GEOMETRIC DIMENSIONING
& TOLERANCING (GD & T)
ME 250: Design & Manufacturing I
School of Mechanical Engineering
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Outline
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Tolerance
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Definitions
Datums
Form Controls
Rules for Tolerancing
Location Control
Orientation Controls
Runout Controls
Profile Controls
Surface Finish
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Rules for Tolerancing
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RULE-1: For features-of-size, where
only a size dimension is specified, the
surfaces shall not extend beyond a
boundary (envelope) of perfect form at
MMC.
“perfect form” means perfect flatness,
straightness, circularity, cylindricity.
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Rule-1 for Tolerancing
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Rule-1 for Tolerancing
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Rule-1 applies to all feature-of-size (and only
to feature-of-size) on a drawing.
Rule-1 says that a feature-of-size must have
perfect form if the feature-of-size is at MMC.
If the part was at less than MMC, a form error
equal to the amount of departure would be
permissible.
If the part was at LMC, a form error equal to
the amount of departure would be permissible.
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The Term of RFS
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The Regardless of Feature Size (RFS) is
when a geometric tolerance (or datum)
applies independent of the feature size.
That is, the geometric tolerance is limited
to the stated amount regardless of the
size of the feature-of-size.
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Rules for Tolerancing
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Rules for Tolerancing
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Exercise in Class
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If the block was at MMC, what would
 its straightness be limited to?
 the flatness of surface A be limited to?
If the block was at LMC, the flatness of surface A would be
limited to
and the flatness of surface B would be
limited to
. Could both of these flatness conditions
exist simultaneously? Why?
If the block was at LMC and the flatness of surface A was 0.3,
what would the flatness of surface B would be limited to?
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Location Control
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Boundary for Surface of Hole
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Location Control
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Location Control
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Orientation Controls
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Perpendicularity
Angularity
Parallelism
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Always uses a datum surface
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Perpendicularity
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Perpendicularity
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Runout Controls
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Circular runout
Total runout
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Always uses a datum surface
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Runout Tolerance
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Circular runout
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Total runout
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Profile Controls
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Profile of a line
Profile of a surface
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May uses a datum surface
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Profile of a Surface
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Profile of a Surface
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Surface Finish
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We control the sizes and shape (GD&T)
of parts in order to make them fit.
We control surface textures in order to
make the parts work
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Fatigue
Fiction
Wear
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Surface Roughness
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Surface Roughness refers to the fine
irregularities (scale smaller than that of
waviness) on the surface and is a result
of the manufacturing process used.
It is measured (by surface profilometers)
in terms of height, width and distance
on the surface.
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Surface Finish
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Surface finish impacts:
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How well a surface will withstand pressure, friction
and wear
How well it will hold and disreibute a lubricant
How it will change over use
How it will seat or seal
How it will conduct electricity, reflect light, accept
paint/plating, etc
How it will change by further manufacturing
processes
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Terminology
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Ra (AA): Arithmetic mean/average
Rq (RMS): Root mean square
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Considerations for Surface Roughness
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Reasons/ Considerations for different surface
roughness in design
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Precision requirements on mating surfaces (seals,
fittings, bearings, etc.)
frictional considerations (effect of roughness on
friction, wear, lubrication)
Fatigue and notch sensitivity
Resistance (electrical/thermal contact resistance
corrosion resistance)
Subsequent processing (painting, coating, etc.)
Appearance
Cost
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