GD&T

PART PRODUCTION COMMUNICATION MODEL

MANAGEMENT

DESIGN

TOOLING

PRODUCTION

INSPECTION

ASSEMBLY

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PLANNING

PRICING

SERVICE

PURCHASING

SALES

CU

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SV

EN

DO

RS

Geometric Dimensioning and Tolerancing (GD&T)

Geometric Dimensioning and Tolerancing (GD&T)

Dimensioning can be divided into three categories:

•general dimensioning,•geometric dimensioning, and•surface texture.

The following provides information necessary to begin to understand geometric dimensioning and tolerancing (GD&T)

Three Categories of Dimensioning

Three Categories of Dimensioning

Limit Tolerancing Applied To An Angle Block

Limit Tolerancing Applied To An Angle Block

Geometric Tolerancing Applied To An Angle Block

Geometric Tolerancing Applied To An Angle Block

Geometric Dimensioning & Tolerancing (GD&T)

Geometric Dimensioning & Tolerancing (GD&T)

• GD&T is a means of dimensioning & tolerancing a drawing which considers the function of the part and how this part functions with related parts .

– This allows a drawing to contain a more defined feature more accurately, without increasing tolerances.

GD&T cont’dGD&T cont’d

• GD&T has increased in practice in last 15 years because of ISO 9000.– ISO 9000 requires not only that something be required, but

how it is to be controlled. For example, how round does a round feature have to be?

• GD&T is a system that uses standard symbols to indicate tolerances that are based on the feature’s geometry.

– Sometimes called feature based dimensioning & tolerancing or true position dimensioning & tolerancing

• GD&T practices are specified in ANSI Y14.5M-1994.

For Example• Given Table Height

• However, all surfaces have a degree of waviness, or smoothness. For example, the surface of a 2 x 4 is much wavier (rough) than the surface of a piece of glass.– As the table height is dimensioned, the following table would

pass inspection.

• If top must be flatter, you could tighten the tolerance to ± 1/32.– However, now the height is restricted to 26.97 to 27.03

Assume all 4 legs will be cut to length at the same time.

or

Example cont’d.

• You can have both, by using GD&T.– The table height may any height between 26

and 28 inches.

– The table top must be flat within 1/16. (±1/32)

27

.06

26

.06

28

.06

WHY IS GD&T IMPORTANT

• Saves money– For example, if large number of parts are

being made – GD&T can reduce or eliminate inspection of some features.

– Provides “bonus” tolerance• Ensures design, dimension, and tolerance

requirements as they relate to the actual function

• Ensures interchangeability of mating parts at the assembly

• Provides uniformity• It is a universal understanding of the symbols

instead of words

WHEN TO USE GD&T

• When part features are critical to a function or interchangeability

• When functional gaging is desirable• When datum references are desirable to

insure consistency between design• When standard interpretation or tolerance is

not already implied• When it allows a better choice of machining

processes to be made for production of a part

TERMINOLOGY REVIEW • Maximum Material Condition (MMC): The

condition where a size feature contains the maximum amount of material within the stated limits of size. I.e., largest shaft and smallest hole.

• Least Material Condition (LMC): The condition where a size feature contains the least amount of material within the stated limits of size. I.e., smallest shaft and largest hole.

• Tolerance: Difference between MMC and LMC limits of a single dimension.

• Allowance: Difference between the MMC of two mating parts. (Minimum clearance and maximum interference)

• Basic Dimension: Nominal dimension from which tolerances are derived.

T H I S M E A N ?W H A T D O E S

S I Z E D I M E N S I O N

2 . 0 0 72 . 0 0 3

LIMITS OF SIZE LIMITS OF SIZE

S I Z E D I M E N S I O N

M M C

L M C

E N V E L O P E O F S I Z E

( 2 . 0 0 3 )

( 2 . 0 0 7 )

E N V E L O P E P R I N C I P L E


A variation in form is allowed between the least material condition (LMC) and the maximum material condition (MMC).

Envelop Principle defines the size and form relationships between mating parts.

ENVELOPE PRINCIPLE

LMCCLEARANCE

MMCALLOWANCE



The actual size of the feature at any cross section must be within the size boundary.

ØMMCØLMC

No portion of the feature may be outside a perfect form barrier at maximum material condition (MMC).


P A R A L L E L P L A N E S

P A R A L L E L P L A N E S P A R A L L E L P L A N E S C Y L I N D E R Z O N E

P A R A L L E L L I N E S P A R A L L E L L I N E S P A R A L L E L L I N E S

P A R A L L E L P L A N E S P A R A L L E L P L A N E S

Other Factors I.e., Parallel Line Tolerance Zones

Other Factors I.e., Parallel Line Tolerance Zones

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

Characteristics & Symbols cont’d.

– Maximum Material Condition MMC– Regardless of Feature Size RFS– Least Material Condition LMC– Projected Tolerance Zone– Diametrical (Cylindrical) Tolerance Zone or

Feature– Basic, or Exact, Dimension– Datum Feature Symbol

– Feature Control Frame

THE

GEOMETRIC SYMBOLTOLERANCE INFORMATION

DATUM REFERENCES

FEATURE CONTROL FRAME

COMPARTMENT VARIABLES

CONNECTING WORDSMUST BE WITHINOF THE FEATURE

RELATIVE TO

Feature Control FrameFeature Control Frame

Feature Control Frame• Uses feature control frames to indicate

tolerance

• Reads as: The position of the feature must be within a .003 diametrical tolerance zone at

Feature Control Frame

• Uses feature control frames to indicate tolerance

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

• The of the feature must be within a tolerance zone.

• The of the feature must be within a tolerance zone at relative to Datum .

• The of the feature must be within a

tolerance zone relative to Datum .

• The of the feature must be within a

zone at

relative to Datum .

• The of the feature must be within a tolerance zone relative to datums .

Reading Feature Control Frames

Placement of Feature Control Frames

� May be attached to a side, end or corner of the symbol box to an extension line.

� Applied to surface.

� Applied to axis

Placement of Feature Control Frames Cont’d.

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

Ø .500±.005

Basic Dimension• A theoretically exact size, profile, orientation, or

location of a feature or datum target, therefore, a basic dimension is untoleranced.

• Most often used with position, angularity, and profile)

• Basic dimensions have a rectangle surrounding it.

1.000

Basic Dimension cont’d.

Form FeaturesForm Features• Individual Features• No Datum Reference

Flatness Straightness

CylindricityCircularity

Form FeaturesExamplesForm FeaturesExamples

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

.003

0.500 ±.005

.0030.500 ±.005

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

Form FeaturesExamplesForm FeaturesExamplesStraightness applied to the surface of a diameter: The straightness of the feature must be within .003 tolerance zone.

.003

0.5000.505∅

Straightness of an Axis at MMC: The derived median line straightness of the feature must be within a diametric zone of .030 at MMC.

.0300.5000.505∅ M∅

1.0100.990

B E Z E LC A S E

C L A M P

P R O B E

6

8

1 0 1 2 1 0

8

6

4

22

4

Dial IndicatorDial Indicator

Verification of Flatness

Activity 13

• Work on worksheets GD&T 1, GD&T 2 #1 only, and GD&T 3 – (for GD&T 3 completely dimension. ¼” grid.)

Features that Require Datum Reference

Features that Require Datum Reference

• Orientation– Perpendicularity – Angularity– Parallelism

• Runout– Circular Runout– Total Runout

• Location– Position– Concentricity

– Symmetry

Datum• Datums 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.– As you know, not every GD&T feature requires a

datum, i.e., Flat

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.

• Datums on a drawing of a part are represented using the symbol shown below.

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

Placement of Datums• Datums 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

Placement of Datums• Feature sizes, such as holes

• Sometimes a feature has a GD&T and is also a datum

Ø .500±.005

A

Ø .500±.005

A Ø .500±.005

6 ROTATIONAL6 LINEAR AND

FREEDOMDEGREES OF

UP

DOWN

RIGHT

LEFT BACK

FRONT

UNRESTRICTED FREEMOVEMENT IN SPACE

TWELVE DEGREES OF FREEDOMTWELVE DEGREES OF FREEDOM

Example Datums• Datums must be perpendicular to each

other– Primary

– Secondary

– Tertiary Datum

Primary Datum• A primary datum is selected to provide

functional relationships, accessibility, and repeatability. – Functional Relationships

• A standardization of size is desired in the manufacturing of a part.

• Consideration of how parts are orientated to each other is very important.

– For example, legos are made in a standard size in order to lock into place. A primary datum is chosen to reference the location of the mating features.

– Accessibility • Does anything, such as, shafts, get in the way?

Primary Datum cont’d.

– Repeatability For example, castings, sheet metal, etc.• The primary datum chosen must insure precise

measurements. The surface established must produce consistent

• Measurements when producing many identical parts to meet requirements specified.

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

Primary DatumPrimary Datum� Restricts 6 degrees of freedom

Secondary & Tertiary Datums

• All dimension may not be capable to reference from the primary datum to ensure functional relationships, accessibility, and repeatability.– Secondary Datum

• Secondary datums are produced perpendicular to the primary datum so measurements can be referenced from them.

– Tertiary Datum• This datum is always perpendicular to both the primary

and secondary datums ensuring a fixed position from three related parts.

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

Secondary DatumSecondary Datum� Restricts 10 degrees of freedom.

Tertiary DatumTertiary Datum� Restricts 12 degrees of freedom.

90°

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

MEASURING DIRECTIONS FOR RELATED DIMENSIONS

Z

D A T U MR E F E R E N C EF R A M E

S U R F A C EP L A T E

G R A N I T E

P R O B E

C O O R D I N A T E M E A S U R I N G M A C H I N EB R I D G E D E S I G N

Coordinate Measuring MachineCoordinate Measuring Machine

SIMULATED DATUM-SMALLEST

CIRCUMSCRIBEDCYLINDER

THIS ONTHE DRAWING

MEANS THIS

PARTDATUM AXIS

A

Size Datum(CIRCULAR)



SIMULATED DATUM-LARGEST

INSCRIBEDCYLINDER

THIS ONTHE DRAWING

MEANS THIS

DATUM AXIS APART

A

Orientation TolerancesOrientation Tolerances

–Perpendicularity –Angularity

–Parallelism� Controls the orientation of individual features

� Datums are required

� Shape of tolerance zone: 2 parallel lines, 2 parallel planes, and cylindrical

PERPENDICULARITY:• is the condition of a surface, center plane, or axis at a right angle

(90°) to a datum plane or axis.Ex:

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.

Practice Problem

• Plane 1 must be perpendicular within .005 tolerance zone to plane 2.

BOTTOM SURFACE

Practice Problem

• Plane 1 must be perpendicular within .005 tolerance zone to plane 2

BOTTOM PLANE

2.00±.01

.02 Tolerance

Practice Problem

Without GD & T this would be acceptable

2.00±.01

.02 Tolerance

.005 Tolerance Zone

With GD & T the overall height may end anywhere between the two blue planes. But the bottom plane is restricted to the red tolerance zone.

PERPENDICULARITY Cont’d.

• Location of hole (axis)

This means ‘the hole (axis) must be perpendicular within a diametrical tolerance zone of .010 relative to datum A’

ANGULARITY:• is the condition of a surface, axis, or median plane

which is at a specific angle (other than 90°) from a datum plane or axis.

• Can be applied to an axis at MMC.

The surface is at a 45ºangle with a .005 tolerance zone relative to datum A.

±0.01

PARALLELISM:• The condition of a surface or center plane equidistant at all

points from a datum plane, or an axis.• The distance between the parallel lines, or surfaces, is specified

by the geometric tolerance.

Activity 13 Cont’d.

• Complete worksheets GD&T-2, GD&T-4, and GD&T-5– Completely dimension.

– ¼” grid

Material Conditions

• Maximum Material Condition (MMC) • Least Material Condition (LMC) • Regardless of Feature Size(RFS)

Maximum Material Condition• MMC • This is when part will weigh the most.

– MMC for a shaft is the largest allowable size.• MMC of Ø0.240±.005?

– MMC for a hole is the smallest allowable size.• MMC of Ø0.250±.005?

• Permits greater possible tolerance as the part feature sizes vary from their calculated MMC

• Ensures interchangeability• Used

– With interrelated features with respect to location– Size, such as, hole, slot, pin, etc.

Least Material Condition

• LMC • This is when part will weigh the least.

– LMC for a shaft is the smallest allowable size.

• LMC of Ø0.240 ±.005?

– LMC for a hole is the largest allowable size.

• LMC of Ø0.250 ±.005?

Regardless of Feature Size

• RFS • Requires that the condition of the

material NOT be considered. • This is used when the size feature does

not affect the specified tolerance.• Valid only when applied to features of

size, such as holes, slots, pins, etc., with an axis or center plane.

Location TolerancesLocation Tolerances

– Position– Concentricity– Symmetry

Position TolerancePosition Tolerance• A position tolerance is the total permissible

variation in the location of a feature about its exact true position.

• For cylindrical features, the position tolerance zone is typically a cylinder within which the axis of the feature must lie.

• For other features, the center plane of the feature must fit in the space between two parallel planes.

• The exact position of the feature is located with basic dimensions.

• The position tolerance is typically associated with the size tolerance of the feature.

• Datums are required.

Coordinate System PositionCoordinate System Position• Consider the following hole dimensioned with coordinate

dimensions:

• The tolerance zone for the location of the hole is as follows:

• Several Problems:

2.000

.750

Coordinate System PositionCoordinate System Position• Consider the following hole dimensioned with coordinate

dimensions:

• The tolerance zone for the location (axis) of the hole is as follows:

• Several Problems:

2.000

.750

Center can be anywhere along the diagonal line.

Position TolerancingPosition Tolerancing• Consider the same hole, but add GD&T:

• Now, overall tolerance zone is:

• The actual center of the hole (axis) must lie in the round tolerance zone. The same tolerance is applied, regardless of thedirection.

MMC =.500 - .003 = .497

Bonus Tolerance• Here is the beauty of the system! The specified

tolerance was:

This means that the tolerance is .010 if the hole size is the MMC size, or .497. If the hole is bigger, we get a bonus tolerance equal to the difference between the MMC size and the actual size.

Bonus Tolerance Example

??Ø .504

.016.006Ø .503 (LMC)

.015.005Ø .502

.013.003 (.010 + .003 = .013)Ø .500 (.500 - .497 = .003)

.012.002 (.010 + .002 = .012)Ø .499 (.499 - .497 = .002)

.0100Ø .497 (MMC)

Φ of Tol. ZoneBonus Tol.Actual Hole Size

This means that the tolerance is .010 if the hole size is the MMC size, or .497. If the hole is bigger, we get a bonustolerance equal to the difference between the MMC size and the actual size.

.503

.497 = BONUS 0

TOL ZONE .010

.499 - .497 = BONUS .002

BONUS + TOL. ZONE = .012

Shaft

Hole

.501 - .497 = BONUS .004


.503 - .497 = BONUS .006


• What if the tolerance had been specified as:

Since there is NO material modifier, the tolerance is RFS, whichstands for regardless of feature size. This means that the position tolerance is .010 at all times. There is no bonus tolerance associated with this specification.

• VIRTUAL CONDITION : The worst case boundary generated by the collective effects of a size feature’s specified MMC or LMC material condition and the specified geometric tolerance.

GT = GEOMETRIC TOLERANCE

PERPENDICULARITY Cont’d.

Means “the hole (AXIS) must be perpendicular within a diametrical tolerance zone of .010 at MMC relative to datum A.”

2.0022.003

2.0012.0001.9991.9981.997 (MMC)

Ø of Tol. ZoneBonus Tol. Actual Hole Size

Vc =

Activity 13 Cont’d.

• Worksheet GD&T 6

Documents

GD&T