Optimal sheet metal geometry

[UBbaja]

I searched around and couldn’t find anything, my apologies if this has already been posted.
Does anyone know any good resources that cover optimal sheet metal geometry?
My application involves something similar to the bottom of a soda can. It is about the same size as the soda can, only differences are that its made out of soft stainless, the bottom is convex instead of concave, and the bottom "convex piece" is welded to the cylinder, not extruded from one piece like a pop can. My lid looks likes if you took the lid of a can of paint and instead of having it flat, made it into a subtlety domed shaped lid. Note the lid of the can of paint has a “U” shaped bend geometry around the edge (the lip). I am trying to figure out what type of geometry would result in the high moment of inertia aka resistance to bending. The radius of my “U” bend around the edge (aka the lip) is currently 2.5 times larger than the minimal bend radius (assuming for my material the min band radius is .5 x thickness).
A secondary issue is that despite being 2.5 times larger than needed, the vendor sometimes delivers the product with cracks around the bend radius. I am thinking it is a tooling issue on his end; the dies are wearing out or something. It doesn’t occur often, but I see runs where it does occur sometimes. Any other ideas on what would cause that?

 

[GregLocock]

Bending in what plane? A diagram might help.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[CorBlimeyLimey]

Could be variations in material hardness/temper.

 

[MikeHalloran]

If there's any quantity involved, you might be able to correlate the cracks with a particular operator; maybe one doesn't like spreading lard on the sheet.

Or, it could be a material issue, or a die finish issue.

Have you evaluated metal spinning?



Mike Halloran
Pembroke Pines, FL, USA

 

[ajack1]

2.5 x material thickness does not seem unreasonable for the “U” however even that depends on the length of the sides of the “U”, .5 x material would be very hard to achieve.

As for why some “crack” there could be any number of reasons, some things to look at.

The material used for the parts, consistent specs, consistent thickness, age hardening etc.

Consistent use of lubricant.

Is the tooling well maintained? All forming tooling gets “pick up” on it eventually and needs to be cleaned and polished. The time scales for this can be reduced by coating any high working parts in tit nit or similar, ensuring all material is well lubricated and free from any foreign bodies.

Finally the process might not be the best, would adding an extra forming stage improve things, stretching more material early on, or forming the “U” in two parts, by forming the inside first and then the outside this allows metal to “flow” from the outside once you have passed the centres of the “U” everything after that is metal stretching if formed as one process.

 

[UBbaja]

Thanks for the comments everyone. My apologies, I am having trouble with attaching my diagram to explain better. Any ideas on optimal geometry in terms of curved top vs. flat top design? Convex vs concave? Based on my knowledge, if I think about the product in terms of a pressure vessel with internal pressure, the convex design is better versus flat or concave. Ideas?