Why flatness tolerance associated with a FOS must be less than the size tolerance?

[Vincent PW]

In my understanding, when a flatness tolerance is associated with a FOS, the Rule #1 is overridden and perfect form at MMC is removed. So, flatness tolerance associated with a FOS can be greater than the size tolerance.

However, ASME Y14.5-2009 specified in section 5.4.2 that, with flatness of a surface, where the considered surface is associated with a size dimension, the flatness tolerance must be less than the size tolerance.

Why must the flatness tolerance be less than the size tolerance?

 

[Garland23]

Vincent PW, paragraph 5.4.2 states, "With flatness of a surface, where the considered surface is associated with a size dimension, the flatness tolerance must be less than the size tolerance."
That sentence lays out that we're dealing with surface flatness. The part saying that it's associated with a size tolerance just distinguishes a surface on a "feature of size" from a surface that might not be a FOS (in other words, not directly opposed from another surface).
But again, it's all within the realm of surface flatness. It's only when you get to paragraph 5.4.2.1 that they're talking about flatness on a feature of size itself.

 

[3DDave]

So, flatness tolerance associated with a FOS can be greater than the size tolerance.

That's because in an association with a FOS flatness is not controlling the flatness of a (singular) surface.

They are separate results from separate conditions.

 

[ghostdigital]

In my understanding, when a flatness tolerance is associated with a FOS, the Rule #1 is overridden and perfect form at MMC is removed. So, flatness tolerance associated with a FOS can be greater than the size tolerance.

However, ASME Y14.5-2009 specified in section 5.4.2 that, with flatness of a surface, where the considered surface is associated with a size dimension, the flatness tolerance must be less than the size tolerance.

Why must the flatness tolerance be less than the size tolerance?

This is correct but only for selected form tolerances applied to the size dimension or FOS. Same applies for straightness specified to the FOS but not the surface exclusively.

Excludes cylindricity...

When a flatness is specified to a surface it is a refinement of the surface, thus the limits of size Rule #1 is intact.

 

[drawoh]

In my understanding, when a flatness tolerance is associated with a FOS, the Rule #1 is overridden and perfect form at MMC is removed. So, flatness tolerance associated with a FOS can be greater than the size tolerance.

However, ASME Y14.5-2009 specified in section 5.4.2 that, with flatness of a surface, where the considered surface is associated with a size dimension, the flatness tolerance must be less than the size tolerance.

Why must the flatness tolerance be less than the size tolerance?

The size tolerance controls flatness. Why call up flatness when the size tolerance applies a tighter control?