Fig Part Where Angular Orientation Is Important

fied as a datum feature, the corresponding datum is simulated by a plane contacting points of that surface. See Fig. 4-10. The extent of contact depends on whether the surface is a primary, a secondary, or a tertiary datum feature. See para. 4.4. If irregularities on the surface of a primary or secondary datum feature are such that the part is unstable (that is, it wobbles) when brought into contact with the corresponding surface of a fixture, the part may be adjusted to an optimum position, if necessary, to simulate the datum. See para. 4.3.3.

4.5.2 Datum Features Subject to Size Variations. Datum features, such as diameters and widths, differ from singular flat surfaces in that they are subject to variations in size as well as form. Because variations are allowed by the size tolerance, it becomes necessary to determine whether RFS, MMC, or LMC applies in each case. See para. 2.8.

4.5.3 Specifying Datum Features RFS. Where a datum feature of size is applied on an RFS basis, the datum is established by physical contact between the feature surface(s) and surface(s) of the processing equipment. A machine element that is variable in size (such as a chuck, mandrel, vise, or centering device) is used to simulate a tnie geometric counterpart of the feature and to establish the datum axis or center plane.

(a) Primary Datum Feature — Diameter RFS. The simulated datum is the axis of the true geometric counterpart of the datum feature. The true geometric counterpart (or actual mating envelope) is the smallest circumscribed (for an external feature) or largest inscribed (for an internal feature) perfect cylinder that contacts the datum feature surface. See Figs. 4-11 and 4-12.

(b) Primary Datum Feature — Width RFS. The simulated datum is the center plane of the true geo-

Primary datum feature A

Primary datum feature A

Secondary Datum Plane
Secondary datum feature B

Tertiary datum feature C

(a) Datum features

True geometric counterpart of datum feature B (MMC virtual condition cylinder perpendicular to datum plane A)

Secondary Datum Plane

True geometric counterpart of datum feature B (MMC virtual condition cylinder perpendicular to datum plane A)

Datum plana A (True geometric counterpart of datum feature A)

Datum plana A (True geometric counterpart of datum feature A)

True geometric counterpart of datum feature C (MMC virtual condition width perpendicular to datum plane A. Center plane aligned with datum axis B)

(b) True geometric counterparts

Datum plane A

Datum plane A

Datum axis B

(c) Datum planes and axis established from the true geometric counterparts

Datum axis B

Datum Reference Frame
Datum reference frame

Datum center plane C

(c) Datum planes and axis established from the true geometric counterparts

(d) Datum reference frame

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