Geometric dimensioning and tolerancing (GD&T)

GD&T
(Geometric Dimensioning & Tolerancing)

Unit II ( 13½ Hours)
Geometric Dimensioning & Tolerancing (GD&T) -Introduction -Dimensioning and
Tolerancing Fundamentals – Dimensioning Features – Features Control Frame – Datum
Feature – Dimensioning Characteristics & Symbols, - Form - Orientation- Position, General -
Position, Location- Position, Coaxiality- Concentricity and Symmetry- Runout- Profile –
Rules to GD&T - Strategy for tolerancing Parts-tolerance of various mould elements, Cost
impact on GT.
Context
DPMT
SEMESTER – VI

I n c r e a s e d
u n d e r s t a n d i n g
Mfg Design Quality

• It is the science of the properties and relations of
lines, surfaces and solids.
Geometry
• It is a measurable extent, as length, breadth and
depth.
• A numerical value expressed in appropriate units
of measure and used to define the size, location
geometric characteristics or surface texture of a
part.
Dimension
3
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Dimension
• It is a allowable variation in any measurable
property.
• The total amount that features of the part are
permitted to vary from the specified dimension.
• The tolerance is the difference between the
maximum and minimum limits.
• Two common methods to specify tolerances
– limit tolerances
– plus-minus tolerances
Tolerance
4

Limit tolerances
Plus Minus Tolerances
5

Dimension Anatomy
• ASME Y14.5M-1994 - The national standard
for dimensioning and tolerancing in the
United States.
• ASME stands for American Society of
Mechanical Engineers.
• The Y14.5 is the standard number. "M" is to
indicate the standard is metric, and 1994 is
the date the standard was officially
approved.
What is GD&T
6

•Geometric Dimensioning and tolerancing (GD&T)
is a language used on mechanical engineering
drawings composed of symbols that are used to
efficiently and accurately communicate geometry
requirements for associated features on component
and assemblies.
•A method to specify the shape of a piece of
hardware on an engineering drawing.
What is GD&T
• A set of fourteen symbols used in the
language of GD&T. It consists of well-
defined of symbols, rules, definitions and
conventions, used on engineering
drawings to accurately describe a part.
• GD&T is a precise mathematical language
that can be used to describe the size,
form, orientation, and location of part
features.
• GD&T is also a design philosophy on how
to design and dimension parts.
What is GD&T
7

What is GD&T
8

9

• Use of this language or tool “can provide economic and
technical advantage” stated the ASME.
• Maximizes quality of the products.
• Provides uniformity of specification and interpretation
(reducing guesswork and controversy)
Advantages of GD&T
• Geometric dimensioning dramatically reduces the need for
drawing notes to describe complex geometry requirements
on a component or assembly by the use of standard
symbology.
Notes Vs Symbols
10

• Have multiple sources on various parts of an
assembly.
• Make valid engineering calculations.
• Have common parts across similar assemblies.
• Design subassemblies in different locations and
have them function correctly.
• Do tolerance analysis to study the effect of part
tolerances on the assembly.
• Use state of the art software tools to analyze parts in
an assembly.
• Use state of the art software tools to inspect the
parts.
• Reduce the risk caused by vague specifications.
• And finally saves money.
Other Advantages
01
Key Terms
•Feature
The general term applied
to a physical portion of a
part, such as a surface, pin,
hole, or slot.
•Feature of size
One cylindrical or spherical surface, or a set of two
opposed elements or opposed parallel surfaces,
associated with a size dimension.
Examples: Cylinder, sphere, slot, etc.
15

Geometric Characteristic Symbols
Form Tolerances
Profile Tolerances
Orientation Tolerances
Runout Tolerances
Location Tolerances
3

Foundations of Mechanical Accuracy
The Four Mechanical Arts
Geometry
Standards of Length
Dividing the Circle
Roundness
Wayne R. Moore
15

Key Definitions
Datum – Theoretically exact point, axis, or plane
derived from the true geometric counterpart.
Datum Feature – Actual feature on a real part used
to establish a datum.
Datum Feature Simulator – A surface of sufficient
precision to establish a simulated datum.
Simulated Datum – A point, axis, or plane
established by processing or inspection equipment.
Datum Target – A specified point, line, or area on a
part used to establish the datum scheme.
21

Key Definitions
Feature of Size – A cylindrical or spherical surface,
or two opposing elements or parallel surfaces.
Least Material Condition – This occurs where a
feature of size contains the least material allowed by
the stated limits of size.
Maximum Material Condition – This occurs where
a feature of size contains the most material allowed
by the stated limits of size.
Regardless of Feature Size – A term that indicates
that a geometric tolerance or datum reference
applies for any increment of size within its size
tolerance.
22

Key Definitions
Tolerance – The total permissible variation in size
for a specified dimension.
Bilateral Tolerance – A tolerance zone where the
boundary conditions contain the specified dimension.
Geometric Tolerance – A general term that refers
any of the 14 symbols used to control form,
orientation, profile, runout, or location.
Unilateral Tolerance – A tolerance zone that only
exists on one side of the specified dimension.
True Geometric Counterpart – The theoretically
perfect boundary or best fit (tangent) plane of a
specified datum feature.
23

Fundamental Rules
Each dimension shall have a tolerance.
(except for those dimensions specifically identified
as reference, maximum, minimum, or stock)
Ensure full understanding of each feature.
Show the detail needed and no more.
Serve function needs, no misinterpretation.
Manufacturing methods are not specified.
Non-mandatory dimensions are OK.
Designed of optimal readability.
24

Fundamental Rules
Dimension materials made to gage numbers.
90o apply when features are shown as .
90o apply when centerlines are shown .
Dimensions apply at 20oC (68oF).
Dimensions apply in a free state.
Tolerances apply for full size of feature.
Dimensions and tolerances only apply at the drawing
level where they were specified.
25

Limits of Size
Actual Size is a general term for the size of a
feature as produced. It has two interpretations.
Actual Local Size is the value of the individual
distance at any cross section of any feature of size.
Actual Mating Size is the dimensional value of the
actual mating envelope.
Limits of Size are the specified minimum and
maximum values for a feature of size.
26

Feature Control Frame Symbols
Statistical Tolerance
Tangent Plane
Free State
Projected Tolerance Zone
Least Material Condition
Maximum Material Condition
Spherical Diameter
Diameter
Feature Control Frame
Description Symbol
.010 A B C
S
M
L
P
F
T
ST
45

Feature Control Frame Elements
Label the elements of the feature control frame using the following terms:
Datum Modifier Geometric Characteristic
Diameter Symbol Primary Datum
Feature Modifier Secondary Datum
Feature Tolerance Tertiary Datum
.014 M A B M C
46

Feature Control Frame Placement
Locate the Feature Control Frame below or attached
to the leader-directed dimension or callout.
Run the leader from the frame to the feature.
Attach a side or an end of the frame to an extension
line from the feature.
Attach a side or an end of the frame to an extension
of the dimension line related to the feature in
question.
49

Form Tolerances
Flatness
Straightness
Circularity
Cylindricity
55

Form Tolerances
Datum references are never made for form
tolerances.
Rule #1 says that limits of size control
variation in form.
Generally, form tolerances are only necessary
to refine (require a tighter tolerance) limits of
size.
Form tolerances are often applied to features
to qualify them as acceptable datum features.
56

Flatness
Definition Flatness exists when a surface has
all of its elements in one plane.
Tolerance Zone Two parallel planes within
which the surface must lie.
57

Proper Application of Flatness
No datum is referenced.
It is applied to a single planar feature.
No modifiers are specified.
Tolerance value is a refinement of other
geometric tolerances or Rule #1.
59

Straightness
Definition Straightness exists when an
element of a surface or an axis is a straight
line.
Tolerance Zone Two parallel lines in the same
plane for two-dimensional applications. A
cylindrical tolerance zone that contains an axis
for three-dimensional applications.
60

Proper Application of Straightness
applied to a Surface Element
It is applied to a surface element.
It is applied in a view where the element to be
controlled is shown as a line.
Tolerance value is a refinement of other geometric
tolerances or Rule #1.
62

Straightness of a Feature of Size
When straightness is applied to a
feature of size:
 Tolerance zone applies to the axis or
centerplane.
 Rule #1 does not apply.
 The tolerance value may be larger that the
limits of size for the feature of size.
63

Proper Application of Straightness
applied to a Feature of Size
It is applied to a planar or cylindrical feature of size.
If a planar feature of size, the diameter symbol is not used.
If a cylindrical feature of size, the diameter symbol is used.
T , and L modifiers are not specified.
Tolerance value is a refinement of other geometric tolerances.
P ,
64

Circularity (roundness)
Definition Circularity exists when all of the
points on a perpendicular cross section of a
cylinder or a cone are equidistant to its axis.
Tolerance Zone Two concentric circles that
contain each circular element of the surface.
Note: Circularity also applies to spheres.
65

Proper Application of Circularity
It is applied to a circular feature.
Tolerance value is a refinement of limits of
size on the diameter or of other specified
geometric tolerances.
67

Cylindricity
Definition Cylindricity exists when all of the
points on the surface of a cylinder are
equidistant to a common axis.
Tolerance Zone Two concentric cylinders that
contain the entire cylindrical surface.
68

Proper Application of Cylindricity
It is applied to a cylindrical feature.
Tolerance value is a refinement of limits of
size on the diameter or of other specified
geometric tolerances.
70

Decisions for Form Tolerances
Form
Tolerances
Consider
Limits of Size
Flatness Straightness Circularity Cylindricity
Surface
Elements
Axis or
Center Plane
Consider
Material Condition
MMCRFS
71

Angularity
Perpendicularity
Parallelism
72

Datum references are always used for orientation
tolerances.
Orientation tolerances applied to a surface control
the form of toleranced surface.
Only a tangent plane may need control.
Orientation tolerances may be applied to control both
features of size and features without size.
Orientation tolerances do not control size or location.
Generally, profile tolerances are used to locate
features without size and position tolerances are
used to locate features of size.
73

Angularity
Definition Angularity exists when all of the
points on a surface create a plane or a feature
axis is at the specified angle, when compared
to a reference plane or axis.
Tolerance Zone Two parallel planes at the
true angle to a reference plane and contain
the entire surface surface.
Note: Applies to median planes and axes too.
DatumFeature
Datum Plane
74

Proper Application of Angularity
Datum reference is specified.
Surface applications may use tangent plane modifier.
Feature of size applications may use MMC, LMC,
diameter, of projected tolerance zone modifiers.
Basic angle defines perfect geometry between the
datum reference and the toleranced feature.
Specified tolerance is a refinement of other geometric
tolerances that control angularity of the toleranced
feature.
76

Perpendicularity
Definition Perpendicularity exists when all of
the points on a surface, median plane, or axis
are at a right angle to a reference plane or
axis.
Tolerance Zone Two parallel planes that are
perpendicular to a reference plane and
contain the entire surface surface.
DatumFeature
Datum Plane
77

Checking for Perpendicularity
78

Proper Application of
Perpendicularity
tolerances that control the perpendicularity of the
toleranced feature.
79

Parallelism
Definition Parallelism exists when all of the
points on a surface, median plane, or axis are
equidistant to a reference plane or axis.
Tolerance Zone Two parallel planes that are
parallel to a reference plane and contain the
entire surface surface.
DatumFeature
Datum Plane
80

Proper Application of Parallelism
tolerances that control parallelism of the toleranced
feature.
82

Decisions for Orientation Tolerances
Orientation
Tolerances
ParallelismAngularity Perpendicularity
Consider
Limits of Size
Consider Limits
Of Location
Feature
of Size
Plane
Surface
Consider
Material Condition
MMCRFS LMC
83

Location Tolerances
True Position
Symmetry
Concentricity
84

Location Tolerances
Datum references are always used for location
tolerances.
Location tolerances are reserved for tolerancing
applications on features of size.
They are always located by basic dimensions back to
the datum scheme.
Location tolerances shown on the same centerline
are assumed to have a basic dimension of zero.
Symmetry and concentricity application are centered
about the datum scheme specified for the controlled
feature.
85

True Position
Definition True position is the exact intended
location of a feature relative to a specified
datum scheme.
Tolerance Zone Most frequently, the
tolerance zone is a cylinder of specified
diameter within which the true axis of the
feature must lie.
Note: True position can also be applied to
median planes relative to specified datums.
86

Positional Tolerancing
Traditional tolerancing (say + .005”) consist
of 2-D rectangular boundaries.
A circular boundary with the same worst-case
conditions increases the area of the tolerance
zone by 57%, prior to any bonus tolerance.
87

Traditional Fastener Tolerances
Threaded Fastener 3/8 – 16
Clearance Hole 13/32
1/64 = .0156 .0015
Perfect Condition Worst-Case Condition 88
Clearance

89
Bonus Tolerances
When tolerancing features of size, bonus
tolerances may be applicable.
With MMC, as the size of a hole increases, so
does the acceptable tolerance zone, provided
the hole does not exceed its limits of size.
Hole at
MMC
Original
Tolerance
Zone
Larger
Hole
Larger
Tolerance
Zone
Larger
Hole

Maximum Material Condition (MMC)
Largest permissible external feature.
 Outside Diameter
 External Feature Size
 Key
Smallest permissible internal feature.
 Holes
 Slots
 Key Way
90

B
C
.014 M A B C
.760
.750
Size
91
Tolerance
MMC
4X
Note: Datum feature A is the back surface.

Least Material Condition (LMC)
Smallest permissible external feature.
 Outside Diameter
 External Feature Size
 Key
Largest permissible internal feature.
 Holes
 Slots
 Key Way
92

B
C
.014 L A B C
.760
.750
Size
93
Tolerance
LMC
4X

Regardless of Feature Size (RFS)
RFS is no longer documented except in rare
cases where it is required for clarity.
RFS is assumed for features of size when
neither MMC nor LMC are specified.
94

Regardless of Feature Size
B
C
.014 A B C
.760
.750
Size
95
Tolerance
4X

Applications of
Material Condition Modifiers
 Used for clearance application.
 Used for location applications.
 Used to protect wall thickness.
Regardless of Feature Size
 Used when size and location do not interact.
M
L
96

Applications for
.503
.501 .002 L
.500
.499
97
The purpose of the hole is to
locate the PLP pin below.
Worst Case Scenario
Hole diameter at .503 (LMC)
Pin diameter at .499 (LMC)
Clearance is .004
Pin can shift .002 in any direction
Tolerance for hole location is Ø .002 at LMC
Hole can be off location .001 in any direction
Pin can be off location .003 in any direction

Applications for
.503
.501 .002 L
.500
.499
98
Hole at MMC – Pin at LMC
Hole diameter at .501 (MMC)
Pin diameter at .499 (LMC)
Clearance is .002
Tolerance for hole location is Ø .004 at MMC

Applications for
.503
.501 .002 L
.500
.499
99
Hole at MMC – Pin at MMC
Hole diameter at .501 (MMC)
Pin diameter at .500 (MMC)
Clearance is .001
Tolerance for hole location is Ø .004 at MMC

Proper Application of Position
Position control is applied to a feature of size.
Datum references are specified and logical for the
application.
Basic dimensions establish the desired true position
of the feature of size.
Tangent plane modifier is not used.
Diameter symbol is used to specify axis control.
Diameter symbol is not used to specify center plane
control.
MMC, LMC, or RFS may be specified.
119

Symmetry
Definition Symmetry defines the location of
non-cylindrical features about a derived
median plane.
Tolerance Zone The tolerance zone is defined
by two planes, equidistant to a datum center
plane. The derived median points must fall
within these two planes.
A
120

Proper Application of Symmetry
A planar feature of size to be controlled uses
the same center plane as the datum scheme.
Diameter symbol is never used to specify the
symmetry tolerance.
MMC, LMC, tangent plane, and projected
tolerance zone modifiers may not be
specified.
122

Concentricity
Definition Concentricity defines the location of
cylindrical features about an axis of rotation.
Tolerance Zone The tolerance zone is defined
as a cylinder about the datum axis that must
contain the median points of diametrically
opposed elements of a feature. A
123

Checking for Concentricity
124

Proper Application of Concentricity
The surface of revolution to be controlled is
coaxial to the axis of the datum scheme.
Diameter symbol is used to specify the
concentricity tolerance.
tolerance zone modifiers may not be
specified.
125

Decisions for Location Tolerances
Location
Tolerances
PositionConcentricity Symmetry
Center
Plane
Axis
Determine
Tolerance
For Position Only
Consider Material Condition
MMCRFS LMC
127

Profile Tolerances
Profile of a Line
Profile of a Surface
2-D Application
3-D Application
128

Profile Tolerances
Profile tolerances are used to control multiple
coplanar surfaces.
Perfect geometry must be defined via basic
dimensions.
The default interpretation for the tolerance zone is
bilateral and equal about the true perfect geometry.
Profile tolerances are not used to control features of
size so MMC, LMC, and RFS do not apply.
Profile features can be used as datum features or
they must be related to a defined datum scheme.
129

Profile
Definition Profile defines the theoretically
exact position of a surface (3-D) or the cross
section of a surface (2-D).
Tolerance Zone A uniform boundary on either
side of the true profile that must contain
either the surface or line.
3-D Application
130
2-D Application

Profile for Cam Application
131

Functional Gaging of Profile
132

Proper Application
of Profile Tolerances
Profile features are used as datum features or
related to a defined datum scheme.
and
Basic dimensions relate the true profile back
to the datum scheme.
or
The profile tolerance value must be a
refinement of dimensions used to locate the
true profile.
133

Decisions for Profile Tolerances
Profile
Tolerances
Consider
Limits of Size
Profile of a
Surface
Profile of a
Line
Consider
Tolerance Zone
BilateralUnilateral
Inside Outside Equal Unequal
134

Runout Tolerances
Circular Runout
Total Runout
2-D Application
3-D Application
135

Runout
Definition Runout is a composite control used
to specify functional relationships between
part features and a datum axis.
Tolerance Zone Circular runout is a 2-D
application that evaluates full indicator
movement on a perpendicular cross section
rotating about a datum axis. Total runout
evaluates full indicator movement of the full
surface rotating about a datum axis.
3-D Application 2-D Application
136

Proper Application of Runout
The surface to be controlled is either coaxial
or perpendicular to the axis of the datum
scheme.
Diameter symbol is never used to specify a
runout tolerance.
tolerance zone modifiers may not be specified
for a runout tolerance.
138

Decisions for Runout Tolerances
Runout
Tolerances
Consider
Limits of Size
Total
Runout
Circular
Runout
139

Geometric Characteristics
for Round Features
Circularity (roundness)
 Evaluates cross section of surface to its own axis
Cylindricity
 Evaluates entire surface to its own axis
Runout
 Evaluates cross section of surface to a defined axis
Total Runout
 Evaluates entire surface to a defined axis
Concentricity
 Evaluates best fit axis of feature to a defined axis
140

Reference Planes
(The Point of Known Return) Ted Busch, 1962
Define the datum reference frame.
Use of mutually perpendicular planes.
The goal is the replication of measurements.
Immobilize the part in up to six degrees of
freedom.
143

Theoretically Perfect
Geometry
Three mutually perpendicular planes.
Datum
144
Point
3 Datum Planes
define the Origin
of Measurement

Criteria for Selecting Datum Features
Geometric Relationship to Toleranced Feature
Geometric Relationship to Design Requirements
Accessibility of the Feature
Sufficient in Size to be Useful
Readily Discernable on the Part
145

Designating Precedence of Datums
Alphabetical order is not relevant.
Order of precedence is shown in the feature
control frame.
 Consider function first.
 Then, consider the process next.
 Finally, consider measurement processes.
146

Datum Features of Size
MMC callouts on a datum features of size can
allow a datum shift on the exact location of
the datum feature.
This applies to:
 Cylindrical Surfaces (internal or external)
 Spherical Surfaces
 A Set of 2 Opposing Elements or Parallel Planes
 A Pattern of Features such as a Bolt Hole Pattern
147

Decisions for Datum Selection
Select
Datum Feature
Feature
of Size
Surface
Axis
Center
Plane
Consider
Material Condition
MMCRFS LMC
Are Other Datums Required?
148

Rational Strategy
149
for Datum Selection
It is reasonable to prioritize the datum selection
process as follows:
1. Functional Requirements
1. Production Requirements
• Measurement Requirements

What Are We Really Interested In?
150
• Error in Geometric Forms
• Size for Features of Size
• Location of Features

Introduction to Datum Workshop
Select datums based on function.
Some features are leaders, others are followers.
Sequence of considerations:
 Establish the datum reference frame (DRF).
 Qualify the datum features to the DRF.
 Relate remaining features to the DRF.
For consistency, assume .005” tolerance zones unless
otherwise specified.
Select and qualify the datum features and identify the
datum point as specified in the following examples.
151

Datum
• It is atheoretically exact plane from whicha
dimensional measurement is made
• Datum system is aset of symbols which is
used to describe the function anddimensions
to the user
• Datum plane, Datum axis and Datum center
plane are used to make measurements of a
part

Section 5
157
Tolerancing Strategies

Process for Tolerance Analysis
Establish Performance Requirements
Develop a Loop Diagram
Convert Dimensional Requirements to
Target Values with Equal Bilateral Tolerances
Determine the Target Value for Requirement
Select the Method of Analysis
Calculate Variation for Performance Requirement
158

Modifiers and itssymbols
Theyare symbols which defines conditions for thetolerance
zone of ageometric tolerance. There are six modifiers. The
symbols of modifiers and their meaning isshown

FeatureControlFrame
It is arectangular box which is divided into compartments within which
symbols, modifiers, tolerances, etc…are placed. Below is an example for
feature control frame

© Professor Britton 1996-2000
When to use geometrictolerancing
• not needed if dimensional tolerances and the
manufacturing process provide adequate
control
• is needed
– when part features are critical to function or interchangeability
– when errors of shape & form must be held within tighter limits
than normally expected from the manufacturing process
– when functional gauging techniques are to be used
– when datum references are required to ensure consistency
between design, manufacture and verification operations
– when computerization techniques in design and manufacture
are used
Use of GD&T to control form of a part increases cost because ext ra control
is required for the manufacturing processes and additional inspection is
required to ensure compliance. Thus it should only be used when necessary.
In order to apply GD&T properly a designer must know about and
understand the manufacturing processes that will be used to make the
part. All processes provide geometric control to some degree. The decision
to apply GD&T or not depends to a large degree on whether the normal
control provided by a process meets the design functional requirements: if it
can meet requirements do not use GD&T, otherwise use GD&T.
In addition GD&T could be used to ensure consistency of datum references
between design, manufacture and verification operations, and also when
computer tools are used during design and manufacture.

Geometric dimensioning and tolerancing (GD&T)

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