Sheetmetal dreams.

[cwilliamrose]

There are a couple of different ways I have seen to do this. First you need to clamp the ends of the blank so the loads you're putting through it don't cause a failure at the clamps. The clamping designs usually involve them being free to move as the part changes shape so there's some type of pivot that allows motion in one plane. If you visualize the two clamps holding the blank from opposite ends and you push a curved punch up into the blank the metal will stretch where first contact is made and the deeper the punch goes into the blank the more the blank stretches and it is progressive. The first section to stretch no longer needs to stretch as the contact area with the punch enlarges. The stretching happens right where the newest contact is happening. Friction between the blank and the punch does not allow the stretch to happen where there is solid contact.

You can also do this from one end which is how I suspect the Christen TE parts were done. You have a fixed clamp at one end and the punch is stationary. The clamp on the other end moves in a path that allows for progressive contact with the punch while also imparting enough tension to the blank to cause it to yield. This is how I see to do it if it comes to that. Instead of some fancy hydraulic system I'd just use a long lever arm and a series of holes in a plate attached to the punch. As I do the first section and it can't go any farther I simply move to the next hole and start again. I haven't done a model of it yet but this is what I visualize. I have seen a sketch of the way Lou Stolp did the TE parts for the Starduster airplanes that was (I believe) a manual process like the one I'd use. I think the punch was made of wood. I did some calculations that say a lever with a 100:1 ratio is a reasonable approach for a manual process.

I was hoping the Pullmax could do a bit of shrinking for me but I'm doubtful now. I made the female plastic so there wouldn't be any stretching from the dies making contact with the blank in between.........

 

[cwilliamrose]

Here's a video of a large stretch forming machine in action with what looks like plywood tooling for the punch;