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Two Holes as Datum Features
(In accordance with the ASME Y14.5-2009 standard)

PDF of this Tip

The first Tip in this series (indexed as "Two Holes as Datum Features") showed a simple part where two holes were used as datum features.  The holes were on a horizontal plane.  On this drawing the two holes are on a diagonal.  So, the datum planes are oriented at the angle established by these holes.  Usually inspectors prefer measuring in the horizontal and vertical directions.  No problem!  A 30° basic angle is used to create auxiliary vertical and horizontal measurement planes.  Once you locate and orient the part using the two holes that are datum features B and C, you simply rotate the set-up by 30° to establish the vertical and horizontal measurement planes.

As long as basic dimensions are used, it is possible to establish auxiliary measurement planes that are translated and/or rotated relative to the datum reference frame without changing the meaning of the drawing.

Back to Tips Tip added Aug 2010