Chapter 6 Datums and Datum Feature References 175
Copyright Goodheart-Willcox Co., Inc.
The variations in the datum feature and the
simulation method have the potential to cause
some error between the actual datum plane and
the simulated datum plane established by simula-
tion methods. Different simulation methods may
result in a different amount of error between the
simulated datum and the theoretical datum. Keep-
ing the simulation error small prevents this from
becoming signifi cant.
Datum features on a workpiece are used to
locate datum planes. A datum plane is located by
the high points on a datum surface (feature). In
the given fi gure, the location of the datum plane
is approximated by resting the datum surface on
a datum simulator. The datum simulator is often
a highly accurate device such as a surface plate or
angle block, but there are some variations in the
surface. So, there is a small difference between the
theoretical and simulated datum. For most situa-
tions, the high degree of accuracy in these devices
permits measurements from them as if the simula-
tors are the theoretical datum planes. However, if
datum simulation is not highly accurate relative to
the size of the required tolerances, the simulation
error must be considered.
Material Boundary Modifi ers on Flat
Datum Features
Prior to the approval of ASME Y14.5-2009,
material condition modifi ers were not used when
datum feature references were made to fl at datum
feature surfaces, because material condition modi-
fi ers were only applicable to datum features of size.
That has changed. All datum feature references
are now assumed to be applicable regardless of
material boundary (RMB) and may, when appro-
priate, be modifi ed with the MMB or LMB symbol.
Although a fl at surface does not have a maximum
or least material condition resulting from size vari-
ations, a fl at surface may, in some instances, have a
maximum material boundary created by the appli-
cation of profi le tolerances. See Figure 6-24. The
shown part has a small fl at surface identifi ed as
datum feature C, and that surface has a profi le tol-
erance applied that is referenced to datum features
A primary and B secondary. The profi le tolerance
creates a maximum material boundary and a least
material boundary at the limits of the profi le toler-
ance. This creates a condition where datum feature
C may be applicable at RMB, MMB, or LMB. Any
tolerance referencing datum feature C defaults to
being applicable regardless of material boundary
(RMB).
Generally, the MMB and LMB symbol should
be avoided when datum features are fl at surfaces.
Never use the MMB or LMB modifi er on a primary
datum feature reference when the feature is a fl at
surface. Never apply the MMB or LMB modifi ers
on datum feature references to fl at datum features,
regardless of order of precedence, unless there are
appropriate geometric tolerances on the datum
features to establish material boundaries. Gen-
erally, the material boundaries must be created
from MMB toward
LMB to make
MMB
LMB
max. contact
LMB
Tertiary datum feature
reference at RMB
Tertiary datum feature
reference at MMB
Drawing
Simulator progresses
location at MMB
Simulator fixed in
of .240
Datum feature
in contact
Goodheart-Willcox Publisher
Figure 6-24. A non-size feature may be referenced
at RMB, MMB, or LMB when a profi le tolerance
establishes material boundaries for that feature.