How to define a sheet metal ring? (Flat pattern? Free state? Restrained State?) 1

[flash3780]

Hello, I'm struggling with the best way to define a thin sheet metal part (.010" thk). My part is a fairly simple stamping which is rolled into a "C-shape". The stamping looks like a long-thin strip with some features... you could think of the part as a rolled up Chevrolet logo. The design requires the part to have a given amount of compression when installed, so the free state diameter is important.

Originally, we defined the part formed in its free state, but we got feedback from the manufacturer that the definition would be difficult to produce because the part is so flexible.

I think that it may be more appropriate to describe the cutout as a flat pattern, and then define the formed shape in the free state. However, our PDE group is recommending that we define the part in the as-installed constrained state and add a free-state diameter requirement. I'm wondering which method is more conventional?

In my mind, the relationship between the flat pattern and the installed state is straightforward, and defining a flat pattern would allow the vendor to inspect the cutout after stamping with a comparator, rather than making up some jig to do so. I don't do a ton of work with sheet metal, so any suggestions would be greatly appreciated.

 

[drawoh]

flash3780,

I do not show flat patterns unless there is some feature that cannot be shown on the as-formed part.

ASME Y14.5 has a section on how to dimension flexible parts. Your drawing should show how you will fixture the part for inspection. On flat sheet metal parts, I routinely attach a note to my primary datum...

DATUM[ ]A APPLIES WHEN THE FACE IS CLAMPED TO A FLAT REFERENCE SURFACE.

In your case, this will be a bit more complicated. You may need to make a fixture.

--
JHG

 

[flash3780]

Thanks for the input. I've read through ASME Y14.5-1994 on the issue and there's not a lot of great examples of flexible parts in there. I was thinking that a separate flat-pattern and a formed shape would be clearer in the case of my part (again, it's a sort-of "Chevy bow-tie" rolled into a 2.5 inch diameter "C" shape). However, it seems like the popular opinion is that it's not the modus operandi for this sort of thing.
Maybe defining the part in the constrained (installed) state and specifying a free-state diameter is the way to go after all. Unfortunately that leaves me defining things with angles rather than linear dimensions. Ahh well, as long as the part gets made the way we need it, it's just semantics, really. There's no difference in the tolerance stackup that I can tell.