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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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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