Step 4: Finish Up
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.