Why You Should Focus on Improving the Meaning of Symmetry.


Meaning of symmetry is very much equivalent to concentricity.

Concentricity is basically used on a surface of revolution while symmetry is used on a planar surface.


The condition in which all the median points of two or more features are in the same axis or center plane of a datum feature.

The part features that are shown to be symmetrical with respect to the datum center plane are the cases in which symmetry control is applied.

The tolerance zone is basically two plane which are at a distance equal to the symmetry tolerance value.

The median points must be within these planes.

The necessary tolerance and datum references must be applied on an RFS basis while using a symmetry control.

Symmetry control tolerance zone

Symmetry control tolerance zone


These controls are used for unique applications when the parts need to be balanced, equal wall thickness, or any another functional requirement is needed then this control is used.

Otherwise positional tolerance is recommended to be used.

The following conditions are applicable when a symmetry control is stated.

  1. Slot must meet its size and Rule 1 requirements.
  2. Two parallel planes that are centered about the center plane is the symmetry control tolerance zone.
  3. Distance between parallel planes is the tolerance value of the symmetry control.
  4. All the median points must be within the tolerance zone. The maximum possible distance between the median points of feature and the datum center plane is half the symmetry tolerance value.
Symmetry applications

Symmetry applications


Symmetry and tolerance of position can be used for tolerance symmetric part features.

Designer often gets confused on which geometric control to use in a given situation.

Understanding the differences would help in finalizing the control which can be used by the designer.

The location of the median points of a part feature is controlled by a symmetry.

The location of the center plane of a part feature is controlled by positional tolerance.

Positional tolerance is a more economical tolerance to produce and verify.

Comparison between symmetry and tolerance of position is as follows:


Two parallel planes is the tolerance zone.

Tolerance zone is applied to median points of toleranced FOS.

Orientation and location are controlled of the part.

The cost to produce a part is greater as compared to the TOP.

The cost of inspection is also very high.

Tolerance of position (RFS):

Two parallel planes is the tolerance zone.

Tolerance zone is applied to the center plane of the AME.

Orientation and location are controlled of the part.

The cost to produce a part is lesser as compared to symmetry control.

The cost to inspect is also lesser.


Following conditions must be satisfied for the symmetry control to be a legal specification.

  1. Application of feature control frame must be done on the planar FOS.
  2. Requirement of datum references is present.
  3. No modifiers must be used in the feature control frame.

If the above mentioned conditions are not fulfilled, then the symmetry specification is incomplete or incorrect.


Inspecting symmetry

Inspecting symmetry

The above figure shows a part with symmetry control.

For inspecting the parts three checks are required:

  1. Size of the slot.
  2. Rule 1.
  3. Symmetry requirements.

Location must be verified for establishing accurate symmetry.

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