176 GD&T: Application and Interpretation
Copyright Goodheart-Willcox Co., Inc.
using profi le tolerances and those tolerances must
be referenced to higher precedence datums.
As stated above, all datum feature references
are applicable regardless of material boundary
(RMB) unless otherwise specifi ed. If there are no
material boundaries for the fl at surface, the appli-
cability of RMB has no real impact.
Explanations of datum simulation methods
given in the preceding pages were based on the
datum feature reference default to applicability at
RMB. As previously explained, when the datum
feature reference is at RMB, the datum feature is
brought into contact with its datum simulator or
the datum simulator brought into contact with the
datum feature.
The applicability of RMB on a datum feature
reference does have an impact if the datum fea-
ture has material boundaries. See Figure 6-24. The
middle illustration in the fi gure shows the datum
C reference at RMB (no modifi er shown). For a fl at
surface such as datum feature C, material condi-
tion boundaries exist. RMB applicability on datum
feature C simply means that the datum simulator
progresses from the MMB toward the LMB until
the simulator makes maximum possible contact
with the datum feature.
A fl at datum feature that has material con-
dition boundaries may be referenced at RMB,
MMB, or LMB, and there are applications where
a datum feature reference to a fl at surface at maxi-
mum material boundary (MMB) is appropriate.
When a datum feature reference is made at MMB,
the simulator is fi xed in location at the MMB. See
Figure 6-24. The lower illustration in the fi gure
shows the part with the datum feature C reference
at MMB. The datum feature C simulator in the
shown illustration is fi xed in location at the MMB
of the datum feature. Based on ASME Y14.5-2009,
the datum feature C must be able to make contact
with the datum simulator that is fi xed in location
at the MMB.
A comparison of RMB and MMB application
to a tertiary datum can be seen in Figure 6-24. To
achieve design intent, the tertiary datum feature
C is meant to stop rotation of the shown part to
ensure the two holes are correctly positioned. The
tertiary datum feature C has a profi le tolerance
applied to it and the tolerance zone establishes
maximum and minimum material boundaries.
When the tertiary datum reference is at RMB, the
simulator for the tertiary datum is oriented to the
higher precedence datums and it progresses from
the MMB toward the LMB to make full contact
with the feature. The part may rotate into the posi-
tion that results in the full contact. The tertiary
datum feature must not move outside its profi le
tolerance boundary.
When the tertiary datum reference is at MMB,
the simulator is fi xed in location at the maximum
material boundary established by the profi le toler-
ance. When the datum feature referenced at MMB
is offset from a higher precedence datum axis as in
Figure 6-24, the tertiary datum feature must make
at least one point contact with the simulator. The
photograph in Figure 6-25 shows datum simula-
tors that are at the MMB locations.
If it is desired for the simulator to be fi xed at
the nominal location of the tertiary feature, the
datum feature reference is followed with the term
BASIC or the abbreviation for basic enclosed in
brackets [BSC]. This fi xes the simulator at the basic
location of the feature and the feature is brought
into contact with the simulator. It is also permissi-
ble to enter a number with brackets to indicate the
required location of the simulator. In Figure 6-24,
the datum feature C reference would be followed
by either [BSC] or [.250]. Further explanation is
provided later in this chapter.
The default applicability of RMB on all
datum feature references is relatively simple to
Goodheart-Willcox Publisher
Figure 6-25. The shown tool locates the secondary and
tertiary datum features at MMB.