![]() ![]() Tertiary Datum should be perpendicular to both the primary and secondary datums and must only form one point of contact (a point).This datum often establishes the orientation of the second axis. Secondary Datum should be perpendicular to the primary datum and have a minimum of two points of contact (a line).It must have at least three points of contact with the part which are not in a straight line (a plane). ![]() Primary Datum should be selected on a plane that contains features that are critical to the function and/or inspection of the feature (think mating surface).The three datums should be mutually perpendicular to each other and selected in an order of precedence according to the following requirements: The purpose of the datum reference frame is to restrict your component by fixing it with respect to the six degrees of freedom: X translation, Y translation, Z translation, X rotation, Y rotation, and Z rotation. The datum reference frame consists of primary, secondary, and tertiary datums. Step One: Setting up the Datum Reference Frameĭefining the datum reference frame is the first step to ensuring you have a drawing with proper GD&T. Now let’s put theory to practice and actually step through the GD&T process. Applying Geometric Symbols to Engineering Drawings.To learn the basics of GD&T, check out our GD&T 101 article which includes the definitions and utilization of: Geometric Dimensioning and Tolerancing (GD&T) is the building block of modern engineering drawings.
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