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.