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Thread: position control of hole pattern without datum & modifier

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    Post position control of hole pattern without datum & modifier

    I would like to know whether position control of hole pattern (circular) without datum and modifier in FCF is ligal or right ?

    for example : a flange having circular pattern hole on the top face. FCF with positional tolerance of dia.0.2 mm without datum & modifier assigned to the hole dia..

    how do we can inspect this positional variance?

  2. #2
    Administrator Kelly Bramble's Avatar
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    The position tolerance controls the hole-to-hole location tolerances. What you have a pattern of cylindrical tolerance zones or boundaries located together at the basic dimensions. The axis of each as-built hole feature within the pattern must fall within these tolerance boundaries from the start to the end of the hole. The tolerance boundaries are applied at RFS (Regardless of Feature Size).

    There should be some sort of separate location control for the hole pattern. Likely a limit tolerance tying one or more of the holes back to other features on the component or assembly.

    To inspect, simply zero on one or more of the holes (doesn't matter which) to establish an implicit datum or datum reference then measure the locations of the other holes relative to that implied datum(s).

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    thanks kelly,

    can you make me clear, what is implicit datum?

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    A side point -- the idea given in the OP is not permitted in the ASME Y14.5 standard; it would require at least one datum reference (probably the surface of the part that the holes are perpendicular to). But having a position tolerance on a hole pattern without any datum reference is permitted in the ISO style of geometric tolerancing.

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    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by Belanger View Post
    A side point -- the idea given in the OP is not permitted in the ASME Y14.5 standard; it would require at least one datum reference (probably the surface of the part that the holes are perpendicular to). But having a position tolerance on a hole pattern without any datum reference is permitted in the ISO style of geometric tolerancing.
    Where in the ASME standard is this concept specified as "not permitted"? Fig. 4-33.

    When I use the words "Implicit” - for ASME standard this is just a way of saying that a datum has been created out of necessity, usually for manufacturing or quality reasons. ISO standard allows the declaration of Datum’s and the order of precedence to be "Implicit".
    Last edited by Kelly Bramble; 11-09-2014 at 04:32 PM.

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    While it's not airtight evidence of it being disallowed, there is first of all the fact that the standard doesn't show any such example of position on bolt circle holes that lack a datum reference. I believe in the 1982 or 1973 standard there was such an example.

    But the best I can point to is probably paragraph 7.2.1.3: "It is necessary to identify features or features of size on a part to establish datums for dimensions locating true positions except where the positioned features establish the primary datum. (The exception is explained in para. 7.6.2.3.)"

    So for the OP's situation to follow the letter of the law, it would have to be the case of coaxial holes (which it's not) or else be a pattern that forms a primary datum (don't think I've ever seen that).

    My own opinion is the same as yours, Kelly ... a datum reference shouldn't be "necessary" -- why pigeonhole people? However, I just mentioned it because the OP might be from a part of the world that observes ISO rather than ASME.

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    Thanks for discussion on my query...

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    Can anyone tell which ISO standard allows position specification for circular holes pattern without specifying datum? please

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    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by mhk665 View Post
    Can anyone tell which ISO standard allows position specification for circular holes pattern without specifying datum? please
    ISO 1101 or any derivative.. For that matter the US standard ASME Y14.5-2009

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    Quote Originally Posted by Kelly Bramble View Post
    ISO 1101 or any derivative.. For that matter the US standard ASME Y14.5-2009
    I have studied both the standards, and cannot find any single example using implied datum concept. Except for COAXIAL feature example in Y14.5 and Bidirectional T.P in 1101. I may be missing something then, could you please give me some detail?

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    Administrator Kelly Bramble's Avatar
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    implied datum concept.

    Your original question was not about "Implied Datums".

    Conceptually, a pattern of features specified with a position tolerance and without a datum reference is sound. What one has is a pattern of tolerance boundaries located at the Basic (or TED) Dimensions relative to each other yet not oriented or located to any features by Datum reference.

    Be aware though that a limit dimension or other method would need be specified to locate the pattern of features to other feature(s) on the part or assembly.

    Don't get hung up on the GD&T or G&T standards not showing an explicit example of an application. Learn the concepts and apply the principles.
    Last edited by Kelly Bramble; 12-05-2014 at 09:44 AM.

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    Quote Originally Posted by Kelly Bramble View Post


    Your original question was not about "Implied Datums".

    Yeah, I thought No datum specified is same as Implied Datum? What do you think of this? In my view position without datum can only be specified for Lower Compartment of Composite True Position.

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    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by mhk665 View Post
    Yeah, I thought No datum specified is same as Implied Datum? What do you think of this?
    Implied datums are included in the ISO standard -where datum features are identified without letter designations and the order of precedence applied is up to manufacturing interpretation. There is not an implied datum definition available within any ASME or ANSI standard. There are some trainer/consultants whom reference the "implied datum" as a matter industry tribal knowledge.


    Quote Originally Posted by mhk665 View Post
    In my view position without datum can only be specified for Lower Compartment of Composite True Position.
    No such rule exists in any dimensioning and tolerancing standard.

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