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RS

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

Limit Tolerancing Applied

To An Angle Block

Geometric Tolerancing

Applied To An Angle Block

Geometric Dimensioning Geometric Dimensioning

& Tolerancing (GD&T)

GD&T is a means of

dimensioning & tolerancing a drawing which considersthe 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 contd

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 features 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 meaning good tables would be rejected.

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

or

Example contd.

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.

THIS MEAN?WHAT DOES

SIZE DIMENSION

2.0072.003

LIMITS OF SIZE

SIZE DIMENSION

MMC

LMC

ENVELOPE OF SIZE

(2.003)

(2.007)

ENVELOPE PRINCIPLE

LIMITS OF SIZE

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

LMC

CLEARANCE

MMCALLOWANCE

LIMITS OF SIZE

LIMITS OF SIZE

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

MMC

LMC

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

LIMITS OF SIZE

PARALLEL PLANES

PARALLEL PLANES PARALLEL PLANES CYLINDER ZONE

PARALLEL LINES PARALLEL LINES PARALLEL LINES

PARALLEL PLANES PARALLEL PLANES

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 & Symbolscontd.

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 SYMBOL

TOLERANCE INFORMATION

DATUM REFERENCES

FEATURE CONTROL FRAME

COMPARTMENT VARIABLES

CONNECTING WORDS

MUST BE WITHINOF THE FEATURE

RELATIVE TO

Feature 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 maximum material conditionrelative to datums A, B, and C.

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 Contd.

May be below or closely

adjacent to the dimension or note pertaining to that feature.

.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 contd.

Form Features

Individual Features

No Datum Reference

Flatness Straightness

CylindricityCircularity

Form Features Examples

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 Features Examples

Straightness 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

BEZELCASE

CLAMP

PROBE

6

8

1012

10

8

6

4

22

4

Dial 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. grid.)

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 contd.

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 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

.500.005

A

.500.005

A .500.005

6 ROTATIONAL6 LINEAR AND

FREEDOMDEGREES OF

UP

DOWN

RIGHT

LEFT BACK

FRONT

UNRESTRICTED FREEMOVEMENT IN SPACE

TWELVE 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 contd.

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 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 Datum

Restricts 10 degrees of freedom.

Tertiary Datum

Restricts 12 degrees of freedom.

90

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

MEASURING DIRECTIONS FOR

RELATED DIMENSIONS

Z

D AT U MR EF E R E N C EF R A M E

SU R F A C EP LA T E

G R A N IT E

P R O B E

B R ID G E D ES IG N

Coordinate Measuring

Machine

SIMULATED DATUM-

SMALLEST

CIRCUMSCRIBED

CYLINDER

THIS ONTHE DRAWING

MEANS THIS

PART

DATUM AXIS

A

Size Datum(CIRCULAR)

Size Datum(CIRCULAR)

SIMULATED DATUM-LARGEST

INSCRIBEDCYLINDER

THIS ONTHE DRAWING

MEANS THIS

DATUM AXIS A

PART

A

Orientation 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 Contd.

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.

Typically must have a basicdimension.

The surface is at a

45 angle 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 Contd.

Complete worksheets GD&T-

2, GD&T-4, and GD&T-5

Completely dimension.

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 0.240.005?

MMC for a hole is the smallest allowable size.

MMC of 0.250.005?

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 0.240.005?

LMC for a hole is the largest

allowable size.

LMC of 0.250.005?

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 Tolerances

Position

Concentricity

Symmetry

Position 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 Position

Consider the following hole dimensioned with coordinate dimensions:

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

Several Problems:

Two points, equidistant from true position may not be accepted.

Total tolerance diagonally is .014, which may be more than was intended.

2.000

.750

Coordinate 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:

Two points, equidistant from true position may not be accepted.

Total tolerance diagonally is .014, which may be more than was intended. (1.4 Xs >, 1.4*.010=.014)

2.000

.750

Center can be anywhere along the diagonal line.

Position 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 the direction.

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 bonustolerance equal to the

difference between the

MMC size and the actual

size.

Bonus Tolerance Example

This system makes senseY the larger the hole is, the more it can deviate from true position and still fit in the mating condition!

Actual Hole Size Bonus Tol. of Tol. Zone

.497 (MMC) 0 .010

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

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

.502 .005 .015

.503 (LMC) .006 .016

.504 ? ?

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.

.503

.497 = BONUS 0

TOL ZONE .010

.499 - .497 = BONUS .002

BONUS + TOL. ZONE = .012

Shaft

Hole

.501 - .497 = BONUS .004

BONUS + TOL. ZONE = .014

.503 - .497 = BONUS .006

BONUS + TOL. ZONE = .016

What if the tolerance had been specified as:

Since there is NO material modifier, the

tolerance is RFS, which stands 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 features specified MMC or LMC

material condition and the specified geometric

tolerance.

GT = GEOMETRIC

TOLERANCE

PERPENDICULARITY Contd.

Means the hole (AXIS) must be perpendicular within a

diametrical tolerance zone of .010 at MMC relative to datum

A.

Actual Hole Size

Bonus Tol. of Tol. Zone

1.997 (MMC)

1.998

1.999

2.000

2.001

2.002

2.003

Vc =

Activity 13 Contd.

Worksheet GD&T 6