Chapter 6 Datums and Datum Feature References 185
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to the axis. The three planes are mutually perpen-
dicular. In simpler words, the datum axis locates
two of the planes and establishes the orientation of
all three planes of the coordinate system. Datum
feature A only constrains translation of (locates)
the third plane.
The given fi gure shows only one of the sev-
eral possible position tolerance specifi cations that
could be provided for the four holes. Selecting the
proper specifi cation requires consideration of the
application for the part. The primary reference to
datum feature B at RMB is appropriate for a situ-
ation where the shaft presses into a hole. RMB is
applicable for a press fi t between parts, because
regardless of the feature sizes, the axes of a hole
and shaft will coincide when the parts are pressed
together.
A practical application of the above explana-
tion is shown in Figure 6-40. The tool is a collet
and the workpiece is inserted and clamped into
the collet to establish the primary datum axis. The
workpiece may also be bumped up against the face
of the collet to locate the secondary datum plane.
MMB on a Primary Datum Feature
Reference
Figure 6-41 shows the same part as the pre-
vious fi gure, except the reference to datum B has
an MMB modifi er. The MMB modifi er applied to a
primary datum feature reference has a signifi cant
impact on how the datum axis is established. The
result is that the datum simulator for datum fea-
ture B, on the example part, has a fi xed diameter
equal to the maximum material size. The MMB
and maximum material size are the same for this
datum feature on this part.
Three conditions must be met for the MMB
of a datum feature to be equal to the maximum
material size of that feature. One condition is that
the datum must be referenced as a primary datum.
The second condition is that the reference must
include an MMB modifi er. The third condition
is that no derived median line (axis) or derived
median plane (center plane) form tolerance may be
applied to create a virtual condition for the feature.
Goodheart-Willcox Publisher
Figure 6-40. A collet may be used to establish a datum
axis.
Only one point
contact required
Establishes two planes on axis B
Establishes the third plane
.878 fixed diameter
datum simulator
Workpiece can move in the gage if
datum feature B departs from MMB
MMB modifier
indicates how primary
datum B is established
Goodheart-Willcox Publisher
Figure 6-41. A primary datum feature referenced at
MMB requires the datum be simulated by a tool that
has a fi xed size equal to the maximum material size of
the datum feature if only size tolerance is applied to
the feature.