unit1_intro

```Geometric Dimensioning and
Tolerancing
Unit 1 – Introduction, Symbols, and Terms
EML 2023
Computer Aided Design
2
Geometric Tolerancing Workbook
Everyone should have a
It contains necessary
reference information along
with class exercises.
Page numbers in yellow on
the slides match pages in
the workbook.
EML 2023
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.
congress by building 10 guns and
assembling and disassembling
them claiming the same exact
parts and mechanisms.
Eli Whitney 1765 - 1825
EML 2023
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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Computer Aided Design
5
Tolerance Notes
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Limit Tolerancing
EML 2023
Computer Aided Design
Limit Tolerancing
EML 2023
Computer Aided Design
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
EML 2023
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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Computer Aided Design
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Dimensioning Symbols
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Dimensioning Symbols
Diameter 
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Dimensioning Symbols
Spherical Diameter S
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Dimensioning Symbols
Number of Places 
‘reversals’, but is a smooth curve
within the tolerance zone.
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Computer Aided Design
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Dimensioning Symbols
Counterbore/Spotface
Countersink
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Computer Aided Design
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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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Computer Aided Design
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Dimensioning Symbols
Square
Reference Dimension
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Computer Aided Design
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Dimensioning Symbols
Dimension Origin
Arc Length
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Computer Aided Design
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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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Computer Aided Design
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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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Computer Aided Design
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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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Computer Aided Design
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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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Computer Aided Design
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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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Computer Aided Design
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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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Computer Aided Design
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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
EML 2023
Computer Aided Design
Common Symbols
units are mm
applied to a Drawing
25
10
18
±1°
1.8
Indexer Assembly
1.17
EML 2023
Computer Aided Design
Indexer Plate
1.17
EML 2023
Computer Aided Design
Workshop Exercise 1.1
12
1.23
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