Chapter Chapter Chapter Chapter Learning Objectives After studying this chapter, you will be able to: Explain the purpose of datums. Defi ne common terms related to datum features. Interpret information provided by datum feature symbols. Describe methods for representing datum surfaces on drawings. Explain how to establish datum reference frames. Identify the degrees of freedom of a part. Explain how to establish datum targets. Identify datum target points, areas, and lines. Describe methods for representing datum axes on drawings. Describe methods for representing datum center planes on drawings. Technical Terms Actual mating envelope Coaxial Coaxial datum features Coplanar surfaces Datum axis Datum feature Datum feature simulator Datum feature symbol Datum plane Datum reference frame (DRF) Datum reference order Datum target line Datum target point Datum targets Datums Degrees of freedom High points Movable datum targets 77 Normal Order of precedence Primary datum Primary datum plane Rotational Secondary datum Secondary datum plane Simulated datum Simulated datum axis Simulated datum plane Tangent plane Tertiary datum Tertiary datum plane Three degrees of rotation Three degrees of translation Translational Datums are considered to be theoretically perfect planes, surfaces, points, lines, axes or a combination thereof derived from the datum feature simulator, which is described in this chapter. Datums 3
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