Chapter 6 Datums and Datum Feature References 199
Copyright Goodheart-Willcox Co., Inc.
frame. Envelope size is based on the maximum
material condition for the dimensioned feature.
The envelope has perfect orientation with respect
to the noted datum reference frame.
The requirement for perfect orientation to the
datum reference frame could potentially increase
product cost above the requirements of Rule #1,
which does not by itself force perfect orientation
relative to the datum reference frame. Relating
dimensions to a datum reference frame should
only be used when design requirements dictate
that it is necessary.
The impact on the least material condition
requirements when dimensions are referenced to a
datum reference frame is somewhat controversial.
A generally accepted approach is to verify least
material limits by direct two-point measurement
between surfaces. This approach ensures that the
part features do not violate the minimum size
limit allowed by the dimension tolerance.
Advanced Datum Concepts
Advanced datum concepts allow a new level
of clarity in specifying complex part requirements.
Also, defi ning how datums are established from
the datum features that are identifi ed and refer-
enced is improved.
Datum Feature Translation
It is possible to indicate in the datum feature
references that the datum simulator may trans-
late. See Figure 6-61. The drawing of the plate
shows one hole as secondary datum feature B and
another hole as tertiary datum feature C. The hole
identifi ed as datum feature B has an orientation
tolerance that is related to datum feature A. The
hole identifi ed as datum feature C has two geo-
metric tolerances applied to it. It has an orientation
tolerance relative to datum A and a position toler-
ance that references datum feature A primary and
datum feature B secondary at MMB.
Three holes in the plate have a position tol-
erance that references datum feature A primary,
datum feature B secondary at MMB, and datum
feature C at MMB with the translation modifi er
shown. Primary datum feature A is simulated
with a fl at plate. Secondary datum feature B is sim-
ulated at the applicable MMB size of .375″ diam-
eter, which is the result of the maximum material
size and the orientation tolerance that is applied to
the datum feature B hole. The tool drawing shows
a round bushing is used to locate a .375″ diam-
eter tooling pin that would be inserted through
the datum feature to establish the orientation and
location of datum axis B.
Simulation of the tertiary datum feature must
be done using the applicable MMB. Because the
translation symbol is shown on the tertiary datum
feature reference, the simulator does not need to
accommodate the position tolerance because the
simulator is permitted to translate. The applicable
MMB for datum feature C is determined using the
maximum material size and the perpendicular-
ity tolerance. The maximum material size of .378″
diameter and the perpendicularity tolerance of
.003″ diameter combine to establish an MMB size
of .375″ diameter for the simulator.
In the given fi gure, a slotted bushing is used
to locate the .375″ diameter tooling pin that would
Primary datum
(three point contact)
Secondary datum
(two point contact)
Tertiary datum
(one point contact)
Perfect form envelope at MMC
related to the datum reference frame
.380
1.758
1.990
MIN
1.742
MIN
.370
MIN
2.010
Goodheart-Willcox Publisher
Figure 6-60. Dimensions related by note to the datum
reference frame establish an envelope at the MMC size
of the dimensioned feature.