Differences -perpendicularities-

[greenimi]

ASME Y14.5-2009

On both figures 7-3 and 7-4 some perpendicularities are to be added (just for the sake of this discussion to make the drawing “complete")

On 7-3:
- on datum feature B: perpendicularity .xxx tol to A primary
- on datum feature C: perpendicularity .xxx tol to A primary and B secondary

On 7-4:
- on datum feature B (middle plane): perpendicularity .xxx tol to A primary
- on datum feature C (middle plane): perpendicularity .xxx tol to A primary and B secondary

Should I understand that is a significant difference between the squareness allowed (parallelogram effect) between these two schemes?

 

[CheckerHater]

Squareness between what and what?
We both know datums are already in "basic" relationship, you cannot improve that.
So you will be adding "squareness" to datum features, providing it is functional.
Other then that, it may not be absolutely necessary, you don't have to add squareness to datum features to make datums square:

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[chez311]

greenimi,

Theres a few obvious answers to your question - apologies if they are not quite as nuanced as you were looking for or too simplistic.

Adding perpendicularity controls to B/C of 7-3 only places a constraint on deviation of each single planar surface. Adding perpendicularity to B/C of 7-4 now places a constraint on deviation of each PAIR of planar surfaces (aka FOS) - which leads into another difference, since they are FOS in 7-4 one could apply RFS (resolved geometry - controls midplane) or MMC (surface interpretation).

All in all, its sort of comparing apples and oranges, isnt it?

 

[chez311]

Other then that, it may not be absolutely necessary, you don't have to add squareness to datum features to make datums square

Well, yeah if by that you mean your theoretical datum planes are always perfectly mutually perpendicular, and your theoretical datum feature simulators are always perfectly located/oriented at their basic location/orientation and with perfect form (fun fact, the term Theoretical Datum Feature Simulators is now actually changed to True Geometric Counterpart in 2018). I interpreted the question to be about controlling the allowable deviation of the feature(s) themselves away from this theoretically perfect geometry. As I said though the question as stated seems to be trying to compare two things which are fundamentally different.