Symmetric dimensioning
Symmetric dimensioning
(OP)
A simple situation, but I have not found explicit reference to it:
- Entirely symmetric part with many mirrored features.
- I believe functional (mating) emphasis requires horizontal dimensioning relative to a part center-line (planar primary and secondary datums, with tertiary datum plane of symmetry passing through the axis of a central datum FOS).
Is a basic dimension from center-line to feature required for each feature AND its mirorred counterpart, or is it acceptable to dimension in one direction from center-line with a "2X FEATURE and feature tolerance control frame" callout, with the dimension to counterpart on the opposite side of center-line being dictated by symmetry?
Any help or comment is appreciated.
- Entirely symmetric part with many mirrored features.
- I believe functional (mating) emphasis requires horizontal dimensioning relative to a part center-line (planar primary and secondary datums, with tertiary datum plane of symmetry passing through the axis of a central datum FOS).
Is a basic dimension from center-line to feature required for each feature AND its mirorred counterpart, or is it acceptable to dimension in one direction from center-line with a "2X FEATURE and feature tolerance control frame" callout, with the dimension to counterpart on the opposite side of center-line being dictated by symmetry?
Any help or comment is appreciated.





RE: Symmetric dimensioning
On symmetric parts, I usually show the dimension across the symmetrical faces. These are real, measurable dimensions, unlike dimensions from centrelines.
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JHG
RE: Symmetric dimensioning
If you've set up your datums properly, and are using GD&T/FCF correctly then I'd say the 2X approach is just fine. (Remember center planes can't be datum features as such, the datums need to derived from FOS - which I think you already said you were doing.)
The alternative (for features located by GD&T) is instead of dimensioning to the center line, dimension symmetrically across the center line from feature to feature. This is shown in figure 5-4.
(Some folks argue this isn't explicitly clear and you're assuming symmetric and that this figure is a case of an incomplete one, however it looks good to me.)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Symmetric dimensioning
My impression of using dimensions that span between the symmetric features is that doing so implies that in the inspection process you are physically measuring from one feature to the opposite. The problem then is that the location of each feature has (presumably) been given a cylindrical tolerance zone, but that which of the two features is to be considered the "reference" for placement of the tolerance zone of the other is not stated, nor is how the "reference" feature is placed/toleranced (presumably relative to the center-line). However, I am no expert in GD&T.
Thanks for the posts.
RE: Symmetric dimensioning
The FCF invokes the relevant datum structure, so no your aren't directly dimensioning from one feature to the other.
You're giving the theoretically perfect location with the basic dims, then the FCF invokes the datum structure and the permissible variation from that perfect location.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Symmetric dimensioning
Sorry. I'm a bit obsessive, and this kind of thing has been tripping me up.
Thanks again.
RE: Symmetric dimensioning
My take is that no you don't explicitly need the 'centering' dimension if you are showing clearly symmetrical across the datum and the datum is referenced in the FCF.
A few people argue otherwise.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Symmetric dimensioning