practical application of Independency symbol

[Goppinath]

Hi
i am aware that #Rule 1 in GD&T,
to break rule 1 we can use Independency symbol as per ASME.
ca any one know the real application (naming of parts or function) where Independency symbol is required.

 

[Belanger]

Perhaps a part, though rigid, will be sandwiched in between two other plates when assembled. Therefore, it can be bowed/bent and it doesn't affect the function because it will be flattened by the strong assembly forces.
I'm not saying this is a good application of I, but it is a real-world reason that you could consider.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[pmarc]

Any application where forcing a feature surface to not extend beyond a boundary of perfect form at MMC is too restrictive from functional standpoint.

Imagine a shaft containing multiple cylindrical features of which only a few are interfaces with other items in the assembly. For the remaining cylinders, their form might as well be only controlled through some more generous geometric controls, such as circular or total runout.*

* Unfortunately, ASME's independency principle does not really make form and size of a cylindrical feature size fully independent. Because of the default swept spheres interpretation of limits of size given in Y14.5.1 (the math standard for the dimensioning and tolerancing principles defined in Y14.5), form error of the feature in each cross-section is still limited by the LMC and MMC spheres.

 

[Burunduk]

The "I" modifier could be used on an external width dimension that has a direct tolerance applied to it, and that width feature only mates to something in assembly on one side while the other side is free in the air. The side that is used as a contact face would have a flatness tolerance applied to it because that's what makes sense functionally. The combination of flatness on one side and the direct width tolerance would restrict the form variation because as a width type feature of size, you can only be that much deformed when one of your sides needs to be flat within some tolerance and the local distances to opposed side are controlled by the size limits.