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Thread: Hole Pattern Identified as Datum

  1. #1
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    Hole Pattern Identified as Datum

    I haven't found many detailed discussions on hole patterns as datums (Fig. 4-26 from ASME Y14.5-2009), but please point me to them if this subjected is previously posted.

    Can anyone help me explain to my customer why this isn't the best way to call out the datum structure. They try to use a hole pattern as a datum on nearly every single part. (Or can anyone enlighten me as to why this IS the best option?)
    I've attached a picture of an example part with datum structure. Please ignore all format, dimensions, and the rest. I just want to talk about datum structure.

    I'd really appreciate more information and examples on hole patterns establishing datums if there are any good resources on the internet.


    P.S. First post.
    P.P.S. I took Mr. Brambles GD&T course in 2012.
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  2. #2
    Technical Fellow Kelly_Bramble's Avatar
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    Well, this specification is NOT wrong and is correct as a geometric datum reference frame. Without a deep dive into what your customers’ requirements are for this component it is not possible to demonstrate “why this isn't the best way to call out the datum structure” where facts may be that this may be the best specification.

    I do suspect that a simpler secondary datum single feature and tertiary (single) feature may be more traditional - arguing against a correct specification is not an effort one should pursue.

    To better understand this dimensioning and Datum specification see:

    Overview as mentioned - ASME Y14.5-2009, paragraph 4.12.3, 4.11.9 and 4.9 and related figure 4-26 “Hole Pattern as a Datum”.

    The ASME-2018 standard also provides specific interpretation guidance for a hole pattern identified as a datum.
    Paragraph 7.12.3, 7.12.4
    Figure 7.18, 7-19, 7-20,

    This concept is not in any of my published books - though it should be.
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  3. #3
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    Agreed that using a pattern of features of size at MMB is acceptable, and also agreed that understanding the form/fit/function would be beneficial, but they don't want to make it too easy on me.
    I guess I was initially hoping to hear something like a rule of thumb for the use of this datum reference frame. A sort of "only use this when..."

    So a few more direct questions:
    1. With an irregular hole pattern, I am curious where datum axis B would lie. Equidistant from all holes?
    2. What is the rule for orienting the second and third planes of the datum reference frame. (in this instance, sure, go vertical and horizontal, but what if the pattern didn't lend to that assumption as easily?)
    3. Lastly, displacement. Suppose the hole is .250 +/-.005. The displacement is up to .010? So there could be ten thousandths of slop just from the datum set up?

  4. #4
    Technical Fellow Kelly_Bramble's Avatar
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    1. The datum created would be an axis and the related two datum planes (secondary and tertiary) and though there is some debate on the location I am of the view that the location would be at the medium location of all the features in the pattern.


    Note that that the relative location of the datum created by the pattern is not going to change the measurement relative to the datum pattern. What does matter is that the location and orientation of the secondary and tertiary datum is established and that the basic dimensions are tolerance-less. So, consistency is important.


    2. The orientation of the datum created by the pattern is perpendicular to the primary datum and as noted above consistent with the feature pattern datum. Horizontal and vertical to the drawing view given.


    3. I think your question is related to the secondary pattern datum thus no, there would not be relative displacement from that datum pattern features as it is specified at regardless of feature size. This means that our datum simulators are at the maximum size for the hole features and movement is not possible. A CMM might be programmed to measure the top and bottoms of each hole feature and thur mathematically derived that datum location from the actual sizes of the each hole.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  5. #5
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    I appreciate the responses. Ok, I think I may have a misunderstanding at #3.

    Correct any of the following if wrong.
    The maximum material condition of a part is when a hole (or some feature of size) is at the smallest possible diameter. Applying maximum material boundary as a datum modifier when the feature is a hole means the datum simulator is the smallest possible diameter.
    Given the two datums established, the part is underconstrained and has the ability to move.
    So if the holes in the part are larger than the minimum allowed diameter (and perfectly located), then the part can "wiggle" within the limits of the difference of diameter.

    I found this from FARO's website and thought it was a good example. (it just doesn't directly explain for a pattern)

    Maximum Material Boundary.pdf

  6. #6
    Technical Fellow Kelly_Bramble's Avatar
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    Oops,

    Yes, I didn't see the MMC modifier that is applied to datum B within the feature control frame for the ten holes.

    So, the datum simulator could be a flat plate with six pins at the datum B MMC size (dia. Dia. .250 - .001) projecting perpendicular from datum A surface and at the basic dimension location and orientation from one another - manufactured at gage tolerance accuracy.

    From that datum simulator each of the ten holes are verified to a position tolerance of .001 RFS while the datum reference plane is allowed to shift within the boundaries set by any divergence from datum B MMC size – collectively.

    So, the datum reference frame origin could shift a maximum of .001 diameter – on a perfect as-manufactured at MMC size datum located part.

    I suppose one could argue that since a tertiary datum is not specified for both the FARO example and the 6 hole datum and ten hole pattern example you posted that the datum reference frame is under constrained.

    However, I would point out that both examples (faro and yours) are adequate to orient and locate the pattern of holes referenced to the datums A and B. What is under constrained is the orientation of the hole pattern and datum to the rest of the part geometries/surfaces but - both drawings are incomplete and additional specifications are needed.

    In effect the argument is - here are two incomplete drawings and they are under constrained therefore they are wrong and need tertiary datums. - not true..

    Both engineering drawings could use limit tolerances to the other features (outer edge, etc..) to create a dimensional relationship as datums (tertiary) are not always required to fully constrain features to each other.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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