Print Reading for IndustryBRX 210 – Module 1Unit 13 – Geometric Dimensioning and Tolerancing (GD&T)

Geometric Dimensioningand TolerancingGD&T is an advanced system of print annotation…

That specifies the geometry of parts (circular)

Control symbols

Datum identifiers

Feature control frames

Modifiers

Learning ObjectivesDescribe 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.

Learning Objectives

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.

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.

The “Boxes” of GD&T• Basic dimension

The “Boxes” of GD&T• Datum identification symbol and feature

control frame

The “Boxes” of GD&T• Feature control frame

GD&T Controls• There are 14 geometric controls available

Datum Identification• Datum identification symbols specify parts

of the object to be used as references

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

Basic Dimensions

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

Feature Control Frames

Datum Reference Framework• A datum reference framework is necessary

for checking certain geometric characteristics

Datum Targets• Datum targets can be used as a more

precise way of establishing a datum plane or axis

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

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

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

Straightness• Regardless of any datum surface,

elements of the feature must be straight

Flatness• Regardless of any datum surface,

elements of the feature must fall between two planes (must be flat)

Roundness (Circularity)• Regardless of any datum reference,

elements of the feature at any given cross section must be round

Cylindricity• Regardless of any datum reference,

elements of the feature must be cylindrical

Parallelism• With respect to a datum reference,

elements of the feature must be parallel to the datum

Perpendicularity• With respect to a datum reference,

elements of the feature must be perpendicular to the datum

Angularity• With respect to a datum reference,

elements of the feature must be oriented at a specified angle to the datum

Profile of a Line• With respect to datum references, 2D

elements of the feature must fall within a specified profile zone

Profile of a Surface• With respect to datum references, 3D

elements of the feature must fall within a specified profile zone

Circular Runout• With respect to a datum axis, circular

elements of the feature must not exceed the specified dial indicator movement

Total Runout• With respect to a datum axis, all elements

of the total feature must not exceed the specified dial indicator movement

True Position• With respect to a datum reference

framework, all features must be located within specified tolerance zones

Symmetry• With respect to a datum center plane, all

features are symmetrical (balanced on each side), regardless of feature size

Concentricity• With respect to a datum center axis, all

features are balanced about the axis, regardless of feature size

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

Composite Tolerancing

• What is the objective of GD&T?

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

• What does the symbol mean?

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

• What does an encircled M mean?

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

• 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.

• 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.

• 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.

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

An axis datum

• What is the purpose of a basic dimension?

It is considered theoretically exact

• 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.

• 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.