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Thread: Help with identifying datums

  1. #1
    Associate Engineer
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    Oct 2014
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    Help with identifying datums

    Hello Everyone!,

    I would really appreciate it if someone could help me identify what are the datums R and R-S in this drawing.

    Many thanks!
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  2. #2
    Administrator Kelly Bramble's Avatar
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    Feb 2011
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    Bold Springs, GA
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    I would really appreciate it if someone could help me identify what are the datums R and R-S in this drawing.
    Datum “R” is the large hole feature (dia. 24 H7) and datum “S” is the smaller hole (dia. 10 H5).

    The datum specification Datum-Datum indicates that a single datum reference is to be established from separate features as identified. So, datum R-S is meant to indicate a single datum element from which the measure from.

    Datum R and S are parallel and offset by 180 mm and this particular specification is controversial in some circles. The best way to visualize this datum feature is with the datum simulator fixture that will be utilized to establish and measure from these features.
    The fixture will have two simultaneously expanding pins located 180 mm separated. The fixture is built to gauge tolerance (very accurate). The as-built part is placed on the fixture and the pins are expanded until limiting contact is made with the as-built hole features.

    Now the interpretation of the established datum is not clear – some would say that the part is locked down in two axis, however I suggest that only a single axis (with accompanying to datum planes) has been establish since the fixture can rotate.

    I believe the intent of the designer was to define the hole feature dia. 15 H6 as symmetrical about datum’s R and S. There are easier to understand methods of doing this.

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