Chapter 6 Datums and Datum Feature References 183
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The MMB is outside the material of the part
whenever there is an applicable geometric tolerance.
The MMB is equal to the MMC only when there is
no applicable tolerance other than size tolerance.
Datum Feature References LMB
Regular features of size, irregular features of
size, and surfaces may have multiple boundaries
that are created by size and geometric tolerances.
In the following paragraphs, the applicable mate-
rial boundary for a datum feature reference that
includes an LMB modifi er is explained.
For a regular feature of size, the size limit
or tolerance boundary that contains the least
amount of material is known as the least material
condition (LMC). That boundary is impacted by
any applicable tolerance. If there are no geometric
tolerances applied, then the LMC is equal to the
least material size (LMS) (which is only affected
by the size tolerance). When a datum reference is
to a regular feature of size without geometric tol-
erances and that reference includes the least mate-
rial boundary modifi er, the applicable boundary is
equal to the LMC.
When an irregular feature of size or a sur-
face has a profi le tolerance applied, two material
boundaries are created. The boundary resulting
in the least material is known as the least material
boundary.
A datum feature reference that includes the
LMB modifi er is applicable to the least material
boundary (LMB). For a regular feature of size
without geometric tolerances, the LMB is equal to
the LMC. For an irregular feature of size or sur-
face, the LMB is the profi le boundary that contains
the least material.
The LMB is always inside the material of the
part except when the produced feature is at the
least material size. Calculations to determine LMB
are similar to those for MMB, except the tolerances
are applied on an LMC basis and the datum simu-
lation boundaries are established by determining
the applicable boundary that is within the material.
Further Considerations of Modifi ers on
Datum Feature References
Careful attention is needed for the selection
of the appropriate material boundary modifi er for
each reference to a datum feature of size. Gener-
ally, the application of MMB on a datum feature
reference can reduce detail part cost, because
it increases allowable variation. However, the
application of the MMB modifi er does not always
reduce assembly cost because the additional detail
part variation may make assembly diffi cult. The
application of MMB on a datum feature reference
may also adversely affect function because of the
additional allowable variation. The MMB modifi er
should be used on datum feature references only
when design function and assembly requirements
can tolerate the additional variation that is allow-
able as a result of the MMB modifi er.
RMB on a Primary Datum Feature
Reference
Placing a datum feature symbol on a diameter
dimension establishes a feature of size as a datum
feature. See Figure 6-39. The shaft on the given
part is identifi ed as datum feature B, and it is ref-
erenced as the primary datum feature in a position
tolerance. Datum feature B provides a physical fea-
ture from which the location of theoretical datum
axis B can be determined.
The given fi gure shows one surface of the fl ange
as datum feature A. The shaft that acts as datum
feature B has a perpendicularity tolerance of .003″
diameter relative to datum A, with the tolerance
applicable at MMC. No material boundary modifi er
is shown on the datum feature reference, so RMB is
applicable to the datum feature A reference.
MMB = MMC + Position
.882
Tertiary datum reference
Size and position tolerance applicable
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Figure 6-38. A tertiary datum reference at MMB
results in a simulator sized to the virtual condition of
the feature relative to any higher order precedence
datums.