I think it's different than hammer forming into a die. I've heard the sheet blank holder you mention described as a "slip ring" But this controls the metal, much like a thumbnail tool, trapping the metal to persuade it to shrink as the metal is pulled over a radius in a corner, so to speak.
I believe Jaroslav is advocating for leaving an angled ring or flange around your part to "trap" the center so when stretching (by EW for example) the center will rise the material up to meet your sweeps.
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Marc
Last edited by Marc Bourget; 05-06-2021 at 06:16 AM.
Reason: added the reference to slip ring
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