1. GEOMETRIC DIMENSIONING & TOLERANCING (GD & T)
ME 250: Design & Manufacturing I
School of Mechanical Engineering
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GD & T Outline
Dimensioning
Principles
Standards
Tolerancing
Definitions
Datums
Form Controls
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Working Drawing
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
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
Extension Lines
Dimension Lines
Dimension
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Threads
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
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
Why do we need the tolerance?
Increasingly exact dimensions
Interchangeable
Manufacturing costs increase as tolerances become smaller
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
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
Per ANSI Y14.5 standard symbols are:
used to define part features and tolerances on drawings.
interpreted in a consistent way for manufacturing.
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Definitions
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
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
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
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
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
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
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
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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37. Five categories of ANSI symbols
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Form Controls
Flatness
Straightness
Circularity
Cylindricity
Does not use any datum surface
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Straightness
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Form Controls
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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