ASME GD&T
Symmetry and Concentricity symbols are no longer supported by ASME Y14.5-2018, “Both have been eliminated because other characteristics provide more direct control of features and establish requirements that have a well-defined meaning.
Concentricity & Symmetry were a unique case in ASME GD&T, since they were the only characteristic controlling “Median Points”.
“Median Points” are centroids of corresponding point elements.
The section in figure-1 shows the section median points derived from the corresponding point elements, each arrow line represents a point pair.
the graph below shows the locus of median points at consecutive sections.
Figure-2 illustrates Symmetry Median Points of a single section
In ASME GD&T Concentricity & Symmetry are not equivalent to Position since Position controls location of an AXIS Or CenterPlane not Median Points.
Since the case were control of Median Points is needed is extremely rare, and there are better alternatives Symmetry and Concentricity were abandoned
Note:
- ISO GPS definitions are different!
- The tolerance feature is the extracted median Line for coaxiality or the Median surface for Symmetry.
- Symmetry or Coaxiality specification can be used in all cases where a position specification can be used, provided all the relevant locating dimension are zero.
Bottom Line
Geometric specifications (ASME GD&T and ISO GPS) are the tools to prescribe geometric variation limits to assure functionality of a manufactured part.
Standard Geometric Specifications allow us to specify even complex requirements in clear unambiguous manner, no need to “reinvent the wheel”.
In cases of uncertainty, consult an expert (save the time and cost of “reinventing the wheel”, misinterpretation, rework…)
Geometric Tolerances are a tool to map Functional Requirements to Geometric Limits
State Standard & Revision to assure proper Decoding
Write to get it Right
