Looking for feedback--this has bugged me awhile. The pic here is how I would dimension this. I think most people would put the datums C-A-B, but I think it is practically impossible to establish datum C first here. This is just one I dashed off for this post, but I've seen more egregious examples--e.g., people trying to establish the edge of a piece of sheet metal first. I think I am in the minority here.
I've attached the pic also, since I can't figure how to resize it.
GDT.PNG
Datum order of precedence most correctly is selected based on each datums influence or fit and function with respect the features being referenced back to the datums.
Chances are in this scenario Datum C is the most influential and datums B and A are likely interchangeable with respect to the hole pattern.
"but I think it is practically impossible to establish datum C first here." <- not at all..
It all depends on the functionality of the part and assembly but generally speaking, yes, it is impractical to use a thin edge as a primary datum because the primary datum requires 3 points of contact. It's actually not that unusual for the edge of a piece of plate to be angled relative to the overall flat surface of the plate because of the way the plate may have been cut. Waterjet cutting for instance often leaves an angled edge. Shearing can too. If you use that thin edge as a primary datum it would throw the rest of the plate off at an angle. Probably not what you want. People are used to thinking that the primary datum MUST be the surface that is perpendicular to the hole because that's mostly the way we've learned it in school or training and that's how you'll see it most often in real life. However, it doesn't need to be the case at all. The primary datum should be what makes the most sense in regards to how the part will be made and inspected in addition to how it will fit and function.
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