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Tertiary Datums Often Orient a Higher Order Datum Axis
(In accordance with Y14.5M-1994 standard)

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I recently received the following question: Will the drawing below produce a part that most resembles Figure A or Figure B?

The correct answer is "B".

Datum A removes one translation and two rotations from the mobility of the part. Datum axis B removes the last two translational degrees of freedom of the part relative to the datum reference framework. Datum C is the axis of datum feature C. This axis is perfectly oriented to the datum reference framework by the basic dimensions. The only purpose of datum C is to remove the last rotational degree of freedom of the part relative to the datum reference framework. The figure below illustrates the datum reference frame associated with the part. This framework made up of three mutually perpendicular planes could have been drawn at the angle shown so that one of the planes passes through the axis of datum feature C without changing the meaning. The hole in the top of the part is shown perpendicular to vertical and horizontal center lines on the drawing. These center lines represent the datum reference frame illustrated. There is an implied 90° basic angle between the hole and one of the planes of the datum reference frame.

Back to Tips Tip added Feb 2002