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  • Is it fine to interchange primary and secondary datums in a drawing

    Discussion in 'Engineering Drawings' started by vinodbarathi, Apr 16, 2014.

    1. vinodbarathi

      vinodbarathi New Member

      Apr 2014
      Likes Received:

      I have come across one drawing, which I need to use it as reference to make new one.
      But in reference drawing one feature has been given as in below image.
      In another location some other feature has a control like shown in below image.

      My doubt is, can datum 'E' be moved from teritary datum to primary datum? Can datums be interchanged in GD&T frame?

      Vinod. B
    3. Erich

      Erich Well-Known Member

      Feb 2012
      Likes Received:
      Datums are just ways of labeling features.

      They become primary, secondary, tertiary by how they are located in individual feature control frames.

      A round feature can be a primary datum for the control of some feature. It can be a tertiary feature for the control of yet another feature.

      It all depends on how the part functions and how you the designer need to control the allowable variation in the part.

      GD&T is a language, it has many letters and words. You are the author and how you put them together depends on what you want to say.

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