Symmetry tolerance with reference to a datum axis

[Burunduk]

If a drawing references ASME Y14.5-2009 and specifies symmetry tolerance applied on a width feature with references to a cylindrical datum feature ( which imposes an axis as a datum) - was this supported or not?

Para. 7.7.2 clearly states:

"Symmetry is that condition where the median points of all opposed or correspondingly located elements of two or more feature surfaces are congruent with a
datum axis or center plane."

I used to think that the tolerance zone in such a case should be two parallel planes equally disposed about a datum axis and rotated about it until obtaining a best-fit orientation to include the median points of opposed elements of the actual feature surfaces.
However, ASME Y14.5.1M-1994 doesn't seem to provide a definition for how the median points should be derived in this case, because according to "TABLE 5-12 SYMMETRY PATTERNS FOR OBTAINING CORRESPONDING FEATURE ELEMENTS" The patterns of symmetry rays used to derive the median points (centroids) have to be:

"Rays from, and perpendicular to, the datum plane"

This seems to imply an ambiguous direction of rays when a datum axis is used. Is this another discrepancy between the standards?

 

[Belanger]

I think yes, it's simply a minor discrepancy between the two standards.
The Y14.5 document should prevail, since it drives the Y14.5.1 document (with the correct year as the driving force, obviously).

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

 

[Burunduk]

Belanger, thank you.
After thinking of it some more, could one of the two planes intersecting on the datum axis and always associated with a primary cylindrical datum feature be used for the purpose of the definition of a direction for the symmetry rays? These theoretical axis-related planes are sometimes referred to in the Y14.5 standard as "datum planes", for example in the "means this" portion of fig. 4-8 in the 2009 version. This perhaps could suggest an explanation for the Y14.5.1 definition of symmetry mentioning only a "datum plane" whereas a datum feature that provides an axis is also valid, but I hesitate about it because these axis-related planes are not real "datums" in the same sense as the axis derived from a cylindrical datum feature or the center plane of a width datum feature (the axis-related planes are established as part of the DRF based on the datums).

 

[chez311]

I know both of you are well aware that concentricity and symmetry have been removed in the 2018 version, I'm just putting it as a disclaimer for anyone who might happen upon this.

Its worth noting a few things. First theres no requirement that concentricity be applied only to circular features or a surface or revolution. In fact, it is recommended in Y14.5-2009 for noncircular features in conjunction with a datum axis.

[...]where the coaxial control of noncircular features is a design requirement, concentricity tolerancing is recommended.

The only practical limitation would be the feature should have at least 2-fold rotational symmetry. I say should because 1-fold is possible if the tolerance zone is wide enough to fit the median points/centroids however I would be extremely (emphasis on extremely) suspect that someone has an actual practical application for it - even more so than I would already be with any use of symmetry/concentriticity.

Another point is admitted in Y14.5 that the concepts of symmetry and concentricity are essentially the same concept, with different applications:

[...]symmetry and concentricity controls are the same concept, except as applied to different part configurations.

At first brush, we assume that this means of the controlled feature itself - and your note that Y14.5-2009 includes "datum axis or center plane" under symmetry might lead us to this conclusion. However if we read more closely in the math standard, we find that the distinction actually has to do with the derived datum involved.

Concentricity is that condition where the median points (centroids) of all diametrically opposed elements of a figure of revolution (or correspondingly located elements of two or more radially disposed features) are congruent with a datum axis or center point. Symmetry is that condition where one or more features is equally disposed about a datum plane. A symmetry tolerance is used for the mathematical concept of symmetry about a plane and a concentricity tolerance is used for the mathematical concept of symmetry about a point or symmetry about an axis.

Concentricity is used when your datum is an axis or point. Symmetry is used when your datum is a plane. Note the bolded portion in parenthesis - contentricity applies not just to surfaces of revolution but also "correspondingly located elements of two or more radially disposed features".

This is the reason there is no provisions for axis under symmetry in Y14.5.1-1994 table 5-12, the standard makes a clear distinction on what control should be utilized with what derived datum type. One could utilize symmetry with a datum axis in light of the verbiage in Y14.5 and have a valid argument why this is acceptable, however the distinction in the math standard is much more purposeful than a simple discrepancy.

 

[Burunduk]

Thank you, chez311.
Y14.5.1's purpose is to provide a mathematical definition to the concepts established by Y14.5, so I think that purposeful or not, discrepancies have no place. I don't see a good reason why the 94' math standard didn't cover the option of using a datum axis as reference for symmetry tolerance. Also, the math standard's definition of symmetry when considered by its wording didn't close the door on application of symmetry to cylindrical features (such as symmetry of a cylinder with reference to a width-shaped datum feature, and I've seen this on drawings), whereas the Y14.5 (94' and 09') standard clearly rejects it by the wording: "Symmetry is that condition where the median points of all opposed or correspondingly-located elements of two or more feature surfaces are congruent
with the axis or center plane of a datum feature." A cylindrical feature has only one surface but it can potentially be a feature "equally disposed about a datum plane" per Y14.5.1's para. 5.7 which you quoted. These problems no longer exist in the new versions of the standards simply because the concepts were removed altogether, but they are still relevant for users that work per the previous editions.