325Geometric Dimensioning and Tolerancing (GD&T)
At this point, beyond the ofcial denition of a datum, the user of GD&T
must be aware of two more concepts. The rst is the relationship of datum
features and the second is what goes in the datum reference frame (DRF) box.
A datum feature is a physical feature that acts as an acceptable substitute for
a datum. Datum features relate the various features of the part to each other.
Three relationships of the datum feature must be considered. They are
1. Primary datum features establish basic stability for the part with
respect to a functional mating feature and typically control three
degrees of freedom.
2. Secondary datum features establish orientation to the primary
datum features and typically control two degrees of freedom.
3. Tertiary datum features establish orientation and location to both
the primary and secondary datum features and typically control
one degree of freedom.
On the other hand, a DRF is three imaginary planes perpendicular to
one another that are mapped onto the part to relate features to each other.
Specic requirements that should be kept in mind are
• A maximum of three datums can be referenced.
• Specify enough datums to sufciently constrain the part.
• Use as many letters as needed. The letters do not have to follow
alphabetical order.
• Letters I, O, and Q cannot be used.
In any discussion of datums, one must also consider the effects of datum
feature shift. This happens when
• FoS datums are referenced at MMB or LMB, datum feature shift
occurs where the part is allowed to move relative to the datum fea-
ture simulators
• Datum feature shift is an additional tolerance that must be con-
sidered when establishing gauging or when calculating tolerance
stackups of component alignment or minimum sealing surfaces of
mating parts
• Datum feature shift can be easily evaluated on functional gauging; it
is more difcult to assess on a CMM
Datum Targets
ASME Y14.5-2009 4.24 denes datum targets as designated points, lines, or
areas that are used in establishing datum controls. A datum target represents
326 Quality Assurance
the datum feature simulator (otherwise known as the gauge) contacting the
part. It is not on the part itself. The part, however, must contact all datum
targets. Datum targets can be xed or moveable. They should be used when
(1) only a portion of a feature is functional as a datum feature, and (2) inher-
ent irregularities cannot be effectively used to establish a datum. The datum
target is identied with the symbol . The top half species a point, line, or
area. The circle contains size and shape. The bottom half of the circle usually
has a letter and a number. The letter designates the datum, followed by the
number of the target in sequence beginning with 1 (e.g., A1).
Sometimes when dealing with datum targets, one will come across cus-
tomer-specic requirements that are associated with datum target dimensions
and moveable datum targets. For example, a customer may state the location
and size of datum targets to be dened with basic dimensions, except when
(1) the datum target is moveable, (2) expandable, or (3) collapsible; (4) the cus-
tomer may clarify that for basic dimensions, established tooling or gauging
tolerances apply; or (5) in the exception cases, the motion is limited by the part/
assembly tolerance. Note: For information on datum feature simulator toler-
ances and tolerance relationships between the simulators, see ASME Y14.43.
In the case of moveable datum targets, the ASME Y14.5-2009, paragraph
4.24.6 tells us that when the direction of movement is not normal to the true
prole or when the direction is unclear, dimensions and/or notes indicating
direction of movement shall be added to the drawing. This symbol (
H1
), also
known as the birdbeak, indicates the movement of the datum target feature
simulator. The direction of movement is typically normal to the true prole.
Things to Remember: Datums
• Always verify that the selected datum is a real feature that can be
measured.
• Verify that the selected features have form and size controls applied.
• Ensure that the primary, secondary and tertiary datum features
been related with location and orientation controls.
• Verify that the features selected are permanent. If the features are
not permanent, determine how the relationship will be measured
when they are gone.
• Datum targets may require additional datum references to ensure
repeatability.
Things to Remember: Form
• No datum reference allowed
• Only one feature may be controlled with form. A seal groove may
create two surfaces and prole will need to be used instead of atness
327Geometric Dimensioning and Tolerancing (GD&T)
• Straightness tolerance of an axis RFS and at MMC can override Rule 1
• Of all the form controls, only straightness tolerance of an axis can use
the MMC modier. However, only attribute data will be obtained
from the gauge (ts or does not t)
• Do not use form tolerances greater than the limits of size when spec-
ifying atness at RFS
Orientation
With orientation controls, all tolerance zones are referenced to a datum or
datums with a basic angle of
0°
90°
ALL°
Rules Applied to Orientation
• Lines drawn parallel have a basic angle of 0°.
• Lines drawn perpendicular have a basic angle of 90°.
• In 3D space, angles other than parallel can appear to change depend-
ing on the models rotation and slight angles may be difcult to
detect.
• Angles that are not parallel or perpendicular must be specied on
the drawing or queried in the CAD model to avoid confusion.
Very often customers may have their own special orientation require-
ments. Therefore, it is important that the supplier is aware of them.
Things to Remember: Orientation
• Orientation controls do not provide location for features.
• Location and prole controls or toleranced dimensions are the only
ways to locate surfaces and features.
• Orientation tolerances should be signicantly smaller than the posi-
tion tolerance to make it a cost-effective control.
• Make sure sufcient datums are referenced for full orientation
control.
328 Quality Assurance
Projected Tolerance Zone (
P
)
• Since the positional tolerance zone controls location and orientation,
the projected tolerance zone is required to transfer the tolerance
zone to the area used by the mating part.
• Section [7.4.1] of the standard species where a composite or multiple
segment feature control frame is used. The projected tolerance zone
symbol shall be shown in all applicable segments.
• If a projected tolerance zone is not used, the minimum clearance hole
must be calculated using the formula shown in the ASME Y14.5-2009
appendix B5, or similar method.
The AMSE standard paragraph 7.4.1 also identies an interference, which
can occur where a tolerance is specied for the location of a threaded or
press-t hole, and the hole is inclined within the positional limits. Unlike the
oating fastener application involving clearance holes only, the orientation
of a xed fastener is governed by the inclination of the produced hole into
which it assembles. Therefore, the projected tolerance zone value is dened
as a minimum height, which is equal to the maximum material thickness of
the mating clearance hole. On the other hand, Section [7.4.1.2] (p. 120) states
that the specied position value for the projected tolerance zone is a mini-
mum and represents the maximum permissible mating part thickness, or
the maximum installed length or height of the components, such as screws,
studs, or dowel pins. Furthermore, [7.4.1.2] (p. 121) states that the minimum
extent and direction of the projected tolerance zone are shown in a drawing
view as a dimensioned value with a heavy chain line drawn closely adjacent
to an extension of the center line of the hole.
As for position surface (boundary) interpretation, the Standard states that
(1) whenever a discrepancy occurs between surface (boundary) or axis inter-
pretation, the surface interpretation is the default and (2) while the boundary
notation is not required, it is recommended whenever the axis interpretation
of the FoS is not clear. This concept is especially helpful for irregular FoS,
such as slots and tubes/hoses, and so on.
Things to Remember: Position
• Position is the measurement of permissible variation from theoreti-
cal design intent (exact location).
• Location of one or multiple FoS are relative to one another or to spec-
ied datums.
• Basic dimensions establish the theoretical design intent (exact
location).
• Positional tolerance applies only to the FoS location, not to the basic
dimensions.
329Geometric Dimensioning and Tolerancing (GD&T)
• Positional tolerancing is applied at RFS, MMC, and LMC.
• Axis and surface interpretations:
• Axis: Zone dening variation allowance of center, axis, or center
plane of a FoS from an exact location
• Surface: A VC boundary, located at exact location which may not
violate FoS boundary
Profile Controls
Prole controls are a group of powerful geometric tolerances that control
the size, location, orientation, and form of a feature. Prole tolerances can be
either independent or related. They use basic dimensions or a CAD model
dened as CAD Basic. Prole was originally intended to control irregular
surfaces; however, it has expanded to include many other applications.
Circle U Modifier (
U
)
Whether it is applied to a drawing or CAD model, the Circle U symbol elimi-
nates the need for support geometry and simplies the prole callout. Both
methods are shown in Figure 20.1.
Things to Remember: Profile
• Prole can be used with or without referencing datums.
• The prole tolerance is applied RFS. Currently, no modiers are
allowed in the tolerance zone with the exception of Circle U.
• Unless otherwise specied, the prole tolerance is equal bilateral.
• The prole tolerance can also be unilateral or unequally disposed by
adding support geometry showing the amount of tolerance that is
applied outside of the part or by using the Circle U modier.
• Referenced datums are allowed to have material modiers, yet the
added tolerance needs to be considered.
0.4
0.4
Ⓤ
Drawing calloutDrawing callout
0.1
0.1
A
A
A
A
FIGURE 20.1
Options of drawing callout the circle U modier.
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