Symmetry and Concetricity without MMC or LMC

[man2007]

Hi all,
Pardon me for asking this stupid question. Why Symmetry and Concentricity are used with only RFS?

 

[greenimi]

According to Y14.5-1994 or 2009 symmetry and concentricity (and runout) is by definition applicable on an RFS/RMB basis only, both for the considered feature and for the datum references respectively.

On ISO (GPS) I don't know, but I am sure other folks around will provide you an answer from ISO(GPS) perspective.

 

[Belanger]

The standard being used is of utmost importance to answering the question, but I presume ASME here, because that it where RFS/RMB is an absolute must with these symbols.

The main reason is that the very definition of concentricity and symmetry involves the "median points" of opposing elements. And the only way to find the median points is to take the actual elements from the actual surface, regardless of size. (If we were to take the median points of an envelope, such as that created by MMC, it would negate the definition of the symbol.)

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