13 Geometric Dimensioning and Tolerancing Permission granted to reproduce for educational use only. Describe the purpose and objectives of geometric.

13Geometric

Dimensioning and Tolerancing

Permission granted to reproduce for educational use only.

• Describe the purpose and objectives of geometric dimensioning and tolerancing (GD&T).

• Identify current and former ASME Y14.5 symbols used in GD&T.

• Define terms related to GD&T.• Explain the purpose and function of datums.• Identify proper datum identification techniques on

a print.

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• Explain basic dimensions as featured in drawings that use GD&T methods.

• Read and interpret the use of modifiers as they apply to basic GD&T applications.

• Read and interpret basic applications of feature control frames for each of the GD&T control symbols.

• Explain how composite tolerances are applied in a basic GD&T application.

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Geometric Dimensioningand Tolerancing

• GD&T is an advanced system of print annotation that specifies the geometry of parts with control symbols, feature control frames, basic dimensions, datum identifiers, and modifiers

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The “Boxes” of GD&T

• Basic dimension

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The “Boxes” of GD&T

• Datum identification symbol and feature control frame

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The “Boxes” of GD&T

• Feature control frame

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GD&T Controls

• There are 14 geometric controls available

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Datum Identification

• Datum identification symbols specify parts of the object to be used as references

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Basic Dimensions

• Basic dimensions are indicated by a value inside of a box

• They are theoretically exact• The tolerance is usually found in a feature control

frame• Tolerance may also be determined by a gage

maker or machinist

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Basic Dimensions

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Feature Control Frames

• Feature control frames may be rather simple, but may also be complex

• They may include up to three datum references• They may also include material condition modifiers

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Feature Control Frames

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Datum Reference Framework

• A datum reference framework is necessary for checking certain geometric characteristics

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Datum Targets

• Datum targets can be used as a more precise way of establishing a datum plane or axis

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Additional GD&T Concepts

• Size feature: Feature of the object that has a center axis or center plane, such as a hole or slot

• Material condition modifiers: Size features can be modified for bonus tolerance based on a feature’s departure from MMC or LMC

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Additional GD&T Concepts

• MMC: Maximum material condition• LMC: Least material condition• Bonus tolerance: Additional tolerance received

based on a feature’s material condition; also available as datum features vary

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Bonus Tolerance Applied

• In these examples, a bonus tolerance is given as the controlled-size feature departs from MMC

• The datum setup is also flexible, as long as the setup is within the datum feature MMC

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Straightness

• Regardless of any datum surface, elements of the feature must be straight

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Flatness

• Regardless of any datum surface, elements of the feature must fall between two planes (must be flat)

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Roundness (Circularity)

• Regardless of any datum reference, elements of the feature at any given cross section must be round

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Cylindricity

• Regardless of any datum reference, elements of the feature must be cylindrical

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Parallelism

• With respect to a datum reference, elements of the feature must be parallel to the datum

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Perpendicularity

• With respect to a datum reference, elements of the feature must be perpendicular to the datum

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Angularity

• With respect to a datum reference, elements of the feature must be oriented at a specified angle to the datum

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Profile of a Line

• With respect to datum references, 2D elements of the feature must fall within a specified profile zone

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Profile of a Surface

• With respect to datum references, 3D elements of the feature must fall within a specified profile zone

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Circular Runout

• With respect to a datum axis, circular elements of the feature must not exceed the specified dial indicator movement

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Total Runout

• With respect to a datum axis, all elements of the total feature must not exceed the specified dial indicator movement

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True Position

• With respect to a datum reference framework, all features must be located within specified tolerance zones

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Symmetry

• With respect to a datum center plane, all features are symmetrical (balanced on each side), regardless of feature size

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Concentricity

• With respect to a datum center axis, all features are balanced about the axis, regardless of feature size

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Composite Tolerancing

• A pattern of holes may have a composite tolerance• This allows the pattern to be more tightly

controlled with respect to its members than to the general datum reference framework

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Composite Tolerancing

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• What is the objective of GD&T?

To control the quality of mass-produced parts by specifying the preciseness of the geometry.

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• What does the symbol mean?

It specifies perpendicularity, which means the feature must be perpendicular to the specified datum within the given tolerance.

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• What does an encircled M mean?

It specifies the condition must exist at the maximum material condition.

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• What symbol is used to specify regardless of feature size?

In current standards, there is no symbol for RFS because it is assumed unless otherwise specified. In older standards, RFS was indicated by an encircled S.

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• What is a size feature and how does it differ from a feature?

A feature is a physical portion of a part. A size feature is simply a feature with a center plane or center axis.

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• What are datums and what is the datum reference framework?

Datums are points, axes, or planes assumed to be exact for the purpose of references. The datum reference framework consists of three mutually perpendicular planes that represent features important to the design of the part.

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• If a datum symbol appears on the leader dimensioning the diameter of a hole, what type of datum is specified?

An axis datum

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• What is the purpose of a basic dimension?

It is considered theoretically exact

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• If flatness of .003 is specified for a surface, what does this mean?

All elements of the surface must fall within a zone defined by two parallel planes .003 apart.

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• How is a composite tolerance identified on a drawing?

The feature control frame has two layers, one below the other, usually with a single geometric characteristic specified.