Datums - common vs system

  • Filter
  • Time
  • Show
Clear All
new posts

  • Datums - common vs system


    I've been reading in ISO 5459:2011 about datums and datum systems. It's very informative but somehow leaves me with more questions than I had before reading it...

    I'll just post one of those for now.

    So the standard explains establishing a datum system from a cylinder and a perpendicular plane, where the cylinder is the primary datum (A), perpendicular to the plane (secondary datum B).


    the standard explains establishing a common datum from a plane and a perpendicular cylinder, where the plane is the primary datum (A), perpendicular to the cylinder (B). Only in this example it's a common datum (A-B).

    Why? Why does one use common datums in some cases? What would the difference be if the second example was also a datum system?

    As far as I can see the end result is a straight line and a point in both examples. So what is the difference?


  • #2

    Are you referring to this figure? Or does this answer your question? (Actually I can't quite remember where I found this but I think it's ISO)

    It depends on the design intent and how the part will assemble & function.

    Same physical part, but depending how you fit the DRF to it, will dictate where any tolerance zones (which relate to that datum system) will be located.

    Applications Engineer
    Hexagon UK


    • NinjaBadger
      NinjaBadger commented
      Editing a comment
      Yes. So in A|B Dat A takes precedence.

      In B|A Dat B takes precedence.

      In A-B the 'error' is equalised (see where figure '4' is shown in the third example).

    • pernilla
      pernilla commented
      Editing a comment
      Right. Figure 4 represents the same difference in both places...
      Thank you! You've just made my reading of ISO 5459:2011 much more fun!

    • NinjaBadger
      NinjaBadger commented
      Editing a comment
      When you start reading standards it's a bit like 'what the actual $%[email protected]???'

      But once you get into them and they start to make sense it's great!

      (I am such a nerd!)

      GD&T is awesome though (when you understand it).

  • #3
    Here are the examples...




    Related Topics