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I'm looking to get some collective agreement on a question that I have come across a few times now. When referring to features that are technically independent, what is the general consensus on dimensioning them? Take, for example, the part below:

Dimension Example 1

The 8.5" and 7" diameters are technically independent. And when being inspected, would typically be reported back as individual diameters. A metrologist would provide me with measurements of the two different 8.5" diameters and the four different 7" diameters. These measurements would be calculated off of each of the individual arcs.

Engineering drawing practice (as far as I've always learned) is to call out the number of places where that feature exists. However, often it makes much more sense to have the feature reported back based on multiple surfaces. For example, the 8.5" diameter would be much more practical (and likely more inclusive of design intent) if it were based on the combination of the top and bottom (as shown) surfaces - essentially a single diameter. Similarly, the 7" diameters would be based on two surfaces on each side of the part.

Now I cannot find any guidance in any of the ASME Y14 standards that clearly indicate how to dimension this. Is there a recognized difference between saying "2X" vs "2 SURFACES" as shown here:

enter image description here

This scenario comes up quite often especially when considering features like crush ribs, or splines, and as I am often reviewing engineering drawings, I've been looking for a specific reference to point to when explaining the differences to other engineers.

I was taught that stating "2 SURFACES" informs manufacturing/metrology that the feature is based on the combination of both (or multiple) surfaces, where 2X implies that they are unique.

I know that the ASME Y14.5-2009 standard introduces a concept called "Continuous Feature" which MIGHT be a solution to this situation, but the examples and definition are still somewhat unclear and a little ambiguous in my opinion.

Does anyone have different thoughts?

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  • $\begingroup$ when you post images on engineering it may be prudent to include a symbol to indicate if you use first/third rotation ( and wether or not your units are mm or inches though you did that) $\endgroup$
    – joojaa
    Nov 22 '17 at 21:26
  • $\begingroup$ @joojaa: Thanks for the comments. I did, however, consider whether any additional information was relevant to this post and decided it was unnecessary. I intentionally left the units off the drawing since they did not matter and could be anything for purposes of the question. Also, the projection angle is also irrelevant, although the symmetry of the sample part I've shown intentionally makes both 1st and 3rd angle projection views identical (with the exception of the cross-section arrows) - which inherently imply 3rd angle. Hope this helps. $\endgroup$
    – Argonax
    Dec 1 '17 at 21:38

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