Introduction:

Geometric dimensioning and tolerancing (GD&T) is

a method for stating and interpreting design

requirements.

It is an international system of symbolic language,

and is a tool to make engineering drawings a better

means of communication from design through

manufacturing and inspection.

Historic Background:

•American Military standard

•American Standards Association ASA Y 14.5 -

1956

• ANSI - Complete system of Symbology for

geometric form & positional tolerances Y 14.5M -

1983 “Dimensioning & Tolerancing”

• ASME Released Y14.5-1994 (A little closer to

ISO)(1995 Latest). The latest issue of GD&T symbology is Y14.5-2009 released in 2009.

.

GD&T:

•Standardized Method for stating and

interpreting design requirements.

•International system of symbolic language

•Makes engineering drawings a better means

of communication from design through

manufacturing and inspection.

GD&T - Advantages

•GD&T symbols, provide a means of completely specifying uniformity and describing the designer's intent.

•Symbols eliminate most of the drawing misinterpretation by not having notes in drawing margins and by not having complete textual descriptions of features and design requirements.

GD&T - Advantages

• Complete specification of design requirements possible with symbols

•Symbols allow the designer to specify maximum tolerances for parts that must assemble with other parts. These maximum tolerances also ensure the interchangeability of parts.

•The use of symbols for complete specification is becoming increasingly important day-by-day, around the world.

GD&T – Advantages

•Two key principles for applying GD&T

•Functional aspect.

•Relationship of parts in an assembly.

GD&T - Advantages•Uniformity in design practice

•Fewer misinterpretations

•Interchangeability Ensured

•Design requirements specified explicitly

•Latest gaging techniques accommodated

•Lower production costs

•Maximum tolerance allocation

•Greater flexibility in manufacturing

•Higher production yields

•Less rework or scrap

GD&T - Advantages

• Protects the part function by simply shifting to GD & T, we get 57% additional tolerance

• Material condition. Permits bonus tolerance

• Prevents accumulation of tolerance

• Gauge design becomes simple

• Increases productivity & reduces the cost of product

• Eliminates confusion at inspection

Things That can Go Wrong

EXAMPLE

A component is submitted for One-Off Sample approval & found not to perform properly with its interacting components.

WHY ?

1) The component was not made to drawing because:-

a) The Supplier made a mistake

b) The Supplier Mis-interpreted the Drawing

Example (cont’d)

2) The component was made to the Drawing BUT:-

a) The Engineer/Draughtsman made a mistake

b) The Engineer/Draughtsman put INCORRECTinformation on the Drawing because he/she did not understand fully the FUNCTIONAL

RELATIONSHIP with its interacting components.

The above example illustrates issues that can affect effective operations within the Company.

Why should we be so interested in this subject?

BECAUSE, ITS USE SAVES MONEY!

� By providing for maximum producibility of the part through maximum production tolerances.

� It provides “bonus” or extra tolerances in many cases.

FeaturesFeatures

FEATURES

• A feature is a general term applied to a physical portion of part, such as a surface, hole or slots.

• An easy way to remember this term is to think of a feature as a part surface.

FEATURES

FEATURE OF SIZE

• This is one cylindrical or spherical surface, or set of two opposed elements or parallel surfacesassociated with size dimension which has an axis, center line or center plane contained within it.

• Features of size are features, which do have diameter or thickness.

• These may be shafts / holes / slots / projections.

How many feature of size are there?

FEATURE OF SIZE NON FEATURE OF SIZE

FEATURE CONTROL FEATURE CONTROL

FRAMESFRAMES

EXAMPLE

A feature control frame that was specified to

control the position of a feature or group of

features is illustrated below

THE

GEOMETRIC SYMBOL

TOLERANCE INFORMATION

DATUM REFERENCES

FEATURE CONTROL FRAME

COMPARTMENT VARIABLES

CONNECTING WORDS

MUST BE WITHINOF THE FEATURE

RELATIVE TO

Feature Control FrameFeature Control Frame

Feature Control Frame

• Reads as: The position of the feature must be within a .003 diametrical tolerance zone at maximum material condition relative to datums A, B, and C.

May be below or closely adjacent to the dimension or note pertaining to that feature.

Ø .500±.005

DATUMDATUM

DatumDatum

A datum is a starting place for a dimension

WHAT IS A DATUM?•Theoretically exact point / line / axis / area /

surface / that is used as an origin for dimensions.

•These regions are considered perfect for

orientation purposes only.

•During machining processes, the part is resting

against a theoretically perfect or exact datum

surface where features of the part have been

identified as datum features

DATUM FEATURE SYMBOL:

•Square box that containing a capital block letter

with a leader connecting it to the feature with a

triangle.

•The triangle may be filled or not filled.

•Figure 3-4 shows a drawing illustrating the proper

symbol and attachment.

Datum Reference SymbolsDatum Reference Symbols

The datum feature symbol identifies a surface or feature of size as a datum.

A

ISO

A

ANSI1982

ASME

A

1994

Datum Symbol:•Any of the letter of the alphabet may be used

except for I, O and Q which may be confused with

numbers.

•The letters may be used in any order because

alphabetical order is not meaningful.

•The important mental distinction that must

be made is that a datum is theoretically perfect

whereas the datum feature itself is imperfect.

Datum

• Datum's are features (points, axis, and planes) on the object that are used as reference surfaces from which other measurements are made. Used in designing, tooling, manufacturing, inspecting, and assembling components and sub-assemblies.

• Not every GD&T feature requires a datum.

1.000

Datums cont’d.

• Features are identified with respect to a datum.

• Always start with the letter A

• Do not use letters I, O, or Q

• May use double letters AA, BB, etc.

• This information is located in the feature control frame.

Placement of Datum's

• Datum's are generally placed on a feature, a centerline, or a plane depending on how dimensions need to be referenced.

A AOR

ASME 1994

A

ANSI 1982

Line up with arrow only when the feature is a feature of size and is being defined as the datum

Three- Plane Concept-Flat:•Theoretical datum planes or surfaces are

established from a perfect three-plane reference

frame.

•This frame is assumed to be perfect with each

plane oriented exactly 90deg to each other, referred

to as the datum reference frame.

•This reference frame, with mutually perpendicular

planes, provides the origin and orientation for all

measurements.

Datum Plane

• Positioning the part with relation to three mutually

perpendicular planes.

5.4

Example Datum's

• Datum's must be perpendicular to each other

–Primary

–Secondary

–Tertiary Datum

FIRST DATUM ESTABLISHEDBY THREE POINTS (MIN)CONTACT WITH SIMULATEDDATUM A

Primary DatumPrimary Datum

PRIMARY DATUM PLANE:

•The primary datum feature must make contact

with the theoretically exact datum plane in a

minimum of three points not in a line.

•The required contact is to prevent the part from

“rocking” during manufacturing or inspection

processes.

Primary Datum Plane

PRIMARY DATUM PLANE:

•This three-point contact is not difficult to achieve;

•If the designer has any concern about surface

irregularity, a surface control may be specified

SECOND DATUMPLANE ESTABLISHED BYTWO POINTS (MIN) CONTACTWITH SIMULATED DATUM B

Secondary DatumSecondary Datum

SECONDARY DATUM PLANE:

•Must be at a 90deg angle to the primary datum

plane.

•Usually selected as the second most functionally

important feature.

•This feature must be perpendicular to the primary

datum feature.

•There is only a two-point minimum contact

required for this plane.

SECONDARY DATUM PLANE:

•These two points establish the part in the other

direction to prevent it from rocking about the

primary datum plane.

•This plane may be a stop, fence, or angle plate

on processing or inspection equipment.

Tertiary DatumTertiary Datum

90°

THIRD DATUMPLANE ESTABLISHEDBY ONE POINT (MIN)CONTACT WITHSIMULATED DATUM C

MEASURING DIRECTIONS FOR RELATED DIMENSIONS

TERTIARY DATUM PLANE:

•Exactly at 90 deg angle to primary & secondary

planes

•The part must contact this plane at least at one

point.

•This contact is required for dimension origin and

to prevent any back-and-forth movement in the

third plane.

TERTIARY DATUM PLANE:

•The tertiary plane could be a locating or stop-pin

in a processing or inspection process.

•All measurements, setups, and inspections are to

be made from these three mutually perpendicular

planes.

Figure 3-7 is an illustration of the theoretical

datum reference frame.

ALL 3 DATUM PLANES

BASIC DIMENSION

INDIVIDUAL (No Datum

Reference)

INDIVIDUAL or RELATED

FEATURES

RELATED FEATURES

(Datum Reference Required)

GEOMETRIC CHARACTERISTIC CONTROLS

TYPE OFFEATURE

TYPE OFTOLERANCE CHARACTERISTIC SYMBOL

SYMMETRY

FLATNESS

STRAIGHTNESS

CIRCULARITY

CYLINDRICITY

LINE PROFILE

SURFACE PROFILE

PERPENDICULARITY

ANGULARITY

PARALLELISM

CIRCULAR RUNOUT

TOTAL RUNOUT

CONCENTRICITY

POSITION

FORM

PROFILE

ORIENTATION

RUNOUT

LOCATION

14 characteristics that may be controlled

Examples for symbols

Flatness as stated on drawing: The flatness of the feature must be within .06 tolerance zone.

Straightness

SYMBOL

Figure 6-1

DEFINITION

Straightness is the condition where one line

element of a surface or axis is in a straight line.

.003

0.500 ±.005

.0030.500 ±.005

Straightness applied to a flat surface: The straightness of the feature must be within .003 tolerance zone.

DEFINITION OF CIRCULARITY:

Circularity is roundness. Circularity is a condition

of a cylindrical surface, other than a sphere, at any

cross-sectional measurement where all points of

the surface are perpendicular to and equal distance

from a common axis during one complete

revolution of the feature.

Circularity of a sphere is a condition where all

points of the surface intersected by any plane

passing through a common center are equal

distance from that center.

Cylindricity:

Condition of an entire feature surface during one

revolution in which all surface points are an

equal distance from a common axis.

LINE PROFILE

Line profile tolerancing is a method of

specifying a two-dimensional control for a

single line element along the true profile of a

surface.

This control is usually specified for the shape

of cross-sections or cutting

planes of parts.

SURFACE PROFILE

Surface profile is a method of specifying a three-

dimensional control along the entire surface to be

controlled.

This control is usually applied to parts having

constant cross-section, surfaces of revolution,

weldments, forgings, etc., where an "all over"

requirement may be desired.

Surface control may apply all around.

Perpendicularity:

Condition of an entire surface, plane, or

axis at a right angle to a datum plane or

axis.

PERPENDICULARITY:

The tolerance zone is the space between the 2 parallel lines. They are perpendicular to the datum plane and spaced .005 apart.

The perpendicularity of this surface must be within a .005 tolerance zone relative to datum A.

perpendicularity

Angularity is the condition of an axis or plane

other than 90deg to another datum plane or axis.

Angularity

Angularity Orientation

Poor G D & T condition

Angularity Orientation

Good G D & T condition

parallelism

±0.01

Parallelism is the condition of a

surface, center plane or axis that is an

equal distance at all points from a

datum plane or axis.

RUNOUT:

•A composite form & location control of

permissible error in the desired part

surface during a complete revolution of the

part around a datum axis.

Concentricity:

Symbol:

Definition: Condition where the median points

of all diametrically opposed elements of a figure

of revolution (or correspondingly located

elements of two or more radially disposed

features) are congruent with the axis (or center

point) of a datum feature

Concentricity vs Runout

RUNOUT VS CONCENTRICITY

• Runout is inspected with a single indicator

while concentricity is inspected with two indicators placed opposite to each other.

• Both concentricity & runout are on diameters.(Eccentricity (no symbol) is on the radius).

• Runout considers form error while concentricity ignores form error.

• Runout is easier to inspect as compared to concentricity.

WHEN IN DOUBT – USE RUNOUT

SYMBOL FOR SYMMETRY

Symmetry:Condition where a feature or part

has the same profile on either side of the

central

(median plane) of a datum feature.

Position:

Position is the condition where a feature or group of features is located (positioned) in relation to another feature or datum feature

Positioning

EXAMPLE OF G.D. & T. ADVANTAGEGREATER FLEXIBILTY IN MANUFACTURING

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Do’s & Don’t s of Dimensioning

Never dimension hidden lines

Do’s & Don’t s of Dimensioning

Avoid Over-Dimensioning

Avoid Over-Dimensioning

Basic Dimensioning Style

Continue dimensioning

Baseline dimensioning

Ordinate dimensioning