued and screwed onto the top of the substrate, thus trapping the balls in place -- no glue or mechanical fasteners needed. The second photo also shows piece number three attached. You can just see the edges of the plywood behind , showing how the smaller holes make an internal ridge to retain the balls.
Step 4: Finish Up
15. Drill a total of twelve 1/8â diameter holes through the chair frame from top-to-bottom, again taking care to align them as straight as possible. The holes should be centered on the rows of holes in the MDF, the two edge rows and the center row.
16. Drill a matching set of 1/8â diameter holes through the pieces of plywood.
17. Take piece number one, the bottom of the seat pan, and attach it to the underside of the seat piece of MDF with glue and stainless steel screws. It should be aligned so that all the holes in the two pieces are centered on one another, allowing for a 1/4â overhang of the MDF on each end front-to-back, and 3/4â of an overhang side-to-side, which overlaps the steel frame.
18. Do the same with piece number three, the back of the back cushion.
19. Insert a tennis ball into all fifty holes. They are available used, quite cheap, on eBay. If you have done everything right so far, all of the balls should fit snugly and none should push through.
20. Fit piece number two over the balls in the seat pan. Again, everything should fit snugly, trapping the balls in between two retaining edges created by the plywood. Glue and screw into place.
21. Do the same with piece number four, the front of the back cushion.
22. Now use twelve 1/8â diameter machine screws with washers to go through the holes drilled in step 15 from top-to-bottom through the frame and both pieces of plywood. Tighten nuts and washers on the underside, snug against the frame. Use a Dremel or angle grinders to trim any excess screw length down flush with the nut.
23. Cut four strips of plywood that are 3/4â by 16-1/4â. Sand, stain, and seal as desired. Glue on the four remaining exposed edges of MDF.
24. Sit down and take a well-deserved rest.
The first picture shows the first three pieces in place. The next couple are close-ups of some of the joints, and alternate views of the chair. The last photos, the black version, is the prototype I made where the balls did not contour. It used masonite instead of plywood, and an external bracing system instead of the internal MDF substrate.