Specifying Datum Features at MMC

Where a datum feature of size is applied on an MMC basis, machine and gaging elements in the processing equipment that remain constant in size may be used to simulate a true geometric counterpart of the feature and to establish the datum. In each case, the size of the true geometric counterpart is determined by the specified MMC limit of size of the datum feature, or its MMC virtual condition, where applicable.

4.5.4.1 Size of a Primary or Single Datum Feature. Where a primary or single datum feature

Straightness Datum

of size is controlled by a roundness or cylindricity tolerance, the size of the true geometric counterpart used to establish the simulated datum is the MMC limit of size. Where a straightness tolerance is applied on an MMC basis, the size of the true geometric counterpart is the MMC virtual condition. See Fig. 6-3. Where a straightness tolerance is applied on an RFS basis, the size of the true geometric counterpart is the applicable inner or outer boundary. See Fig. 6-2.

4.5.4.2 Size of a Secondary or Tertiary Datum Feature. Where secondary or tertiary datum features of size in the same datum reference frame are controlled by a specified tolerance of location or orientation with respect to each other, the size of the true geometric counterpart used to establish the simulated datum is the virtual condition of the datum feature. See para. 2.11.1 and Fig. 4-16. This example illustrates both secondary and tertiary datum features specified at MMC but simulated at virtual condition.

4.5.4.3 Determining Size. An analysis of tolerance controls applied to a datum feature is neces sary in determining the size for simulating its true geometric counterpart. Consideration must be given to the effects of the difference in size between the applicable virtual condition of a datum feature and its MMC limit of size. Where a virtual condition equal to MMC is the design requirement, a zero geometric tolerance at MMC is specified. See para. 5.3.3 and Fig. 6-41.

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  • Angelika
    What is the difference between a datum feature and a simulated datum?
    5 years ago

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