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Thread: Is this a method of GD&T simultaneous referencing?

  1. #1
    Associate Engineer
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    Is this a method of GD&T simultaneous referencing?

    Not sure how to interpret this. (see what is in green oval)
    At first I thought it was wrong.
    That possibly datum -c- should be by itself with its own FCF using just datums A & B, and then the other three holes with a feature control frame referencing back to A, B & C.

    But then someone told me that was legal because it was using simultaneous reference positioning.
    However, after reading up on simultaneous referencing, I'm not sure.

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    Last edited by Kelly Bramble; 03-15-2017 at 08:43 PM. Reason: Spelling

  2. #2
    Administrator Kelly Bramble's Avatar
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    ASME Y14.5-2009, paragraph 4.9 "Simultaneous Requirements" ...

    No, this does not appear to be a valid definition. I'm sure that there may be some folks whom might argue a gray interpretation where all of the hole features are Datum "C" and then measured to each other but - don't. You'll likely confuse the issue.

    One of the holes should be exclusively specified as Datum "C" and the other three oriented and located back to Datum's A, B and C.
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  3. #3
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    If all holes are the same size and of the same importance, I wouldn't even use just one hole as the datum feature. Just make the 4-hole pattern create datum C. This can be done by dumping the reference to C(M) in the green oval, and then moving the datum feature symbol to hang under that feature control frame. (Then, you'd probably want to and the MMB modifier to datum reference C in the all-around profile callout.)

  4. #4
    Administrator Kelly Bramble's Avatar
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    I knew you was going to say that...

    So, let me corner you. Per. your recommendation, how does one establish datum "C" from the four holes using a CMM?

    And... what are the chances that the quality metrologist will get confused with the specification? and then just do what they want..


    Quote Originally Posted by Belanger View Post
    If all holes are the same size and of the same importance, I wouldn't even use just one hole as the datum feature. Just make the 4-hole pattern create datum C. This can be done by dumping the reference to C(M) in the green oval, and then moving the datum feature symbol to hang under that feature control frame. (Then, you'd probably want to and the MMB modifier to datum reference C in the all-around profile callout.)
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  5. #5
    Associate Engineer rpolleys's Avatar
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    CMM will need to construct a hole pattern as a set an the set can have a hole pattern as a datum , the tricky part comes in when the software needs to act in this case as a gage simulator and software I use does this.
    As far as clarity for interpretation add note to drawing "C" Datum is 4 ID pattern

  6. #6
    Administrator Kelly Bramble's Avatar
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    Let's examine the two specifications..

    Specifically, the CMM will establish Datum A by contacting many areas/points (minimum of three) and deriving a datum "A" plane. Then we establish Datum B by contacting a minimum of six areas (points) on Datum feature 'B". Three at approximately 120 degrees apart toward the top of the cylinder and three at approximately 120 degrees apart towards the bottom of the cylinder. This will establish an axis and with that axis we get two datum planes - which are not constrained and can rotate about datum axis “B”.

    At this point we have three axis and three datum planes where two of the datum planes can rotate about datum axis "B".

    Datum "C" will now be used to orient the two datum planes that can rotate about datum feature "B" and therefore lock down the entire six degrees of freedom for our coordinate system or DRF.

    If only one hole is used as datum "C" – we would then establish three areas of contact 120 degrees apart toward the top of the hole and then three areas of contact 120 degrees apart toward the bottom of the hole to establish an axis which the existing datum’s planes rotating about datum B will be oriented and locked down to.

    The location of Datum C can be simultaneously verified while establishing the DRF. Then, the other three holes are measured and verified to the DRF.

    Straight forward…


    Now, let’s examine the four hole datum “C” scenario.

    If all four holes are used as datum “C” – the setup will be different. First datum’s A and B will be established as stated above. Then for each of the four holes a minimum of six areas of contact (three towards the top and three towards the bottom) will need be contacted and then a single derived axis from the four hole axis’s is established. Most modern CMM’s can handle this.

    This single axis is then used to orient and lockdown the rotating the datum B axis/planes and thus lock all six degrees of freedom for the DRF. Finally after establishing our DRF the orientation and location of the four holes are measured and verified relative to the DRF.

    For me – I think the definition is correct however the resulting DRF is different than the single hole Datum C DRF. Moreover, this definition may ~ confuse the metrologist and ultimately require extra time and work – depending on the technology capabilities of the CMM.

    I don’t see the four hole datum specification as an improvement or advantageous
    .
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