GEOMETRIC DIMENSIONING
& TOLERANCING (GD & T)
ME 250: Design & Manufacturing I
School of Mechanical Engineering
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GD & T Outline
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Dimensioning
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Principles
Standards
Tolerancing
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Definitions
Datums
Form Controls
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Working Drawing
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Three orthographic views
Auxiliary views, section views
Dimensions
Special lines/symbols/notations
Tolerances
Title block
Bill of materials
Other notes/labels
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Working Drawing
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Dimensioning
Dimension is used to specify the object
size and its location.
Specify the exact theoretically values for
the size,orientation and location of
features.
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Principles
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Show every necessary dimension.
Never repeat a dimension.
Locate dimensions that are linked to one another with respect to
the same datum.
If possible, avoid crossing two dimension lines.
Arrange a series of dimensions in a continuous line.
Place longer dimensions outside shorter ones so that extension
lines will not cross dimension lines.
Place dimensioning information outside the orthographic views if
possible.
Leaders note dimensions and information.
Fillet are dimensioned with leaders or by an appropriate note.
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Principles
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Standards
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Extension Lines
Dimension Lines
Dimension
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Threads
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Threads are specified by giving the nominal
diameter and threads per inch.
ANSI: ½-13UNC; ½-13UNF
Metric: M8x1.25
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Schematic Symbols
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Simplified symbols
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Chamfer
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Counterbore
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Countersink
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Title Block
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Bill of Material
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GD & T
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Dimensional tolerance is defined as the
permissible or acceptable variation in
the dimension (height, width, length,
angle, etc.) of a part.
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GD & T
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Why do we need the tolerance?
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Increasingly exact dimensions
Interchangeable
Manufacturing costs increase as
tolerances become smaller
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A tolerance should be as lager as possible
without interfering with the function of the
part to minimize production costs.
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GD & T
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In the U.S. GD&T standards are
evaluated, updated and published by
the sub-group Y14.5 of the American
National Standards Institute (ANSI).
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GD & T
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Per ANSI Y14.5 standard symbols are:
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used to define part features and tolerances
on drawings.
interpreted in a consistent way for
manufacturing.
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Definitions
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Feature is a general term applied to a
physical portion of a part, e.g., a surface,
hole, slot, etc.
Feature-of-size is one cylindrical or
spherical surface, or a set of parallel
surfaces, each of which is associated
with a size dimension.
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Definitions
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Location dimension is a dimension
which locates the centerline or
centerplane of a part feature relative to
another part feature, centerline or
centerplane .
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Definitions
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Definitions
Maximum Material Condition(MMC):
When a feature-of-size contains the
most amount of material it is said to be
in its MMC.
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Definitions
Least Material Condition(LMC): When a
feature-of-size contains the minimum
amount of material it is said to be in its
LMC.
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Definitions
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An external feature-of-size is at MMC when it is at its
largest size limit.
An external feature-of-size is at LMC when it is at its
smallest size limit.
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Definitions
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An internal feature-of-size is at MMC when it is at its
smallest size limit.
An internal feature-of-size is at LMC when it is at its
largest size limit.
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Definitions
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Regardless of Feature Size(RFS) is
when a geometric tolerance applies
independent of the feature size.
The geometric tolerance is limited to the
stated amount regardless of the size of
the feature-of-size.
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Terminology
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Basic size: the exact theoretical
size
Actual size: the measured size of
the finished part
Tolerance: the difference between
the limits prescribed for a single
feature
Limits of tolerance: the extreme
measurements permitted by the
maximum and minimum sizes of a
feature
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Fits
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Fit: the tightness between two assembled
mating parts
Clearance fit: the clearance between two
assembled mating parts
Interference fit: results in an interference
between two assembled mating parts
Transition fit: may result in either an
interference or a clearance between two
assembled mating parts
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Fits
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Datums
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A theoretically exact point, line, axis, or
plane which indicates the origin of a
specified dimensional relationship
between a toleranced feature and a
designated feature on a part.
Datum feature is a part feature which
contacts or is used establish a datum.
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Datums
Datums are used to locate a part in a
repeatable manner for checking
geometric tolerances.
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Datums
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Datums
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Datums
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Five categories of ANSI symbols
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Form Controls
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Flatness
Straightness
Circularity
Cylindricity
Does not use any datum surface
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Straightness
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Form Controls
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Each element of the feature controlled
must lie between the stated tolerance
zone. Additionally, each element of the
feature must be within the specified
limits of size.
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Flatness
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Circularity
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Cylindricity
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