Geometric Dimensioning and
Tolerancing
Chapter 1, Introduction
History
• during the early period of manufacturing there were
seldom any drawings
• one-off manufacturing
• ~1798 – concept of interchange parts
– Eli Whitney 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.
• specifying tolerances became very
important
Eli Whitney 1765 - 1825
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Department of Mechanical and Aerospace Engineering
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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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Department of Mechanical and Aerospace Engineering
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Tolerance Notes
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Department of Mechanical and Aerospace Engineering
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Plus/Minus (Limit) Tolerancing
• Plus/minus tolerancing seemed to work well for many
applications.
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Department of Mechanical and Aerospace Engineering
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Plus/Minus (Limit) Tolerancing
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Department of Mechanical and Aerospace Engineering
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Plus/Minus (Limit) Tolerancing
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Department of Mechanical and Aerospace Engineering
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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
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Department of Mechanical and Aerospace Engineering
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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.
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Department of Mechanical and Aerospace Engineering
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Limit Tolerancing
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Department of Mechanical and Aerospace Engineering
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Limit Tolerancing
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Department of Mechanical and Aerospace Engineering
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Limit Tolerancing
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Department of Mechanical and Aerospace Engineering
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GDT
• Limit tolerancing can give the size of individual features.
• GDT can provide:
– relationships between features
– circular tolerance zone for hole placement
• GDT removes any ambiguity from the drawing
specification
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Department of Mechanical and Aerospace Engineering
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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
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Department of Mechanical and Aerospace Engineering
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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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Department of Mechanical and Aerospace Engineering
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communication
symbols
ASME
clarity
replace
total
form
size & location
function & relationship
tolerance tolerances & interchageability
size
plus/minus
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Department of Mechanical and Aerospace Engineering
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