This is where Datums come in – Datums are derived from the datum features and are exact points, lines, and axes simulated by measurement equipment. To control features, we need exact points, lines, and axes. However, actual manufactured features are not perfect. These features are indicated on your GD&T drawing by a datum feature symbol, and other features reference them in their feature control frames. Datum Features are tangible surfaces or features of size that are usually important functional features of the part. To answer this question, we first need to review datums and datum features. Today we are answering question number 4: Question #4: How can datums be perfectly perpendicular to each other when the features themselves are likely imperfect? Question #3: Can my virtual conditions be the same diameter for a shaft (boss) and bore for the actual parts? Question #2: On a pattern of holes being identified as the datum feature, would the datum feature be the single hole or all the holes? Question #1: Can a single point (without clocking) be used as a secondary datum in a feature control frame? We are now making this available to everyone in a four-part series on our blog and YouTube channel! 6-41.We receive many GD&T questions from our students, and in a recent live instruction webinar, we answered four that are commonly asked. Where a virtual condition equal to MMC is the design requirement, a zero geometric tolerance at MMC is specified. 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. An analysis of tolerance controls applied to a datum feature is neces sary in determining the size for simulating its true geometric counterpart. This example illustrates both secondary and tertiary datum features specified at MMC but simulated at virtual condition.Ĥ.5.4.3 Determining Size. 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. 6-2.Ĥ.5.4.2 Size of a Secondary or Tertiary Datum Feature. Where a straightness tolerance is applied on an RFS basis, the size of the true geometric counterpart is the applicable inner or outer boundary. Where a straightness tolerance is applied on an MMC basis, the size of the true geometric counterpart is the MMC virtual condition. 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. 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.Ĥ.5.4.1 Size of a Primary or Single Datum Feature. 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.
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