What are auxiliary datums in GD&T

How to interpret auxiliary datums on GD&T drawing

What are Auxiliary datums?

Datums which are not part of the primary datum reference frame are called auxiliary datums.

In this example, the primary datum reference frame is formed by A, B and C where as the datum D is additional.

The primary datum reference frame is formed by Datum A which is applied to planar face , Datum B which is side planar face and Datum C which is top planar face.

These three mutually perpendicular planar faces are datum features A, B and C and form the Datum reference frame. Whereas, Datum D is established as a pattern of holes with 12 (nominal) dia.

The centre hole has a secondary datum reference frame formed by primary datum and auxiliary datum (at Maximum material boundary)

Auxiliary datums maybe used to create multiple datum reference frames and alternative geometric controls .

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