this is a 4" disc, with not much concavity, does it have to be aluminum,
does it matter how thick it is? oftentimes (if the project doesn't have a
heavy time restriction on it) i'll just keep my eye open for objects at the
local recycling center/dump. how about *getting* a tank end and cutting
(plasma, jigsaw, etc.) a 4" disc out of it already pre-formed? sometimes i
get lucky and find exactly what it is i need at the dump.

sorry grant if this doesn't help you at all.

b.w.

(at first i thought you were talking about making a whole weber style dome.
i tried to find a link i used to know about a guy making a expansion pipe
for a two stroke motorcycle by hydroforming it, wondered how big of a dome
you could blow up before it started wrinkling
http://en.wikipedia.org/wiki/Expansion_chamber (see links at the bottom)
there are other better links on the web about this. it surprised me how
smooth the blown up shapes can be.)