Fundamentals of GD&T-1.pdf

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 M.S Ramaiah School of Advanced Studies - Bengaluru
K.N. Ganapathi
Asst Professor
Mechanical and Manufacturing Engineering
MSRSAS
Fundamentals of Geometric
Dimensioning and Tolerancing
(Basic GD&T)
Based on ASME Y14.5M

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Session Objectives
At the end of the session delegates should
have understand
• Introduction and need for GD&T
• Terms and definitions
• Symbols and rules of GD&T
• Datum and datum reference frame
• Concept of bonus tolerance

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Types of Tolerances
General
General Tolerances apply to all dimensions on
a drawing.
Linear
Linear Tolerances refer to specific features
that require more accuracy than general
tolerances provide.
Geometric Geometric Tolerances are concerned with a
feature’s shape or profile, not its size or
dimensions.

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What is an Engineering Drawing?
• An engineering drawing is a document that
communicates a precise description of a part. This
description consists of pictures, words, numbers and
symbols. Together these elements communicate part
information to all drawing users
• Engineering drawing information includes
– Geometry (shape, size and form of the part)
– Critical functional relationships
– Tolerances allowed for proper function
– Material, heat treat, surface coatings
– Part documentation information (part number, revision
level)

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The engineering drawing is the
specification for the component or
assembly and is an important
contractual document with many legal
implications, every line and every
comment is important
What is an Engineering Drawing?

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Consequences of Poor Drawing
• Drawing errors
cost the
organization in
four ways
– Money
– Time
– Material
– Unhappy
customers

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Dimensioning can be divided into three categories:
•general dimensioning,
•geometric dimensioning, and
•surface texture.
The following provides information necessary to
begin to understand geometric dimensioning and
tolerancing (GD&T)
Three Categories of Dimensioning

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1935: “American Drawing and Drafting Room Practices”,
18 pages including 2 paragraphs on tolerancing
WWII: High scrap rate (lack of full information and +/- system)
Positional tolerance system (round tolerance zones)
“Dimensional Analysis of Engineering Design”
1940: Draftsman’s Handbook (Chevrolet division)
1945: Ordnance Manual on Dimensioning and Tolerancing,
U.S.Army (used symbols rather than notes)
1946: “SAE Aeronautical Drafting Manual”
(Automotive version in 1952)
1949/53: MIL-STD-8/ MIL-STD-8A (7 basic symbols used)
1966: ANSI Y14.5 (updated in ‘73 replacing notes with symbols)
Geometric Dimensioning & Tolerancing

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Drawing that does not use GD&T
Why GD&T?

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Manufactured part
that conforms to
the drawing
(previous slide)
without GD&T

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Using English to control part features

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Drawing that uses GD&T gives no room for ambiguity and
is very precise and unique

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What is GD&T?
• ASME Y14.5M-1994 GD&T is a language of
symbols used on mechanical drawings to efficiently
and accurately communicate geometry requirements
for features on parts and assemblies.
• GD&T, both ASME Y14.5M-1994 and ISO 8015
series are the only recognized international drawing
standards in use throughout the world.
• GD&T is the language that designers use to translate
design requirements into measurable specifications.

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The Geometric Dimensioning and Tolerancing
System
• Geometric Dimensioning and Tolerancing (GD&T) is an
international language that is used on engineering drawings
to accurately describe a part
• GD&T language consists of a well-defined set of symbols,
rules, definitions and conventions
• GD&T is a precise mathematical language that can be used
to describe the size, form, orientation and location of part
features
• G D & T is an exact language that enables designers to
“say what they mean” through a drawing.
• GD&T is also a design philosophy on how to design and
dimension parts

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GD&T is a means of dimensioning &
tolerancing a drawing which considers the
function of the part and how this part
functions with related parts
– This allows a drawing to contain a more
defined feature more accurately, without
increasing tolerances

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• GD&T has increased in practice in last 15 years
because of ISO 9000
- ISO 9000 requires not only that something be required, but
how it is to be controlled. For example, how round does a
round feature have to be?
• GD&T is a system that uses standard symbols to
indicate tolerances that are based on the
feature’s geometry
– Sometimes called feature based dimensioning &
tolerancing or true position dimensioning & tolerancing
• GD&T practices are specified in ANSI / ASME
Y14.5M-1994

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WHEN TO USE GD&T
• When part features are critical to a function or interchangeability
• When functional gauging is desirable
• When datum references are desirable to insure consistency
between design
• When standard interpretation or tolerance is not already implied
• When it allows a better choice of machining processes to be
made for production of a part

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GD&T Benefits
• GD&T provides better product design
• GD&T increases tolerances with cylindrical
tolerance zones
• GD&T allows additional (bonus) tolerances
• GD&T allows the designer to communicate
more clearly
• GD&T eliminates confusion at inspection

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How Does GD&T Work?
• Identify part surfaces to serve as origins and provide
specific rules explaining how these surfaces establish
the starting point and direction for measurements.
• Convey the nominal (ideal) distances and orientations
from origins to other surfaces.
• Establish boundaries and/or tolerance zones for
specific attributes of each surface along with specific
rules for conformance.
• Allow dynamic interaction between tolerances
(simulating actual assembly possibilities) where
appropriate to maximize tolerances.

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Tolerancing systems
1. Coordinate Tolerancing system
2. Geometrical Dimensioning and
Tolerancing system

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Coordinate Tolerancing System
• Coordinate Tolerancing is a dimensioning system
where a part feature is located (or defined) by means
of rectangular dimensions with given tolerances
Coordinate Tolerancing
has three shortcomings
1. Square or rectangular
tolerance zones
2. Fixed-size tolerance zones
3. Ambiguous instructions for
inspection

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Square or Rectangular Tolerance zone
• The hole can be off its nominal location in the diagonal
direction a greater distance than in the vertical and horizontal
direction
• A more logical and function approach is to allow the same
tolerance for a hole location in all directions, creating a
cylindrical tolerance zone

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Square or Rectangular Tolerance zone

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ROUND TOLERANCE ZONE
Rectangular Tolerance Zone Circular Tolerance Zone
0.707
+/- 0.25
+/- 0.25
57% Larger
Tolerance Zone
Circular Tolerance Zone
Rectangular Tolerance Zone

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COORDINATE V/s ROUND
TOLERANCE ZONE

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Fixed size tolerance zone
Tolerance for distance between two hole is
fixed irrespective of size of hole.

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Ambiguous Inspection

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Notes required to make coordinate dimensional
equivalent to GD&T Drawing

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Comparison between GD&T and
Coordinate Tolerancing

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Coordinate Tolerancing Vs. GD&T
Drawing
concept
Coordinate Tolerancing Geometric Tolerancing
Tolerance
zone shape
Condition
Square or rectangular tol zones for hole locations
Results
Less tolerance available for hole
Higher manufacturing costs
Condition
Can use diameter symbol to allow round
tol zones
Results
57% more tol for hole location
Lower manufacturing costs
Tolerance
zone
flexibility
Condition
Tol zone is fixed in size
Results
Functional parts scrapped
Higher operating costs
Condition
Use of MMC modifier allows tol zones to
increase under certain conditions
Results
Functional parts used
Lower operating costs
Ease of
inspection
Condition
Implied datum allows choices for set up when
inspecting the part
Results
Multiple inspectors may get different results
Good parts scrapped, Bad parts accepted
Condition
The datum system communicates one set
up for inspection
Results
Clear instructions for inspection
Eliminates disputes over part acceptance

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Introduction to Geometric
Tolerancing Symbols

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Symbols
• Anyone, regardless of his or her native tongue, can
read and write symbols.
• Symbols mean exactly the same thing to everyone.
• Symbols are so compact they can be placed close to
where they apply, and they reduce clutter.
• Symbols are quicker to draw and easier for computers
to draw automatically.
• Symbols are easier to spot visually.

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Geometric Characteristic Symbols
• Geometric Characteristic Symbols are set
of fourteen symbols used in the language
of geometric tolerancing

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Geometric Characteristic Symbols

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Modifiers
• Modifiers communicate additional information about the
drawing or tolerancing of a part
• There are eight modifiers used in geometric tolerancing

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Introduction to Geometric
Tolerancing Terms

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Maximum Material Condition (MMC)
The condition in which a feature of size contains
the maximum amount of material within the
stated limits of size
for example, minimum hole diameter, maximum
shaft diameter.
.255
.250 + .005
.245
.250 + .005

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Maximum Material Condition

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Least Material Condition (LMC)
The condition in which a feature of size contains
the least amount of material within the stated limits
of size
for example, minimum shaft diameter, maximum
hole diameter
.245
.250 + .005
.255
.250 + .005

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Maximum Material Condition (MMC)
Least Material Condition (LMC)

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Use the figure to fill the value of the MMC and LMC for each
dimension (or indicate, does not apply).
Review Exercise

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Feature
– The general term applied to a physical portion of a
part, such as a surface, pin, tab, hole, or slot

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• Feature of Size (FOS)
– One cylindrical or spherical surface, or a set of two
opposed elements or opposed parallel surfaces,
associated with a size dimension. An axis, median
plane or center point can be derived from a feature
of size

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Using the figure, indicate if each letter is associated with a feature
of size dimension or a non-feature of size dimension.
Review Exercise

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• Regardless of Feature Size (RFS)
– The term used to indicate that a geometric
tolerance or datum reference applies at any
increment of size of the feature within its size
tolerance.

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Tolerance Zone on RFS Basis
(Straightness of Axis)
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05

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Tolerance Zone on MMC Basis
(Straightness of Axis)
0.11

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Bonus Tolerance
• Bonus tolerance is an additional tolerance for a
geometric control
• Bonus tolerance is only permissible when an
MMC (or LMC) modifier is shown in the
tolerance portion of a feature control frame
• Bonus tolerance comes from the FOS tolerance
• Bonus tolerance is the amount the actual
mating size departs from MMC (or LMC)

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Bonus Tolerance Examples

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Bonus Tolerance

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Radius and Controlled Radius
• A radius is a straight line extending from the center of an arc
or a circle to its surface
• When “R” symbol is specified, flats or reversals are allowed
• When “CR” symbol is specified, flats or reversals are not
allowed

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Controlled radius example
CR should only be used in special cases for eg: when the part
stresses are very high and reversals in the radiused surface would
produce higher additional stresses

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Actual Local Size and Actual Mating Envelope
(AME)
• Actual Local Size is the value of any individual
distance at any cross section of a FOS
• Actual Mating Envelope (AME) is a variable
value, derived from an actual part
– For an external feature, the actual mating envelope is
the smallest perfect feature counterpart that can be
circumscribed about the feature
– For an internal feature, the actual mating envelope is
the largest perfect feature counterpart that can be
inscribed within the feature

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Actual Mating Envelope
Internal Feature
External Feature

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Unconstrained Actual Mating Envelope for an external feature
such as a bent cylinder is shown below:
Actual Mating Envelope

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Actual mating envelope of an external feature of size

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Actual mating envelope of an internal feature of size

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Feature control frame

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THE
GEOMETRIC SYMBOL
TOLERANCE INFORMATION
DATUM REFERENCES
FEATURE CONTROL FRAME
COMPARTMENT VARIABLES
CONNECTING WORDS
MUST BE WITHIN
OF THE FEATURE
RELATIVE TO
Feature Control Frame

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Reads as: The position of the feature must be within
a .003 diametrical tolerance zone at maximum material
condition relative to datums A, B, and C.
Uses feature
control frames
to indicate
tolerance

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Reads as: The position of the feature must be
within a .003 diametrical tolerance zone at
maximum material condition relative to datums
A at maximum material condition and B.
Uses feature
control frames
to indicate
tolerance

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Placement of Feature Control Frames
 May be attached to a side, end or corner of the symbol
box to an extension line.
 Applied to surface.
 Applied to axis

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Placement of Feature Control FramesCont’d.
 May be below or closely adjacent to
the dimension or note pertaining to
that feature.
Ø .500 .005

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Basic Dimensions
• Basic Dimensions
– can be used to define the theoretically exact location, orientation or true
profile of part features or gage information
– that define part features must be accompanied by a geometric tolerance
– that define gage information do not have a tolerance shown on the print

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Basic Dimension
• A theoretically exact size, profile, orientation, or
location of a feature or datum target, therefore, a basic
dimension is untoleranced
• Most often used with position, angularity, and profile
• Basic dimensions have a rectangle surrounding it.
1.000

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Basic Dimension cont’d.

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Basic dimension example

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Rules and Concepts of GD&T

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Geometric Tolerance Rule
RULE – 1 (Limits of Size Rule):
Where only a size dimension is given
a) The size dimensions at any cross section must be
within the size tolerance.
b) The surface(s) shall not extend beyond the perfect
form defined by the MMC Size.
c) The form may vary within an envelope between the
MMC and LMC.
RULE – 2
Geometric tolerances are understood to be applied RFS. If MMC or
LMC is required, it must be placed in the feature control frame.

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Rule#1
• There are two general rules in ASME Y14.5M-
1994. The first rule establishes default
conditions for features of size. The second rule
establishes a default material conditions for
feature control frames
• Rule#1: For features of size, where only
tolerance of size is specified, the surfaces shall
not extend beyond a boundary (envelope) of
perfect form at MMC

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Rule#1 Examples

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Rule#1 Examples

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• How to Override Rule#1
– A straightness control applied to a FOS
– A special note applied to a FOS
• Rule#1 Limitation
– Rule#1 does not control the location, orientation or
relationship between features of size

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Rule#2
• Rule#2 is called “the all applicable geometric
tolerance rule”
• Rule#2: RFS applies, with respect to the
individual tolerance, datum reference or both,
where no modifying symbol is specified.
MMC or LMC must be specified on the
drawing where required

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Sl
no
Size Form
tolerance
Bonus
tolerance
Total
tolerance
1 11
2 10
3 9
Interpretation
Φ10 +/-1
No form tolerance

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Sl
no
Size Form
tolerance
Bonus
tolerance
Total
tolerance
1 11
2 10
3 9
Interpretation
Φ10 +/-1
--- 0.2
Form tolerance-FCF-
attached to Feature

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Sl
no
Size Form
tolerance
Bonus
tolerance
Total
tolerance
1 11
2 10
3 9
Interpretation
Φ10 +/-1
--- 0.2
Form tolerance-FCF-
attached to FOS.
RFS situation

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Sl
no
Size Form
tolerance
Bonus
tolerance
Total
tolerance
1 11
2 10
3 9
Interpretation
Φ10 +/-1
--- 0.2
Form tolerance-FCF-
attached to FOS with
MMC modifier
M

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Sl
no
Size Form
tolerance
Bonus
tolerance
Total
tolerance
1 11
2 10
3 9
Interpretation
Φ10 +/-1
--- 0.2
Form tolerance-FCF-
attached to FOS with
LMC modifier
L

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Datum (Planar)

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Introduction
• The datum system is a set of symbols and rules
that communicates to the drawing user how
dimensional measurements are to be made
– Datum system allows the designer to specify in
which sequence the part is to contact the inspection
equipment for the measurement of a dimension
– Datum system allows the designer to specify
which part surfaces are to contact the inspection
equipment for the measurement of a dimension

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• Datum system benefits
– It aids in making repeatable dimensional
measurements
– It aids in communicating part functional
relationships
– It aids in making the dimensional measurement as
intended by the designer

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Implied Datum
• An implied datum is an assumed plane axis or
point from which a dimensional measurement
is made and it is an old concept from
coordinate tolerancing
• Consequences of implied datum
– Good parts are rejected
– Bad parts are accepted

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Planar Datum
• A datum is a theoretically exact plane, point or
axis from which a dimensional measurement is
made
• A datum feature is a part feature that contacts a
datum
• A planar datum is the true geometric counterpart
of a planar datum feature
• A true geometric counterpart is the theoretical
perfect boundary or best fit tangent plane of a
specified datum feature

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Planar Datum

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Datum Features and Datum
• Datum features are part features and they exist
on the part
• A datum feature simulator is the inspection
equipment that includes the gage elements
used to establish a simulated datum
• Datum are theoretical reference planes or axis
and are simulated by the inspection equipment
• For practical purpose, a simulated datum is
considered a datum

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Datum Feature Symbol
• The method of attaching this symbol to a part
feature determines if it designates a planar or a
FOS datum

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Referencing Datum in Feature Control
Frames

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Datum Reference Frame

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Datum Reference Frame

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Datum Features-Example

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Primary, Secondary and Tertiary Datum

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3-2-1 Rule
• The 3-2-1 rule defines the minimum points of
contact with the primary datum as 3, the
secondary datum as 2 and the tertiary datum as 1

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Coplanar Datum Features

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Datum (Axis and Centerplane)

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• When a FOS is used as a datum feature, it
usually results in an axis or a centerplane as
the datum
• When diameter is used as a datum feature, it
results in a datum axis
• When a planar FOS is used as a datum feature,
it results in a datum centerplane

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Placement of datum feature symbols on
Features of Size

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Datum Targets
• Datum targets are symbols that describe the
shape, size and location of gage elements that
are used to establish datum planes, axis and
points

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Datum Target Symbols

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When to use Datum Targets
• Datum targets should be used whenever
– It is not practical to use the entire surface as a
datum plane
– The designer suspects the part may rock or wobble
when the datum feature contacts the datum plane
– Only a portion of the feature is used in he function
of the part

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Datum target points

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When to use RFS, MMC and LMC ??
• RFS
1. Applied only to those features with an axis or median plane
2. Suitable for dynamic assembly
3. RFS can be applied to interference fit and transitional fit
• MMC
2. Suitable for static assembly
3. MMC can be applied to clearance fit only
• LMC
2. Guarantee minimum wall thickness

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LMC guarantees minimum Wall
thickness

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When to use RFS, MMC and LMC ??

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Virtual Condition and Boundary Conditions
• Virtual Condition (VC) is a worst-case
boundary generated by the collective effects
of a feature of size at MMC or at LMC and
the geometric tolerance for that material
condition.
• The VC of a FOS includes effects of the
size, orientation and location for the FOS.
The VC boundary is related to the datums
that are referenced in the geometric
tolerance used to determine the VC

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• Inner Boundary (IB) is a worst-case boundary
generated by the smallest feature of size minus the
stated geometric tolerance (and any additional
tolerance, if applicable)
• Outer boundary (OB) is a worst-case boundary
generated by the largest feature of size plus the stated
geometric tolerance (and any additional tolerance, if
applicable)
• Worst-case Boundary (WCB) is a general term to
refer to the extreme boundary of a FOS that is worst-
case for assembly. Depending upon the part
dimensioning, a worst-case boundary can be VC, IB
or OB

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Feature Control Frame Placement

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MMC Virtual Condition
• VC= MMC + Geometric Tol in the case of
external FOS such as shaft or pin
• VC= MMC - Geometric Tol in the case of
internal FOS such as hole

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MMC VC Examples

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LMC Virtual Condition
• VC= LMC - Geometric Tol in the case of
• VC= LMC + Geometric Tol in the case of

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LMC VC Examples

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RFS Inner and Outer Boundary
• OB= MMC + Geometric Tol in the case of
• IB= MMC - Geometric Tol in the case of

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RFS Inner and Outer Boundary Examples ???

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Worst-Case Boundary Formulas

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Identify Features of Size (FOS) and determine their MMC and WCB size values
Review Exercise

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Summary
• Introduction and need for GD&T
• Terms and definitions
• Symbols and rules of GD&T
• Datum and datum reference frame
• Concept of bonus tolerance
have been studied
Thank You

Fundamentals of GD&T-1.pdf

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