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Computer Aided Engineering
Applications
6.Geometric Dimensioning and
Tolerancing (GD&T)5.1 Basic tolerancing
5.2 Surface control
Engi 6928 - Fall 2014
5.2 Surface control
5.3 GD&T
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5.1 Basic 5.1 Basic TolerancingTolerancing
An objective of an engineering drawing is to convey
the shape and size of an object.
Dimensioning specifies the required size of features.
Tolerancing specifies the required precision of the
size.
Surface control specifies the required surface finish.
Geometric tolerancing is a specification used to
control the shape of parts
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5.1 Basic 5.1 Basic TolerancingTolerancing
The tolerance determines the manufacturing process
to use.
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5.1 Basic 5.1 Basic TolerancingTolerancing
Cost increases exponentially with the specified
tolerance.
Tighter tolerances should be used only where
necessary for the functionally critical features.
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5.1 Basic 5.1 Basic TolerancingTolerancing
Unilateral tolerance
Bilateral tolerance
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5.1 Basic 5.1 Basic TolerancingTolerancing
MMC maximum material condition
LMC least material condition
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5.1 Basic 5.1 Basic TolerancingTolerancing
Fit refers to how tightly or loosely mating parts must fit
together when assembled
Categories:
Clearance Fit The internal member is always
smaller than the external member so that the parts
will slide together.will slide together.
Interference Fit The internal member is always
larger that the external member so that the parts
must be forced together.
Transition Fit Compromise for applications
where accuracy of location is important but either a
small amount of clearance or interference is
permissible.
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5.1 Basic 5.1 Basic TolerancingTolerancing
The Basic Hole System The
external member (often a hole) is
used as the basic dimension from
which the toleranced dimensions to
achieve the fit are determined.
The Basic Shaft System - Takes The Basic Shaft System - Takes
the size of the shaft as the basic
dimensions from which toleranced
dimensions are derived. Not
commonly used
Allowance The difference
between the smallest hole size and
the largest shaft size.
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5.1 Basic 5.1 Basic TolerancingTolerancing
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5.1 Basic 5.1 Basic TolerancingTolerancing
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5.1 Basic 5.1 Basic TolerancingTolerancing
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5.1 Basic 5.1 Basic TolerancingTolerancing
1. Select basic size 40mm
2. Select shaft or hole basis - Hole
3. Select type of fit - Sliding
4. Select ANSI preferred tolerances
H7/g6
Example
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5.1 Basic 5.1 Basic TolerancingTolerancing
Example
Type of fit MMC Hole MMC Shaft Allowance Loosest fit
A Interference 50 50.1 -.1 -.03
B Transition 49.950 50 -.05 +.15
C Clearance 50 49.95 +.05 +.15
The basic dimension of the hole/shaft is 50 mm
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5.2 Surface control5.2 Surface control
Surface finish is another crucial factor which affects
the cost of parts
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5.2 Surface control5.2 Surface control
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5.2 Surface control5.2 Surface control
Roughness: Measure of the irregularities of the
surfaces finish
Waviness: Measure of the more widely spaced
irregularities of the surface due to warping, machine
vibrations, heat treating or other similar factors
Lay: Direction of the primary surface patternLay: Direction of the primary surface pattern
Roughness Width Cut-Off: Maximum width of surface
irregularities to be included in the measurement of
roughness height
Roughness Width: Maximum allowable spacing
between repetitive units of surface pattern
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5.2 Surface control5.2 Surface control
Perthometer
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5.2 Surface control5.2 Surface control
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5.2 Surface control5.2 Surface control
Example
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5.3 GD&T5.3 GD&T
Uses symbols to specify geometric characteristics
and dimensional requirements
Current GD&T standard: ANSI/ASME Y14.5M-2009
Symbols for:
Form and profile tolerances, orientation
tolerances, location tolerances, runout tolerances
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5.3 GD&T5.3 GD&T
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5.3 GD&T5.3 GD&T
Straightness
The line feature has a tolerance zone which is specified by two
parallel lines spaced 0.002 apart. The tolerance zone is not
necessarily parallel to any datum feature
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5.3 GD&T5.3 GD&T
Flatness
The plane feature has a tolerance zone which is specified by two
parallel planes spaced 0.002 apart.
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5.3 GD&T5.3 GD&T
Parallelism
The line feature has a tolerance zone which is specified by two
parallel lines spaced 0.002 apart. The tolerance zone is parallel to the
Datum feature A.
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5.3 GD&T5.3 GD&T
Circularity
The circle feature has a tolerance zone which is specified by two
concentric circles spaced 0.002 apart. The center is the best fit
center for the profile of the irregular circle.
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5.3 GD&T5.3 GD&T
Cylindricity
The cylindrical face feature has a tolerance zone which is specified by
two concentric cylinders spaced 0.002 apart.
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5.3 GD&T5.3 GD&T
Coordinate Measurement Machines (CMM) allows
automated part inspection
Least squares fit for line, plane, circle and cylinder
Defines the median line of the tolerance zones for
straightness, flatness, circularity and cyclindricity respectively.
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5.3 GD&T5.3 GD&T
Cocentricity
The center of the circle feature has a tolerance zone which is specified
by a circles with diameter 0.005. The tolerance zone is centered with
datum A.
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5.3 GD&T5.3 GD&T
Runout
The circle feature has a tolerance zone which is specified by two
concentric circles spaced 0.005 apart. The tolerance zone is
centered with datum A.
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5.3 GD&T5.3 GD&T
Positional tolerance
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5.3 GD&T5.3 GD&T
Datum preference specifies the order in which the
part is assembled w.r.t a datum reference frame.
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5.3 GD&T5.3 GD&T
Bonus tolerance - This is an extra tolerance zone
applicable to a feature due to the use of feature
frame modifiers.
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5.3 GD&T5.3 GD&T
Example:
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5.3 GD&T5.3 GD&T
Example:
0.02
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ReferencesReferences
ASME Y14.5-2009 Dimensioning and Tolerancing.