Chapter 6 Datums and Datum Feature References 177
Copyright Goodheart-Willcox Co., Inc.
accommodate when the datum features are all fl at
surfaces. Figures 6-21 and 6-31 both show posi-
tional tolerances that include three datum feature
references. All three datum features are fl at sur-
faces. No boundary condition modifi er is shown
on the datum feature references. Therefore, RMB
is applicable.
Regardless of the boundary or size of any
feature on the part, the surfaces will be placed
in contact with fl at datum simulators to locate
datum planes as shown in Figures 6-21 and 6-32.
The given part will be placed on the surfaces so
that three point contact is made on primary
datum feature A, two point contact on second-
ary datum feature B, and one point contact on
tertiary datum feature C.
The three datum planes are mutually per-
pendicular, even if one of the datum features is
made with some orientation or fl atness variation.
Another way to view this is that each simulator is
a plane and each simulator is brought into contact
with its associated datum feature while also main-
taining the relative orientation of each plane in the
datum reference frame.
The three datum planes for the given part are
located by the high points on the given surfaces.
In Figure 6-32, datum plane C is located such that
it extends through a portion of the part. This is
acceptable because there is no requirement to use
the outermost surfaces as datum features.
Datum Features of Size
Features of size are often identifi ed as datum
features. It is common to use cylindrical features,
such as holes and shafts, as datum features of size.
Rectangular slots, rails, and tabs are also used as
datum features of size.
Placement of the datum feature symbol in line
with a size dimension indicates that the feature of
size is the datum feature. It is also common prac-
tice to attach the datum feature symbol to a fea-
ture control frame that is attached to the feature of
size. When a datum feature of size is identifi ed, the
theoretical datum is typically an axis, centerline,
or center plane.
Cylindrical Features
Shafts, holes, counterbores, and other cylin-
drical items may be identifi ed as datum features of
size. See Figure 6-26. The datum feature symbol is
normally placed in line with the diameter dimen-
sion of the cylindrical feature, but it may also be
placed on a leader that extends to the circular view
of the feature. The datum feature symbol should
not be placed on an extension line from the surface
when the intent is to identify a datum feature of
size. A datum feature symbol on an extension line
is used for identifying surfaces as datum features.
The given fi gure shows three examples of
cylindrical datum features. In the fi rst example,
the small diameter on a stepped shaft is identifi ed
as datum feature A. Datum feature A establishes
the datum axis.
The second example shows a hole that is iden-
tifi ed as a datum feature of size. The datum iden-
tifi cation triangle may be attached to the hole or it
may be attached to a leader that points to the hole.
Both methods indicate that the hole establishes a
datum axis.
The third example shows a part with two
holes. A datum feature symbol is connected to one
hole with a leader. This identifi es only one hole as
a datum feature. Placement of the datum feature
symbol on the leader of the callout for the two holes
or on the feature control frame for those two holes
would establish the hole pattern (using all holes in
the pattern, not just one) as the datum feature.
Placement here indicates
the pattern of features
locates the datum
Leader pointing to one
feature of size
Aligned with
dimension
Applied on a
leader
Or
Attachment with a
leader indicates one
datum feature
Goodheart-Willcox Publisher
Figure 6-26. A datum feature of size is identifi ed by
associating the datum feature symbol with the size of
the feature.