UPDATE: This bench was featured in an article by Amanda Kwan of the Associated Press in August of 2009. I completely forgot to post an update until I was in IKEA this weekend! A photo of the painted bench has also been included.
Step 1: Buy the Materials
IKEA Chair - $20
Home Depot Super-strip $2.97 (2)
Home Depot Laminated Pine Panel 1/2" X 18" X 48" $12
2" wood screws (4)
The remaining hardware is included with the chair.
Step 2: Cut the Boards
I used a CNC to cut my benchtop, but the same results can be achieved with a jig saw. I've included an image of my benchtop with the standard 1" square background. This can be printed full size at Kinko's (or similar) for about $10.
This isn't necessary, since the bench has two parallel sides. You can extend this the same way as the rails (by adding 30"). To do this, draw two parallel lines 15-1/2"(in my case) apart and 30" long. Next, mark the centerline for the seat and trace around each side of the seat at each end of the two 30" lines.
Next, the rails will need to be cut. There will be four. Two for the back, and two for the seat. I used the "super strips" because they're a little beefier. Only the tenons needs to be the same as the rails. I ran into one small problem with the seat rails. The original rails did not support the seat. I cut the rails taller(I'd say wider, but that would be even more confusing) to add support for the width of the bench.
The chair back rails will lose the curve of the originals, but over the width of the bench, it will not matter. The images below show the width and height of the tenons. Again, the placement of the tenon will vary from the original chair if you are planning to use it to support the benchtop.
The hole drilled in the center of the tenon will be addressed later.
Step 3: Cutting the Tenons
The tenon length and width do not need to be exact (calipers are not required). Tight fitting mortise and tenon joints are far better than loose joints. The corners of the tenons need to be trimmed (they will not have to be rounded as the original rails).
A router table is the best choice for this step, but a router or even a handsaw can be used. Get the tenons close then finish them up with sandpaper. I did not glue the joints, they are only held together by the mechanical fasteners.
After you have test fit the tenons, dry fit the bench. Drill through the existing holes and into the tenons. If you are marginally accurate with your drill, the finished product will look very similar the original rail.