Chapter 6 Datums and Datum Feature References 181
Copyright Goodheart-Willcox Co., Inc.
tolerance for the four holes references two datum
features. The primary datum feature reference is
to a pilot shaft. Referencing datum feature B as
the primary datum establishes datum axis B and
two intersecting planes at the axis. Referencing a
second datum feature, which is a fl at surface per-
pendicular to the axis, establishes the third plane
in the datum reference frame. Thus, the axis and
plane as used in this example establish all three
planes of the datum reference frame.
Figure 6-34 shows a part on which one sur-
face and two features of size are identifi ed and
referenced as datum features. In this example, the
primary datum feature is a surface. It locates the
primary datum plane of the datum reference frame.
The secondary datum feature is a hole. The
second and third planes of the datum reference
frame are located on the datum axis created by the
hole. The secondary datum axis and the associated
planes are perpendicular to the primary plane.
The tertiary datum feature is needed to pre-
vent rotation of the planes that pass through the
secondary datum axis.
The number of references shown in a feature
control frame is affected by the type of datum fea-
tures and the order of precedence. The required
number of references is also affected by the type
of tolerance being specifi ed, and the desired level
of control.
Material Boundary Modifi ers on Datum
Features of Size
Material boundary modifi ers for regardless
of material boundary (RMB), maximum material
boundary (MMB), and least material boundary
(LMB) are applicable to datum feature references.
The following explains how the material condition
boundaries are applied and how they are simulated.
Datum Feature References RMB
Based on Rule #2 of ASME Y14.5-2009, all
datum feature references, for features of size and
surfaces, are applicable regardless of material
boundary (RMB) unless otherwise specifi ed. For
a datum reference to a feature of size, RMB means
that the simulator (the actual mating envelope)
shall make contact with the datum feature. The
process utilized to establish contact and the extent
of contact will vary depending on the geometry of
the datum feature, the order of precedence of the
datum feature reference, and tolerances that are
applied to the datum feature.
Datum feature references may also be modi-
fi ed with the maximum material boundary (MMB)
or the least material boundary (LMB) symbol.
When a datum feature reference is modifi ed with
the MMB or LMB symbol, the datum simulator
has a fi xed size. The order of datum precedence
and applicable tolerances have an impact on the
size and any orientation or location requirements
for the simulator. See Figure 6-17.
Datum Feature References MMB
Features of size may have multiple bound-
aries that are created by the tolerances that are
applied. The size limit for a feature that contains
the maximum amount of material is known as
the maximum material condition (MMC) and is
within this textbook referred to as either the MMC
(which includes form considerations) or the maxi-
mum material size (which is only the size limit).
When only a size tolerance is applied to a fea-
ture, the MMC for the feature is also the maximum
material boundary (MMB) for that feature. The
MMC and MMB are coincident. See Figure 6-35.
The position tolerance on the two holes reference
the shaft diameter as primary datum A and the
MMB modifi er is included. The shaft diameter is
identifi ed as datum feature A and only has a size
tolerance applied. The MMB of the datum feature
is equal to the 1.005″ diameter MMC of the shaft
because no other tolerances are applied.
If the feature of size has a straightness toler-
ance applied to defi ne the allowable variation of
the derived median line, then the MMC and form
tolerance combine to create a maximum material
boundary. See Figure 6-36. The same part from
Figure 6-35 is shown, except a straightness toler-
ance is applied to the feature of size. The straight-
ness tolerance applied to a feature of size combines
with the MMC size to create a virtual condition.
Goodheart-Willcox Publisher
Figure 6-34. One datum feature surface and two
datum features of size may be used to establish a
datum reference frame.