Woosang
Aerospace
- Dec 18, 2009
- 48
Symmetry is removed from Y14.5-2018. So let's talk about 1994 or 2009 version.
What I understood about the symmetry so far is:
1. symmetry controls median points of tolerance feature
2. datum feature can be either a cylindrical feature of size or a feature of size that is composed of opposed surfaces (e.g. tab, slot, etc.)
3. toleranced feature is ONLY a feature of size that is composed of opposed surfaces, but NOT a cylindrical feature
What bothers me now is the number 3 above. Some argues that symmetry can be applied to a cylindrical feature.
I read again Y14.5-1994 and 2009. Below is from 2009.
7.7.2 Symmetry Tolerancing to Control the Median Points of Opposed or Correspondingly Located Elements of Features
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. Where design requirements dictate a need for the use of a symmetry tolerance and symbol, the method shown in Fig. 7-66 may be followed. The explanation given in para. 7.6.4 applies to the considered feature(s), since symmetry and concentricity controls are the same concept, except as applied to different part configurations. Symmetry tolerance can only be applied RFS and the datum reference can only be applied RMB.
Underlined portion is not clear to me. First I thought it excludes cylindrical features, but now I don't know if it does.
Is it clear to you if that excludes cylindrical feature or not?
Below is ISO 1101:2017, and ISO seems to allow a Symmetry application on cylindrical features.
17.15 Symmetry specification
17.15.1 General
The toleranced feature is either an integral feature or a derived feature. The nature and shape of the nominal toleranced feature is a point, a set of points, a straight line, a set of straight lines, or a flat surface. If the indicated feature is a nominally flat surface, an intersection plane indicator shall be indicated, if the toleranced feature is a set of straight lines in the surface. If the indicated feature is a nominally straight line, the specification element ACS shall be indicated, if the toleranced feature is a set of points on the line. In this case, the datum for each point is also a point in the same cross-section. At least one datum that locks a non-redundant translation of the tolerance zone shall be indicated in the tolerance indicator. The angular and linear dimensions that are locked between the nominal toleranced feature and the datums are defined by implicit TEDs.
A symmetry specification can be used in all cases where a position specification can be used, provided all the relevant linear TEDs are zero.
What I understood about the symmetry so far is:
1. symmetry controls median points of tolerance feature
2. datum feature can be either a cylindrical feature of size or a feature of size that is composed of opposed surfaces (e.g. tab, slot, etc.)
3. toleranced feature is ONLY a feature of size that is composed of opposed surfaces, but NOT a cylindrical feature
What bothers me now is the number 3 above. Some argues that symmetry can be applied to a cylindrical feature.
I read again Y14.5-1994 and 2009. Below is from 2009.
7.7.2 Symmetry Tolerancing to Control the Median Points of Opposed or Correspondingly Located Elements of Features
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. Where design requirements dictate a need for the use of a symmetry tolerance and symbol, the method shown in Fig. 7-66 may be followed. The explanation given in para. 7.6.4 applies to the considered feature(s), since symmetry and concentricity controls are the same concept, except as applied to different part configurations. Symmetry tolerance can only be applied RFS and the datum reference can only be applied RMB.
Underlined portion is not clear to me. First I thought it excludes cylindrical features, but now I don't know if it does.
Is it clear to you if that excludes cylindrical feature or not?
Below is ISO 1101:2017, and ISO seems to allow a Symmetry application on cylindrical features.
17.15 Symmetry specification
17.15.1 General
The toleranced feature is either an integral feature or a derived feature. The nature and shape of the nominal toleranced feature is a point, a set of points, a straight line, a set of straight lines, or a flat surface. If the indicated feature is a nominally flat surface, an intersection plane indicator shall be indicated, if the toleranced feature is a set of straight lines in the surface. If the indicated feature is a nominally straight line, the specification element ACS shall be indicated, if the toleranced feature is a set of points on the line. In this case, the datum for each point is also a point in the same cross-section. At least one datum that locks a non-redundant translation of the tolerance zone shall be indicated in the tolerance indicator. The angular and linear dimensions that are locked between the nominal toleranced feature and the datums are defined by implicit TEDs.
A symmetry specification can be used in all cases where a position specification can be used, provided all the relevant linear TEDs are zero.