Chapter 6 Datums and Datum Feature References 159
Copyright Goodheart-Willcox Co., Inc.
Chapter 6
Datums and Datums and
Datum Feature References Datum Feature References
Objectives
Information in this chapter will enable you to:

Defi ne the difference between a theoretically
perfect datum and a datum feature.

Explain how to create a datum reference frame
through references made in a feature control
frame.

Utilize all methods for identifying datum
features, including the use of target points,
lines, and areas.

Make datum feature references in a feature
control frame using the correct order of
precedence.

Explain how a datum reference frame may be
simulated when three mutually perpendicular
surfaces are referenced as datum features.

Use material boundary modifi ers on datum
feature references and explain the signifi cance
of the modifi ers.

Identify the degrees of freedom constrained
by each referenced datum feature in a datum
reference frame.
Technical Terms
clocking
compound datum feature reference
compound datum features
datum
datum features
datum feature references
datum feature symbol
datum precedence
datum reference frame (DRF)
datum simulation
datum simulators
datum target symbol
degrees of freedom
envelope
equalizing datum targets
least material boundary (LMB)
least material condition (LMC)
least material size (LMS)
maximum material boundary (MMB)
movable datum target symbol
related actual mating envelope
stepped datum surfaces
target areas
target lines
target points
true geometric counterpart (TGC)
unrelated actual mating envelope
Introduction
Tolerance specifi cations may be used to
defi ne relationships between features. Examples
of tolerance specifi cations that defi ne relationships
between features include position, orientation,
and profi le tolerances.
The feature control frames that defi ne these
types of tolerances include datum feature refer-
ences. See Figure 6-1. The orientation and position
tolerances in the gure each include datum feature
references. Those references indicate how to estab-
lish a coordinate system for measuring the loca-
tion of the toleranced features. The datum feature
references (in the feature control frames) indicate
which datum features (features on the part) estab-
lish the theoretical datums (planes that establish
the coordinate system) from which measurements
are made.
Three similar sounding but signifi cantly differ-
ent terms appear in the preceding paragraph. These
terms are datum feature references, datum features,
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