Geometric Dimensioning and
Tolerancing
Unit 1 – Introduction, Symbols, and Terms
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Computer Aided Design
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Geometric Tolerancing Workbook
Everyone should have a
workbook to follow along.
It contains necessary
reference information along
with class exercises.
Page numbers in yellow on
the slides match pages in
the workbook.
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Computer Aided Design
History and Background
Eli Whitney, is the inventor of the
cotton gin and a pioneer in the
use of mass production methods.
Around 1798, he won a contract
to supply muskets to the United
States government. The firearms
manufacture were based on the
concept of interchangeable parts.
He made a presentation to
congress by building 10 guns and
assembling and disassembling
them claiming the same exact
parts and mechanisms.
Eli Whitney 1765 - 1825
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Computer Aided Design
Tolerances
• All dimensions require a tolerance.
• A tolerance should be as large as possible
without interfering with the function of the part to
minimize production costs.
• Consider how your part will be checked to see if
it meets the tolerances.
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Tolerance Notes
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Limit Tolerancing
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Limit Tolerancing
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Limit Tolerancing
• Is the .620-.630 hole horizontal position
measured from a true vertical plane or from the
as built face?
• A .005” tolerance on the horizontal and vertical
position of the hole means that the position
could be off by as much as .007”.
max allowed error
for hole center
.007
perfect location
for hole center
+.005
+.005
EML 2023
Computer Aided Design
Limit Tolerancing
• Limit tolerances don’t have an origin or any orientation or
location relative to datums.
• The datums are usually implied.
• The drawings are subject to different interpretations.
• Plus/minus tolerancing works well for individual features
of size (ex. diameter of a shaft), but does not control the
relationship between individual features very well.
EML 2023
Computer Aided Design
History of Dimensioning and
Tolerancing Standards in the USA
Mil Std 8 1950’s
Mil Std 8A
Mil Std 8B
Mil Std 8C-1963
ASA-Y14.5-1957
USASI Y14.5-1966
ANSI Y14.5-1973
ANSI Y14.5M-1982
ASME Y14.5M-1994
ASME Y14.5-2009
1.3
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Computer Aided Design
Introduction
• GDT is a language of symbols
• We will see an overview of the symbols
associated with:
1. Dimensioning Symbols
2. Geometric Characteristics
3. Modifying Symbols
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Dimensioning Symbols
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Dimensioning Symbols
Diameter 
Radius R
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Dimensioning Symbols
Spherical Diameter S
Spherical Radius SR
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Dimensioning Symbols
Controlled Radius Tolerance CR
Number of Places 
A controlled radius has no
‘reversals’, but is a smooth curve
within the tolerance zone.
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Dimensioning Symbols
Counterbore/Spotface
Countersink
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Dimensioning Symbols
Depth/Deep
Dimensions Not to Scale
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Dimensioning Symbols
Conical taper
Slope
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Dimensioning Symbols
Square
Reference Dimension
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Dimensioning Symbols
Dimension Origin
Arc Length
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Dimensioning Symbols
All Around
All Over
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Dimensioning Symbols
Chain Line
Used to identify a limited region to
be treated differently from the rest
of the part.
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Introduction
• We will see an overview of the symbols
associated with:
1. Dimensioning Symbols
2. Geometric Characteristics
3. Modifying Symbols
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Geometric Characteristics
Explained in detail in Chapters 6 and 8.
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Introduction
• We will see an overview of the symbols
associated with:
1. Dimensioning Symbols
2. Geometric Characteristics
3. Modifying Symbols
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Modifying Symbols
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Modifying Symbols
Maximum Material Condition
MMC
MMC
Refers to the size of the feature
associated with the most material.
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Modifying Symbols
Least Material Condition
LMC
LMC
Refers to the size of the feature
associated with the least material.
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Modifying Symbols
Continuous Feature
Basic Dimension
Used to describe a theoretically
exact size.
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Modifying Symbols
Between
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Geometric Dimensioning and
Tolerancing (GDT)
1. establish a reference coordinate system by
defining datums
2. provide basic dimensions (perfect dimensions)
relative to the datums
3. specify allowable tolerances
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Computer Aided Design
Common Symbols
units are mm
applied to a Drawing
25
10
18
±1°
1.8
Indexer Assembly
1.17
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Indexer Plate
1.17
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Workshop Exercise 1.1
12
1.23