Chapter Chapter Chapter
Learning Objectives
After studying this chapter, you will be able to:
a72 Explain the purpose of datums.
a72 Defi ne common terms related to datum features.
a72 Interpret information provided by datum feature symbols.
a72 Describe methods for representing datum surfaces on drawings.
a72 Explain how to establish datum reference frames.
a72 Identify the degrees of freedom of a part.
a72 Explain how to establish datum targets.
a72 Identify datum target points, areas, and lines.
a72 Describe methods for representing datum axes on drawings.
a72 Describe methods for representing datum center planes on drawings.
Technical Terms
Actual mating envelope
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
Degrees of freedom
High points
Movable datum targets
Order of precedence
Primary datum
Primary datum plane
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
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.
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