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Chapter 9. Geometric Dimensioning and
Tolerancing (GD&T) – An Introduction
Learning Outcomes
At the end of this topic you should be able to:
• Recognize various symbols used in GD&T
• Explain the terms maximum material condition
(MMC) and least material condition (LMC)
• Explain the advantages of GD&T
Chapter 9. Geometric Dimensioning and
Tolerancing (GD&T)
9.1 Introduction
Engineering
DesignProduct
Part
Inspection
What is to be
manufactured
What has beenmanufacturedCompare product
with design
Chapter 9. Geometric Dimensioning and
Tolerancing (GD&T)
9.1 Introduction
Engineering
DesignProduct
Part
Inspection
GD&T
• GD&T binds the three elements (engineeringdesign, the product and part inspection together)
Activity
List the main factors that determine the
manufacturing cost of the stepped shaft shown:
9.2 Meaning of tolerance
• Tolerance is the amount by which a dimension is
allowed to vary
• Tolerances are applied to both position and size:
50 ± 0.5
25 ± 0.1
Tolerance on size
Tolerance on position
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9.3 GD&T
• GD&T is a method of dimensioning and tolerancing
a drawing with respect to the actual function or
relationship of part features that can be most
economically produced
9.3 GD&T
• GD&T is a method of dimensioning and tolerancing
a drawing with respect to the actual function or
relationship of part features that can be most
economically produced
• GD&T is used when:
(a) Features are critical to functionality of part
(b) Datum references are required to ensure
consistency between design, manufacturing
and inspection
“Proper application of GD&T willensure that the allowable part andassembly geometry defined on thedrawing leads to parts that have thedesired form and fit (within limits) andfunction as intended.”
From Wikipedia, the free encyclopedia
9.4 Benefits of GD&T
• GD&T adds clarity to conventional coordinate
dimensioning
• Universal symbols are used to:
i) convey design intent to remote
manufacturing or assembly sites
ii) provides a common standard for
dimensioning practices
iii) enhance repeatability of part orientation
iv) increases interchangeability of parts
• Traditional Cartesian coordinate system creates
square tolerance zone, e.g. tolerance for hole
center:
20 ±±±± 1
20 ±±±± 1
• Permissible machining limits:
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• Location tolerance of ±1 creates tolerance zone for
hole center of 2 mm:
2
2
20
20
Only hatched space is withintolerance
But, shaded areas are also
within same distance from
center
Unnecessary restriction becomes much greater
• Consider what happens when two holes are involved: • GD&T provides a diametrical (circular) tolerance
zone:2
2
9.5 Definitions
• Feature: Physical portion of a part, e.g. hole,
surface, slot etc.
• Datum: Theoretically exact plane, point or axis
from which a dimension is measured
• Datum feature: Part feature that contacts the
datum
• Datum reference frame: Set of three mutually
perpendicular datum planes:
First datum plane(primary plane)
Second datumplane
(secondaryplane)
Third
datum
plane(tertiary
plane)
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• Feature of size: One cylindrical or spherical surface
or a set of two opposed elements or opposed parallel
surfaces associated with a size dimension
9.5 Definitions
• Language of GD&T is a set of symbols, divided into
five types of dimensioning control:
- form tolerance- profile tolerance
- orientation tolerance
- location tolerance
- runout tolerance
• Form tolerance: States how far an actual surface or
feature is allowed to vary from the desired form on
the drawing
18 mm 17.89 mm
??0.055 mm
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Example where
flatness control can
be applied:
• Profile tolerance: States how far an actual surface
or feature is allowed to vary from the desired form on
the drawing or vary relative to a datum
• Form tolerance for lines: Profile
0.02
0.02
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• Orientation tolerance: States how far an actual
surface or feature is permitted to vary relative to a
datum
• Location tolerance: States how far an actual size
feature is permitted to vary from the perfect location
implied by the drawing as related to a datum or other
feature
• Runout tolerance: States how far an actual surface
or feature is permitted to vary from the desired form
implied by the drawing during full 360° rotation of
the part on a datum axis
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9.6 Material conditions
• Maximum material condition (MMC): Condition in
which a feature of size contains the material within
its stated tolerance limits (Symbol )
e.g. maximum material condition for pin:
MMC for pin is φ6.15
e.g. maximum material condition for hole:
MMC for hole is φ6.25
• Least material condition (LMC): Condition in which
a feature of size contains the least amount of
material within its permissible limits (Symbol ), e.g.
minimum shaft diameter or maximum hole diameter.
LMC for hole is
φ6.35
• Regardless of Feature Size (RFS): Geometric
tolerance that applies at any increment of size of
feature within its permissible limits
- RFS is implied on all geometric tolerances, unless
indicated by the presence of a modifier
9.7 Feature control frame
• A feature control frame is used to specify
geometric tolerances on a drawing
• The feature control frame is a rectangular box that
contains the geometric symbols, modifiers and
datum references
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• Examples of different feature control frames:
• Examples of different feature control frames (ctd.): • Form tolerance for lines: Circularity
0.02
0.02
• Form tolerance for surfaces: Flatness
0.05
0.05
• Form tolerance for surfaces: Cylindricity
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• Form tolerance for surfaces profiles
• Orientation tolerance: Parallelism
• Orientation tolerance: Perpendicularity
Describe the type of tolerance control shown in
the figure
ACTIVITY 1
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Describe the type of tolerance control shown in the
figure
ACTIVITY 2
• Orientation tolerance: Angularity
ACTIVITY 3
The language of GD&T is a set of symbols
divided into five types of dimensioning control.
Name the five types of dimensioning control.
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STRAIGHTNESS
‘Straightness is the condition where
all the points on a surface or an axis
are in a straight line. A straightness
tolerance specifies a zone within
which the surface or axis must lie. Inthe example the zone is bounded by
two parallel lines 0.03 mm apart.
When a diameter symbol is added to
the tolerance the derived axis of thefeature must lie within a cylindrical
tolerance zone of 0.03 mm diameter.
When a MMC modifier is added, the
tolerance zone is 0.03 diameter at
18mm diameter and the zone
increases as the feature decreases
from MMC.’
http://www.actphx.com/gd_and_t/gd_and_t.html FLATNESS
‘Flatness is the condition
of a surface having all
points in one plane.
A flatness tolerancespecifies a zone defined
by 2 parallel planes. In the
example shown thesurface must lie between 2
parallel planes 0.18mm
apart and the surface must
be within the specified size
limits.’
(http://www.actphx.com/gd_and_t/gd_and_t.html)
CIRCULARITY (ROUNDNESS)
‘Circularity or roundness is the
condition where all the points
on a surface are in a circle.
Circularity tolerance specifies a
zone bounded by 2 concentric
circles within which the
measured surface must lie.
In the example shown each
circular element must lie
between 2 concentric circles,
one having a radius 0.05mm
larger than the other. Each
circular element of the surface
must also be within the
specified limits of size.’
(http://www.actphx.com/gd_and_t/gd_and_t.html)
CYLINDRICITY
‘Cylindricity is the condition
where all points of a surface
of revolution are equidistant
from a common axis.
Cylindricity tolerance specifies
a zone bounded by 2
concentric cylinders within
which the measured surface
must lie.
The tolerance applies
simultaneously to both circular
and longitudinal elements.’
(http://www.actphx.com/gd_and_t/gd_and_t.html)
OPEN PROFILE (LINE)
‘A profile is the outline of an
object in a given plane. The
tolerance zone established by the
profile of a line tolerance is two
dimensional extending along the
length of the considered feature.
In this example each line elementat any cross section must lie
between 2 profile boundaries
0.6mm apart in relation to the
datum plane A. Additionally the
surface must be within any
specified limits of size. The
tolerance zone may be specified
to be unilaterally disposed either
inside or outside the true profile’
(http://www.actphx.com/gd_and_t/gd_and_t.html)
CLOSED PROFILE (SURFACE)
‘The tolerance zone established by
the profile of a surface tolerance is
three dimensional extending along
the length and width (or
circumference) of the considered
feature.
In the example all points on thesurface must lie between 2 profile
boundaries 0.6mm apart in relation
to datum plane A. Additionally the
surface must be within and
specified size limits.’
(http://www.actphx.com/gd_and_t/gd_and_t.html)
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ANGULARITY
‘Angularity is the condition of
a surface or axis at a specified
angle (other than 90º) from a
datum plane or axis.
The tolerance zone is defined
be 2 parallel planes at the
specified basic angle from a
datum plane or axis. The
surface or axis of the
considered feature must lie
within this zone. In the
example all points of the
surface must lie within the 0.5
mm wide tolerance zone.’
(http://www.actphx.com/gd_and_t/gd_and_t.html)
PERPENDICULARITY
Perpendicularity is the
condition of a surface or axis at
a right angle to a datum plane
or axis.
1) A zone defined by 2 parallel
planes perpendicular to a
datum plane or axis. In the
example shown, the surface of
the feature must lie within this
zone which is 0.15 mm wideand at right angles to datum A.
(http://www.actphx.com/gd_and_t/gd_and_t.html)
2) A zone defined by 2 parallel
planes perpendicular to the
datum axis. In this example,
the axis of the hole must lie
within the zone which is 0.3
mm wide and at right angles to
datum axis A. The feature axis
must also be within the
tolerance of location.
PERPENDICULARITY (ctd.)
(http://www.actphx.com/gd_and_t/gd_and_t.html)
3) A cylindrical tolerance zone
perpendicular to a datum
plane. In this example the axis
of the part must lie within a
cylindrical zone of 0.05 mm
diameter at right angles to
datum A.
This tolerance applies at the
maximum diameter of 14.984
mm (MMC). As the feature size
decreases from MMC, the
perpendicularity tolerance zone
is increased a corresponding
amount. The feature axis must
also be within the tolerance of
location.
PERPENDICULARITY (ctd.)
(http://www.actphx.com/gd_and_t/gd_and_t.html)
PERPENDICULARITY (ctd.)
(http://www.actphx.com/gd_and_t/gd_and_t.html)
4) A zone defined by 2 parallel lines
perpendicular to a datum plane or
axis. In the example shown, each
radial element of the surface must lie
within this zone 0.05mm wide and at
right angles to datum A.
The surface must also be within thespecified limits of size.
PERPENDICULARITY (ctd.)
(http://www.actphx.com/gd_and_t/gd_and_t.html)
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PARALLELISM
The surface of the feature must lie within 2 planes 0.25
mm apart which are parallel to datum A.
Describe the type of tolerance control shown in the
figure
ACTIVITY 4
Describe the type of tolerance control shown in the
figure
ACTIVITY 5
The tolerance applies at the minimum diameter of 9.000
(MMC). As the feature size increases from MMC, the
parallelism tolerance zone is increased a correspondingamount.
TRUE POSITION
If position tolerances are to be modified as features depart from
maximum material condition, the MMC modifier must be specified on the
drawing.
A positional tolerance defines a zone within which the center, axis or
center plane of a feature of size is permitted to vary from the true (exact)
position. Basic dimensions establish the true position.
In the example shown, the center of the holes must lie within circles of
0.5 mm diameter when the holes are at 10.25 mm diameter.
As the diameter of the holes increases to 10.5 mm diameter, the
tolerance zones increase proportionately to 0.75 mm diameter.
(http://www.actphx.com/gd_and_t/gd_and_t.html)
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CONCENTRICITY CONCENTRICITY
CONCENTRICITY
Concentricity is the condition
where the axes of all cross
sectional elements of a surface
of revolution are common to the
axis of a datum feature.
Concentricity tolerance
specifies a cylindrical tolerance
zone whose axis coincides with
the datum axis.
In this example, the zone has a
diameter of 0.2mm and the
feature axis must lie within thiszone.
(http://www.actphx.com/gd_and_t/gd_and_t.html)
CIRCULAR RUNOUT
Runout is a composite tolerance used to control the
relationship of one or more features to a datum axis. The
illustration shows the types of features that can be
controlled by runout tolerances.
(http://www.actphx.com/gd_and_t/gd_and_t.html)
Circular runout provides control of circular elements of a
surface. It can be used to control the cumulative variations
of circularity (roundness) and coaxiality.
(http://www.actphx.com/gd_and_t/gd_and_t.html)
In the example shown, each circular element of the surfaces
toleranced must fall within 0.04mm (Full Indicator Movement)
when the part is rotated 360º about the datum axis.
(http://www.actphx.com/gd_and_t/gd_and_t.html)
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TOTAL RUNOUT
Total runout provides composite
control of all surface elements.
For surfaces around a datum
axis, including:
• Circularity (Roundness)
• Straightness
• Coaxiality
• Angularity
• Taper
• Profile of a Surface
(http://www.actphx.com/gd_and_t/gd_and_t.html)
For surfaces perpendicular to a datum axis it includes:
• Perpendicularity
• Flatness
In the example shown, the entire surface must lie within the 0.04mm
wide (Full Indicator Movement) tolerance zone when the part isrotated 360º about the datum axis.
(http://www.actphx.com/gd_and_t/gd_and_t.html)
Activity 5Name the following symbols: