Primary, secondary, tertiary datum question

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  • Primary, secondary, tertiary datum question

    Hi everybody, i am here again to seek your quidance and knowledge.

    I uploaded a png file i made in paint( i didn't have any other program to use so i hope this is fine )
    Firstly, i did search about my question regarding that clarification from forum & i didn't find a match so i made a new topic.

    Now, the main reason why i am here is that i would like to have a straightforward answer that what i read & understand is correct.
    The issue i am having is do i understand Primary, Secondary & Tertiary datums correctly. (So 3-2-1 rule)

    The 3-2-1 Rule and Points of Contact

    The 3-2-1 rule defines the minimum number of points of contact required for a part datum feature with its primary, secondary, and tertiary datum planes. It only applies when all three plaines are used. The 3-2-1 rule says:
    So there is a rule for minimum points of contact, meaning that i may take 4 points for my plane, 2 points for my say y-axis as a line & 2 points for my x-axis as a line, did i understand this correctly?

    – The primary datum feature has at least 3 points of contact with its datum plane.
    – The secondary datum feature has at least 2 points of contact with its datum plane.
    – The tertiary datum feature has at least one point of contact with its datum plane.
    The 3-2-1 Rule and Points of Contact

    So, in my case my primary should be Datum C, secondary datum B and tertiary datum A ?
    If not then how do i define primary, secondary and tertiary datums?(how do i know which is which?)


    The other question is that i read from forum something about i could use TP instead of datums, does it affect the results in any way(or is it even correct way?)

    PS: Those holes on the part have position tolerance callout diam. 0.4 A B C


    If i need to input more information let me know

    PS: There are no stupid questions, only stupid answers

    As always, thanks in advance!
    Attached Files
    Last edited by Villem89; 07-24-2020, 08:39 AM.

  • #2
    Those holes on the part have position tolerance callout diam. 0.4 A B C
    If your drawing call out says Position|Ø0.4| A|B|C| then A is the primary, B is the secondary and C is the tertiary. Measure all three datum faces as planes using as many points as you feel necessary to capture their form error. If you have the part loaded on your CMM so that when viewed from above it is like the LH view in your .png, you would need to LEVEL ZPLUS to datum A, rotate YMINUS to datum B around Z and then ORIGIN X to DATUM C, Y to DATUM B and Z to datum A.
    Neil Challinor
    PC-DMIS Product Owner

    T: +44 870 446 2667 (Hexagon UK office)
    E: [email protected]

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    • Villem89
      Villem89 commented
      Editing a comment
      This part is loaded on my CMM as the middle view but rotated about its Z, meaning that datum A stays on the left side.
      That way i Leveled datum C(gave it origin), Rotated Datum B & gave Datum B and A an origin. (was it wrong move?)

  • #3
    Datum Reference Frame

    If the callout is ABC
    Primary A
    Secondary B
    Tertiary C

    If the callout is BCA
    Primary B
    Secondary C
    Tertiary A

    If the callout is CAB
    Primary C
    Secondary A
    Tertiary B


    Etcetera, etcetera, etcetera.

    B&S CHAMELEON/PCDMIS CAD++ V2011

    There are no bugs, only "UNDOCUMENTED ENHANCEMENTS!"

    sigpic

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    • #4
      Generally datums are called out in relation to which degrees of freedom are controlled by that surface for the function of, or assembly of the part. There are 6 degrees of freedom total. A primary datum should control 3 of the 6 degrees, a secondary should control 2, and a tertiary datum only controls one. In this case it looks like the piece would maybe sit on datum A during assembly but the part would still be able to rotate about the z axis and move side to side, Datum B would prevent the part from being able to rotate about the Z axis, and datum C would be the mating feature that prevents the part from being able to move side to side.

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      • #5
        Have you looked at all at the ASME Y 14.5? 2009 look at pg 49-50 figures 4-1 4-2 and 4-3 do a good visual depiction of datum precedence and how degrees of freedom are applied here. 3-2-1 Rule is rules of precedence of constraint as it applies to the 6 degrees of freedom -3- You need 3 point minimum or 3 controls of the 6 degrees of freed -a minimum # point’s construct plane has 3 points and controls 2 rotations and can control 1 translation. But a primary can also be a cylinder, the axis can control up to 2 rotation and 2 translation -2- Can be a plane or a line or set features that create a centerline Plane primary-plane secondary the plane secondary controls a rotation that is not constrained by primary and can constrain a translation. This can also be a line on an edge or a line from 1 or more circular features or mid points. -1- Can be any point circle, intersecting point, centroid on a plane. Whatever degree of freedom left a point will now stop the last translation is typical but could possibly stop a rotation also. I have worked a feature control frame with and A-B callout that constrained all 6 degrees of freedom. A plane was primary and 2 post were labeled as B datum or 2 ID’s. With the above, A circle or a line cannot be a primary datum. A plane can be primary, secondary or tertiary but order of precedence can have a huge impact on accept/reject. In a perfect world you will never have an issue but we do not work in perfect, never will. I have seen parts the when measured incorrect datum precedence used they are acceptable, when correct datum precedence is applied the part location now fails. The intent of a feature control frame is to define what order precedence is and minimize error measurement correlation.

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        • #6
          Thank you all for the information.

          As i understand is i need to take in consideration 6 degrees of freedom when i see 3 datums callout on the print & i can also use it to clear that the designer has put the datums in a correct places.

          Meaning: with 3-2-1 rule, primary datum has to constrain 3 degrees of freedom(2 rotational and 1 translational).
          As primary datum is bound to constrain 3 degrees of freedom from the other 3?
          Am i correct?

          I Hope i did understood this concept correctly,
          ps: i am working with iso1101 standard not asme

          PS: if you see me rewriting what everybody told me is because i am Estonian and English is not main language and i want to clarify that nothing went past me nor i understood differently.

          I really like what i do so i want to be updated with the latest information that is correct.( that way nobody can tell me, hey your program is bad )

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          • #7
            Originally posted by Villem89 View Post
            Thank you all for the information.

            As i understand is i need to take in consideration 6 degrees of freedom when i see 3 datums callout on the print & i can also use it to clear that the designer has put the datums in a correct places.

            Meaning: with 3-2-1 rule, primary datum has to constrain 3 degrees of freedom(2 rotational and 1 translational).
            As primary datum is bound to constrain 3 degrees of freedom from the other 3?
            Am i correct?

            Datum precedence is an order of operation. The primary datum can and must control all degrees of freedom that it may. So if the primary datum controls 4 degrees of freedom, it must and does and it doesn't matter what type of freedom it is or how many (IE: your primary datum could be a cylinder which controls 4 degrees of freedom, there it must control all 4 of those degrees of freedom). If the secondary datum shares a degree of freedom with the primary datum, the primary datum has precedence and the secondary datum does not control that degree of freedom. The same holds true for the tertiary datum, it may only control datums that have not already constrained by the primary and secondary.

            3-2-1 is a methodology for controlling 3 degrees of freedom, not how datum precedence is controlled in a feature control frame.
            Last edited by Peter Fuller; 07-26-2020, 01:47 PM.
            Systems Integrator
            Hexagon Manufacturing Intelligence

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