Diffrence between origin and datum

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  • Diffrence between origin and datum

    can someone please explain to me the diffrence between the terms origin and datum because they seem to me the same

    thank you

  • #2
    There can be many differences between both "alignments", but they can also be the same.
    If you construct the alignment from least square features, the alignment should be different from the datums, which use tangent planes or max inscribed / min circumscribed circles / cylinders.
    The alignment is usefull to drive the cmm, the datums are used to dimension the part, it's totaly different.
    In the case below, alignment and datums are different !
    origin 2.JPG

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    • UKCMM
      UKCMM commented
      Editing a comment
      The use of the words origin or datum are interchangeable alignment structures are something else

    • JEFMAN
      JEFMAN commented
      Editing a comment
      UKCMM : my english is not "accurate" enough, but I disagree with this.
      The origin should be unique, the "origin" from datums A-B-C and A-C-B could give different coordinates...

  • #3
    ORIGIN should be where your X, Y, & Z values are zero, DATUM is what you measure to set up the alignment to check the part, and in many, MANY cases, these are NOT where X, Y, or Z are zero.

    https://www.math.usm.edu/lambers/mat.../lecture17.pdf
    sigpic
    Originally posted by AndersI
    I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

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    • UKCMM
      UKCMM commented
      Editing a comment
      I was trying to help the OP as you will very often see drawings with the term datum or origin and they do mean the same thing. Engineers/ draftsmen have been using these words interchangeably for decades, even CMM manufactures use the two terms.

  • #4
    Datum- The feature itself. "This plane is Datum -A-, whole hole probed as a cylinder is Datum -B-", etc.. Engineers call them out on their print.

    Alignment- Telling your software how to "use" features that it has probed to crunch it's dimensional analysis through. Leveling through -A- and making -B- your X and Y origin could be very different than leveling through -B- and making -B- X and Y origin and -A- your Z. As the programmer, the datums on the print should be driving how you set up your alignments. Doing so correctly will yield a program that is "accurate" (meaning you can duplicate the CMM's output via traditional inspection methods).
    SF7107(PCD), SF454(PCD), 152614(Quindos), 9159(Quindos), 7107(Quindos), B&S Manual, M&M Gear Checker

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    • #5
      Origin is where your probe readout would be 0,0,0. Datum's include origin but adds leveling orientation and offset rotation. You will have one unique point as an origin but an infinite number of rotations.

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