I keep thinking I can do this with a few tools. Some how it never seems to work that way. In fact I have ideas for more tools to make this all work better. I need to get a used wooden baseball bat.
Part of the trick to this is which bolts to have tight and which loose. Tight bolts hold the metal from moving and allows the metal to be stretched. Loose bolts let the metal slide so you don't have to stretch it too much and fracture the steel.
The first tool is the rivet gun and narrow tool. the smaller tip area gives higher psi. I start at the intersection of the tubes and work up the edge of each tube, but not along the long straight bottom. I then go across the bottom of the tubes and work toward the top to get the forming started.
The bottom gets formed with a piece of 2" dowel, until I get a dead baseball bat. It forms the bottom as a nice straight curve and pulls the steel out of the form block rather than stretch it, which would be too much work and it would end up wrinkled.
There is a wrinkle starting at the top because I went to far with the bottom dowel.
The top gets formed with a short piece of dowel. If I had worked between the 2 dowels with smaller deformations I think this would have come out smoother in the radius going to the outlet.
We'll see how I do on the next one. At this point it's time to go back to the inlet tubes.
As I form the inlet tubes the inlet itself closes in because the metal slides in the block. Every so often I stop and use the dolly to stretch the excess so the bend doesn't get out of control.
Once the tube is to the correct depth near the inlet, the actual inlet needs to be formed. That's where the modified bolt comes in. I can drive it down to square up the inlet and get it to the right depth.
There is still more work to be done forming the tube and I don't want the inlet to close back up, so I made some blocks of oak to drive down in the inlet. By clamping it in place while I work on the rest of the tube, the metal can't slide in. I also tighten the 3 bolts right at the inlet.
It turned out the anvil on the 6" c-clamp was to big and the clamp kept sliding off the edge of the plywood. The adjustable clamp worked better. Even though you can't get it as tight.
All the forming is done except the outlet. I couldn't get it to form a nice radius so I decided to make a tool to pound into the block to form it in one hit. I wanted Oak but I settled for some treated Yellow Pine from a piece of 4x4. I sawed it to 2 3/4" wide but put a 1 1/8" radius on each corner. The idea was to over bend the radius a little so it would spring back to 1 3/8" radius. It worked great. Problem solved.
It came out great. There is a little roughness in the tubes, but I like it.
The inlets came out the right size without any cracks or tears.
The one mistake I made was to work the tubes from front to back. I should have done the front tube then the back tube, followed by the middle tubes. It all slid slightly toward the front. You can see from the blue lines, which started at the edge of the form block, where the steel slid and stretched.
Here is the comparison of the formed shell with the blank.
You can see from the top view how well it worked to let the steel slide into the bend along the straight bottom edge. The cuts into the corners where the tubes join worked very well to let the steel slide into the tubes.
With a good Left Hand Bottom shell I'm ready to make the opposite shell. I worried that my plan to swap the top and bottom pieces of plywood would never work because there are 32 bolts to align which meant the holes in the block had to be almost perfectly perpendicular to the block. As it turns out the Shop Smith is up to the task. I drilled the bolt holes 1/64" oversize so the bolts would go in easily but still be snug.
They all fit perfectly. I didn't have to re-drill a single hole.
Time to make another shell.
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