Chapter 6 Datums and Datum Feature References 161
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applied on an axis. There are numerous features
and combinations of features that could be used
to establish an axis. Only one method is correct
for the part’s function. The lower segment of the
fi gure shows datum features applied to two diam-
eters. From the feature control frame, it is known
that both diameters are to be used in establishing
the axis.
Although datums cannot be touched or seen,
they are an important part of tolerance application
and verifi cation. Tolerance specifi cations reference
datum features and the tolerances are related to
theoretical datums established from those datum
features. Datum features are identifi ed on the
physical elements of the part to defi ne the features
that are used in the location of the datums from
which to make measurements. In this text, there
are explanations of tolerance zones that are related
to referenced datums or datum reference frames.
The referenced datums are the theoretical datums
established from the datum features in the order of
precedence defi ned by the datum feature references
that are shown in a feature control frame or note.
Because measurements are made with respect
to the theoretical datums, the datums must be
established relative to the physical features of the
part in some manner. The theory will be explained
fi rst, then a practical application will be explained.
The practical approach is intended to provide a
means of putting the theory into practice.
In theory, a datum is established by relating a
true geometric counterpart to a datum feature. A
true geometric counterpart (TGC) is a perfect geo-
metric shape, or a theoretically perfect simulator,
corresponding to the shape of a datum feature on
a part. As an example, a planar simulator may be
brought into contact with a fl at surface on a part to
establish the location of the datum plane. Another
example is a theoretical cylinder that is brought
in contact with a shaft to establish a datum axis.
True geometric counterparts exist only in theory.
The ASME Y14.5-2009 standard replaced the term
true geometric counterpart with the word simu-
lator, meaning a theoretically exact datum feature
simulator. That seems to have created confusion
between a simulator that is theoretically exact and
a simulator that is real and imperfect. To avoid the
confusion between perfect and imperfect simula-
tors, the term true geometric counterpart will be
used here. A true geometric counterpart used to
establish a datum is the same as a theoretically
perfect datum simulator within this textbook.
In practical application, datum features are
simulated using tooling features or they are gener-
ated from measurement data using mathematical
processes. Datum simulators used in the fi gures
of this book are shown as tooling features and
no tolerances are applied to them. The process
of establishing the datums from the imperfect
datum features is referred to in this text as datum
simulation (or simulation) and the tools and pro-
cessing equipment are referred to as datum simu-
lators. The datum simulators (whether a physical
tool or computer generated) provide a surface
or machine-generated plane, axis, or point from
which measurements may be made.
Datum Reference Frame
Orientation, runout, and concentricity toler-
ances always include references to datum features.
Profi le tolerances may or may not include datum
feature references, depending on the required
level of control. Position tolerances almost always
require datum feature references, but may be spec-
ifi ed without them in a very small number of spe-
cial applications. The exceptions are explained in
the chapters covering position and profi le.
Datum feature references are shown on
the right end of the feature control frame. See
Figure 6-3. Depending on the tolerance specifi ca-
tion and the needed level of control, there may be
one, two, or three datum feature references.
The shown order of letters in the feature con-
trol frame signifi es datum precedence. The letter
of the alphabet used to identify a datum does not
affect precedence.
Datum feature references shown in a feature
control frame establish a datum reference frame.
See Figure 6-4. A datum reference frame (DRF) is
made up of three mutually perpendicular planes.
These planes locate the three axes that form an X,
Y, and Z coordinate system.
The X, Y, and Z axes may be identifi ed on the
drawing, and identifying them is required if the
datum feature references include degrees of freedom
M
Primary
Secondary
Tertiary
Datum feature references
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Figure 6-3. Datum feature references are included on
the right end of the feature control frame. They are
read from left to right.