It's important to think about the sequence of operations for a complicated part like this. If I had put the beads and flared holes in the sheet first, the bending would not have come out smoothly.
So, with the seat back curved, I was ready to put the beads on the panel. The beads were designed as curved lines, to fit neatly between the curved rows of flared holes, and my pattern showed the exact location of each bead. Again, the sequence is important; if I had made the holes first, the flared edges would have interfered with the dies used for beading.
You need a deep-throat beading machine for a panel this size. I used my 18-inch throat Lazze machine, and even with a machine this size I had to approach the panel from two different directions to keep the edge of the part from bumping against the frame of the machine.
With the beads completed, I was ready to make the holes. The punch and flare tools are operated by tightening a 1/2-inch bolt that draws the two halves together, so 1/2-inch holes were made for each hole location. I used a Rotabroach for this, but a step-drill would work, too. Next, I started making all the flared holes. With dozens of holes to make, I decided to use a 1/2-inch air impact gun to speed the work. A note of caution here; it's a really good idea to use a felt-tipped marker to write the size of each hole on the metal blank. (Don't ask how I know this!)
I knew that both the beading and the hole flaring would take some of the curvature out of the seat back, so I re-bent the panel freehand to restore the proper contour. It's important to keep the blank rectangular up to this point, since you'll need the leverage provided by the extra material to bend the whole sheet evenly.
With the shape corrected, it was time to trim the edges to final size. I wanted the seat bottom to meet the seat back with a radiused edge rather than a sharp corner, so I used rounding-over dies on a beading machine to put a 1-inch radius 45-degree curl on the bottom edges of the seat back. This could be done with a hammer and dolly, too, but the beading machine makes this operation quick and accurate.
Next I used chip board to make a pattern for the seat bottom. I wanted to get the width right, but I allowed extra material both front and back to give me a little wiggle room for fitting it to the seat back. Thinking about the sequence of operations, I put the beads in the panel first, and then curled the edges while it was still flat.
To make the bends, I made a simple radius die from exhaust tubing, which was held to my workbench with clamps. Clamping the metal against the fixture flattened the curled edge in the corners, but that was easy to correct later. I used a bevel protractor to measure the angle of each bend (note that the front angle is different from the back).
With the seat bottom bent, I positioned it against the seat back and marked the edges for trimming to the final size. The curved back edge was curled with the rounding-over dies, and the panels were tack-welded together. After using a hammer and dolly to get perfect alignment at the joint, they were finish welded with my Miller Dynasty 200 TIG welder.
11. The dies are tightened together with a 1/2-inch bolt. Since I had so many holes to make, I used an impact gun to speed the job. Look at the beautiful holes these dies make!
12. Flaring the holes takes some of the curve out of the seat back, and I left the panel's edges untrimmed so I would have enough leverage to re-bend the part back to the right contour.
13. After correcting the shape, the edges of the panel were trimmed to the finished profile, and I put a 45-degree curl on the edges where the seat bottom will attach.
14. I'm using a piece of chip board to make a pattern for the seat bottom. I want to get the width right, but I'm leaving extra material front and back so I can trim it to size after bending.
15. I made a simple bending die from a piece of exhaust tubing. The seat bottom gets two bends, and I'm checking the angle of the first bend here with a bevel protractor.
16. Here's the seat bottom trimmed to size, beaded, and tack-welded to the seat back.
17. The seat bottom is finish welded and smoothed here, and I'm doing the final layout for the holes. Note how I'm keeping the rows of holes in a consistent pattern.
18. The seat has a reinforcing strip riveted to the edge. Here I'm using a large beading die to start forming a strip of aluminum into the preliminary shape.
19. I discovered that using a large flat-jawed vise is better than hammering for tightening the curl on the edge of the strip. I made a wooden cover for the vise jaw to help reduce marking.
20. With the part formed into a “question mark” shape, I inserted a length of 5/16-inch aluminum round bar, and squeezed the sheetmetal tightly against it—first with the vise and then the beading machine.