Specifying Datum Features at LMC

Where a datum feature of size is specified on an LMC basis, a primary datum may be established as the axis or center plane of the LMC boundary. A secondary or tertiary datum may be established as the axis or center plane of the true geometric counterpart of the feature's virtual condition size. See para. 2.11 and Fig. 4-17. This example illustrates both secondary and tertiary datum features specified at LMC but simulated at virtual conditions.

4.5.6 Effects of Datum Precedence and Material Condition. Where datums specified in an order of precedence include a feature of size, the material condition at which the datum feature applies must be

Primary datum feature A

Secondary datum feature B

Secondary datum feature B

Secondary Datum Plane

(a) Datum features

Tertiary datum feature C

(a) Datum features

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

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

Secondary Datum Plane

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

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

(b) True geometric counterparts

Datum plane A

Datum axis B

Datum axis B

Datum plane A

Datum plane C

Secondary Datum Plane

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

Datum reference frame

Datum plane C

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

Datum reference frame

(d) Datum reference frame
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