geometric dimensioning and tolerancing (gd&t). dimensioning can be divided into three...

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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 ExampleFor 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 cont’d.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 IMPORTANTWHY 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&TWHEN 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 ensure 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 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 LIMITS OF SIZE

SIZE DIMENSION

MMC

LMC

ENVELOPE OF SIZE

(2.003)

(2.007)

ENVELOPE PRINCIPLE


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.

ØMMC

ØLMC

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


PARALLEL PLANES

PARALLEL PLANES PARALLEL PLANES CYLINDER ZONE

GEOMETRIC DIMENSIONING TOLERANCE ZONES

PARALLEL LINES PARALLEL LINES PARALLEL LINES

PARALLEL PLANES PARALLEL PLANES

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.

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 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 FrameFeature 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, B, and C.

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 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 Reading Feature Control Frames

Placement of 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.

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 DimensionBasic 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.

Basic Dimension cont’d.

Form FeaturesForm Features Individual Features No Datum Reference

Flatness Straightness

CylindricityCircularity

Form Features ExamplesForm 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 ExamplesForm 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

DIAL INDICATOR

6

8

1012

10

8

6

4

22

4

Dial IndicatorDial Indicator

Verification of FlatnessVerification of Flatness

Activity 13Activity 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

DatumDatum 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.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 DatumsPlacement 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 DatumsPlacement 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 Example Datums Datums must be

perpendicular to each other– Primary

– Secondary

– Tertiary Datum

Primary DatumPrimary 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.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

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

DATUMREFERENCEFRAME

SURFACEPLATE

GRANITE

PROBE

COORDINATE MEASURING MACHINE

BRIDGE DESIGN

Coordinate Measuring Machine

Coordinate Measuring Machine

SIMULATED DATUM-SMALLEST

CIRCUMSCRIBEDCYLINDER

THIS ONTHE DRAWING

MEANS THIS

PART

DATUM AXIS

A

Size Datum(CIRCULAR)




SIMULATED DATUM-LARGEST

INSCRIBEDCYLINDER

THIS ONTHE DRAWING

MEANS THIS

DATUM AXIS A

PART

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: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 ProblemPractice Problem

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

BOTTOM SURFACE


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

BOTTOM PLANE

2.00±.01

.02 Tolerance


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.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: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 basic

dimension.

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

±0.01

PARALLELISM: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.Activity 13 Cont’d.

Complete worksheets GD&T-2, GD&T-4, and GD&T-5– Completely dimension.– ¼” grid

Material ConditionsMaterial Conditions

Maximum Material Condition (MMC)

Least Material Condition (LMC)

Regardless of Feature Size(RFS)

Maximum Material ConditionMaximum 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 ConditionLeast 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 SizeRegardless 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:– 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 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:– 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 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 the direction.

MMC =

.500 - .003 = .497

Bonus ToleranceBonus 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 ExampleBonus Tolerance Example

This system makes sense… 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. Φ of Tol. Zone

Ø .497 (MMC) 0 .010

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

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

Ø .502 .005 .015

Ø .503 (LMC) .006 .016

Ø .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


.503 - .497 = BONUS .006


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 feature’s specified MMC or LMC material condition and the specified geometric tolerance.

GT = GEOMETRIC TOLERANCE

PERPENDICULARITY Cont’d.PERPENDICULARITY Cont’d.

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.

Ø of Tol. Zone

1.997 (MMC)

1.998

1.999

2.000

2.001

2.002

2.003

Vc =

Activity 13 Cont’d.Activity 13 Cont’d.

Worksheet GD&T 6

geometric dimensioning and tolerancing (gd&t). dimensioning can be divided into three...

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