Tennis Ball Chair

Step 2 Substructure

3. Cut two pieces of 3/4” MDF 16-1/4” by 14-1/2”. 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 16-1/4” dimension is side-to-side, the 14-1/2” dimension is from front-to-back. Draw a grid on the MDF that has lines every 3” from front-to-back and 3-1/4” from side-to-side.

5. Using a hole-cutter bit and a drill press, bore a 2-5/8” 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. 3-1/4” inches down from the end of the chair frame top, drill a 1/8” 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 you’re really drilling two holes – one through each side of a hollow section of tube. If the two holes are misaligned because you didn’t 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 1/4” shy of the ends of the frame and screw stainless steel, 1/8” 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.