Fig Development Of A Datum Reference Frame For Part In

metric counterpart of the datum feature. The true geometric counterpart (or actual mating envelope) is two parallel planes at minimum separation (for an external feature) or maximum separation (for an interna feature) that contact the corresponding surfaces of the datum feature. See Figs. 4-13 and 4-14.

(c) Secondary Datum Feature RFS — Diameter or Width. For both external and internal features, the secondary datum (axis or center plane) is established in the same manner as indicated in (a) and (b) above with an additional requirement: The contacting cylinder or parallel planes of the true geometric counterpart must be oriented to the primary datum (usually a plane) — that is. the actual mating envelope relative to the primary datum. Datum B in Fig. 4-15 illustrates this principle for diameters; the same principle applies for widths.

{d} Tertiary Datum Feature — Diameter or Width RFS. For both external and internal features, the ter-tiary datum faxis or center plane) is established in the same manner as indicated in (c) above with an additional requirement: The contacting cylinder or parallel planes must be oriented in relation to both the primary and the secondary datum — that is, the actual mating envelope relative to the primary and secondary datum. The tertiary datum feature may be aligned with a datum axis as in Fig. 4-15 or offset from a plane of the datum reference frame.

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