ME 407 - Introduction to GD&T (v1.3)

8/9/2019 ME 407 - Introduction to GD&T (v1.3)

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Introduction toGeometric Dimensioning &

Tolerancing (GD&T)Dr. Melik Dölen

Middle East Technical UniversityDepartment of Mechanical Engineering

Ankara 06531, TURKEY1956


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Outline• Definition and Background

– + /- Tolerancing vs. GeometricTolerancing

– Features

– Datums

– Material Conditions Modifiers

– Feature Control Frames

• Major Categories of Tolerances

– 14 Tolerance Measurements

• Bonus Tolerance

• Virtual Conditions

• GD&T with Solidworks

• Summary

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What is GD&T?

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• GD&T is a symbolic language used to specify the size, shape,

form, orientation, and location of features on a part.

– Design tool for communicating design requirements.

• Like other languages, GD&T uses special punctuation andgrammar rules.

– Must be used properly in order to prevent misinterpretation.

– Comparable to learning a new language!


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What is GD&T? (Cont’d)

• GD&T has developed as a method to question and

measure the truth about the form, orientation,

and location of manufactured parts.

– Considers the function of the part and how this part

functions with related parts.

– Allows a drawing to contain a more defined feature

more accurately without increasing tolerances.

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Standards

• Standards on GD&T comefrom two organizations: – ASME (American Society of

Mechanical Engineering)

– ISO (International Organization for

Standardization)

• ASME Y14.5M and ISO

1101 are the written

standards. – Standards are nowhere complete. – Continuously evolving since WWII!

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When to Use GD&T?

• Designers should specify tolerance for the parts

with GD&T when

– Drawing delineation and interpretation need to be the

same, – Features are critical to function or interchangeability,

– Automated manufacturing/inspection equipment is

utilized,

– Functional gauging is required, – It is important to increase productivity.

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Dimensioning

• Dimensioning can be divided into

three categories:

– General dimensioning

• Used since 1800s.

– Limit dimensioning

– Plus/minus dimensioning

– Geometric dimensioning

– Surface texture

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Plus/Minus Tolerancing

• Plus/ Minus tolerancing, or limit

tolerancing is a two-dimensional system.

• When the designer draws the part, using

CAD tools, the lines are straight, angles

are perfect, and the holes are perfectly

round.

• When the part is produced in a

manufacturing process, there will be

errors.

• The variations in the corners and surfaces

will be undetectable to the human eye.

– They can be picked up using precise

measurements such as a Coordinate Measuring

Machine (CMM).

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Plus/Minus Tolerancing (Cont’d)

• In a plus/minus tolerancing system, the datums are

implied and therefore, are open to interpretations.

• Plus/minus tolerancing works well when individual

features are considered.

– However, one can not understand the relationship between

individual features.

• With the dawn of CAD systems and CMMs, it has become

increasingly important to describe parts in three

dimensions (i.e. solid geometric models), and plus/minustolerancing is simply not precise enough.

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Example - Dimensional Tolerancing

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Example – Produced Part

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Example - GD&T Specs

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Example - Feature Control via GD&T

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Example

• Consider the given Table. – Assume all four legs will be cut to

the length at the same time.

• All surfaces have a degree of

waviness (smoothness). – The surface of a 2 by 4 is much

wavier (rough) than the surface of apiece of glass.

– As the table height is dimensioned,the following table would passinspection.

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Example (Cont’d) • 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.

• 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”)

15

26

.06

27

.06

28

.06


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Review of Terminology

• Basic Dimension: Nominal dimension from which tolerancesare derived.

• With Size: A feature said to be “with size” if it is associatedwith a size dimension. It can be cylindrical or spherical orpossibly a set of two opposing parallel surfaces.

• Without Size: A plane surface where no size dimensions areindicated.

• Feature Control Frames: Probably the most significantsymbol in any geometric tolerancing system. Provides theinstructions and requirements for its related feature.

• Radius: Two types of radii can be applied. The radius (R)distinguishes general applications. The controlled radius(CR) defines radius shapes that require further restrictions.

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Terminology - Feature

• Real, geometric shapes thatmake up the physical

characteristics of a part.

– May include one or more elements:

• Holes, Screw threads, Profiles, Faces,Slots

• Can be individual or may be

interrelated.

• Any feature can have manyimperfections and variations

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Material Condition Modifiers

• Have tremendous impact on stated tolerance ordatum reference.

• Can only be applied to features and datums that

specify size (holes, slots, pins, tabs). If applied to

features that are without size, they have noimpact.

• There are three material condition modifiers:

– Maximum material condition (MMC) – Least material condition (LMC)

– Regardless of feature size (RFS)

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Maximum Material Condition M

• This is when part will weigh the most.

– MMC for a shaft is the largest allowable size.• MMC of Ø.250±.005?

– MMC for a hole is the smallest allowable size.

• MMC of Ø.250±.005?

• Permits greater possible tolerance as thepart feature sizes vary from theircalculated MMC

• Ensures interchangeability

• Used with interrelated features with

respect to location: – Size, such as, hole, slot, pin, etc.

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.255

.250 + .005

.245

.250 + .005

M


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Least Material Condition L

• This is when part will weighthe least.

– LMC for a shaft is the

smallest allowable size.

• LMC of Ø.250±.005?

– LMC for a hole is the largest

allowable size.

• LMC of Ø.250±.005?

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.245

.250 + .005

.255

.250 + .005

L


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Regardless of Feature Size

• Requires that the condition of the

material NOT be considered.

• This is used when the size feature doesnot affect the specified tolerance.

• Valid only when applied to features of

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

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Limits of Size

THIS MEAN?

WHAT DOES

SIZE DIMENSION

2.007

2.003

• A variation in form isallowed between the

least material condition

(LMC) and the maximum

material condition(MMC).

• Envelope (Taylor)

Principle defines the size

and form relationships

between mating parts.

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SIZE DIMENSION

MMC

LMC

ENVELOPE OF SIZE

(2.003)

(2.007)

ENVELOPE PRINCIPLE


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Limits of Size (Cont’d)

• The actual size of the feature

at any cross section must be

within the size boundary.

• No portion of the feature

may be outside a perfect

form barrier at maximummaterial condition (MMC).

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ØMMC

ØLMC


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Controlled Features

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INDIVIDUAL

(No Datum

Reference)

INDIVIDUAL or

RELATED

FEATURES

RELATEDFEATURES

(Datum

Reference

Required)

GEOMETRIC CHARACTERISTIC CONTROLS

TYPE OF FEATURE TYPE OF

TOLERANCE 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


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Some Common Symbols

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Feature Control Frame

Datum Reference Frame

Diametral (Cylindrical) Tolerance

Zone or Feature

Basic- or Exact Dimension

Least Material Condition (LMC)

Maximum Material Condition (MMC)

.003 M A

A

.500

M

L


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Feature Control Frame

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GEOMETRIC SYMBOL

TOLERANCE INFORMATION

DATUM REFERENCES

THE

MUST BE WITHIN

OF THE FEATURE

RELATIVE TO


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Example - Frame Control Frame

• Reads as “ The (true) position of the

feature must be within a .01”

diametric tolerance zone at

maximum material conditionrelative to datums A, B (at

maximum material condition), and

C.”

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Application of FCFs

• May be attached to a side, end

or corner of the symbol box to

an extension line or could be

applied to a surface or an axis.

• May be below or closely

adjacent to the dimension ornote pertaining to that feature.

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Ø .500±.005


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Basic Dimension

• A theoretically exact size,

profile, orientation, orlocation of a feature or

datum target, therefore, a

basic dimension is non-

toleranced.

• Most often used with

position, angularity, and

profile• Basic dimensions have a

rectangle surrounding it.

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1.000


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Datum Reference Frame

• GD&T positions every part within a “DatumReference Frame” (DRF).

• The DRF is by far the most important

concept in the geometric tolerancingsystem.

• The skeleton, or frame of reference to

which all requirements are connected.

• Understanding the DRF is critical in order to

grasp the concepts of position and profile.

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DRF (Cont’d)

• Engineering, manufacturing,

and inspection all share acommon “three planes”

concept.

• These three planes are: – Mutually perpendicular

(orthogonal)

– Perfect in dimension and

orientation• This concept is called the

Datum Reference Frame.

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DRF (Cont’d)

• The three main features of the DRF

are the planes, axes, and points.

• The DRF consists of three imaginary

planes, similar to the X, Y, & Z axes

of the traditional coordinate

measuring system.• The planes exist only in theory and

make up a perfect, imaginary

structure that is mathematically

perfect.• All measurements originate from

the simulated datum planes.

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This flat, granite surface plate

and the angle block sitting on

it, can represent two of thethree datum planes.


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DRF (Cont’d)

• The Datum Reference Frame will

accommodate both rectangularand cylindrical parts.

• A rectangular part fits into the

corners represented by the

intersection of the three datum

planes.

• The datum planes are imaginary

and therefore perfect.

• The parts will vary from these

planes, even though the variations

will not be visible to the naked eye.

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DRF (Cont’d)

• The most important concept to understandis that when the part is placed into an

inspection apparatus, it must make contact

with its planes in the order specified by thefeature control frame.

– Primary, then secondary, then tertiary!

• This is the only way to assure uniformity inthe measurement of different parts.

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DRF (Cont’d)

• A cylindrical part rests on the flat

surface of the primary plane and

the center of the cylinder aligns

with the vertical datum axis

created by the intersection of

the planes.• In this case, it becomes very

important to be able to establish

the exact center of the part,

whether it is the center of a solid

surface, or the center of a hole.

• Cylindrical parts are more

difficult to measure.

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Implied Datums

• The order of precedence in the

selection and establishment of

datums is very important.

• The picture shows a part with

four holes, located from the

edges with basic dimensions.• The datums are not called out in

the feature control frame, but

they are “implied” by the

dimensions and by the edges

from which those dimensionsoriginate. Thus, we imply that

these edges are the datums.

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( )


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Implied Datums (Cont’d)

• Problems with implied datums: – We do not know the order in which they are

used.

– We know the parts are not perfect.

– None of the edges are perfectly square.

– The 90o corners will not be perpendicular.

• In theory, even if the corners were out of

perpendicularity by only .0001, the part would still“rock” back and forth in the “theoretically perfect”

datum reference frame.

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d f


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Order of Datums

• GD&T instructions designate which feature of the

part will be the “primary, secondary, or tertiary”

datum references.

• These first, second and third datum features

reflect an order of importance when relating toother features that don’t touch the planes

directly.

• Datum orders are important because the same

part can be inspected in several different ways,

each giving a different measurement.

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d f ( ’d)


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Order of Datum (Cont’d)

• Creating a Datum Reference

Frame and an order of

importance is mandatory in

order to achieve

interchangeable parts.• Improper positioning could

result in measurement errors

unless the preferred

positioning in the inspectionfixture is indicated in the

drawing.

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d f ( ’d)


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Order of Datums (Cont’d)

•The primary datum feature

must have at least three points

of contact with the part and

contacts the fixture first.

• The secondary has two pointsof contact and the tertiary hasthree points of contact with

the part.

• This process correctly mirrors

the datum reference frameand positions the part the way

it will be fitted and used.

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A li i f D


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Application of Datums

• Datums are ideal features (points,

lines, circles, planes spheres,cylinders, cones) on the object that

are used as references 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!

• Datums are imaginary . They are

assumed to be exact for the purpose

of computation or reference.

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1.000

D (C ’d)


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Datum (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.

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.003 M A

Pl f D


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Placement of Datums

• Feature sizes, such as

holes

• Sometimes a feature

has a GD&T and is also

a datum

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A Ø .500±.005

A

Ø .500±.005 Ø .500±.005

I ti i CMM


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Inspection via CMM

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Z

DATUM

REFERENCE

FRAME

SURFACE

PLATE

GRANITE

PROBE

BRIDGE DESIGN

F F t


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Form Features

• Individual features

• No datum (reference) is required.

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Flatness Straightness

CylindricityCircularity

Fl t


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Flatness

• Flatness is a three-dimensionalversion of straightness tolerance.

• Requires a surface to be within two

imaginary (perfectly flat & parallel)

planes.

– Only the surface of the part (not the

entire thickness) is referenced to the

planes.

– Most often used on rectangular or

square parts. – If used as a primary datum, flatness must

be specified in the drawing.

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V ifi ti f Fl t


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Verification of Flatness

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St i ht


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Straightness

• Straightness is a two-dimensional

tolerance.

• Edge must remain within two

imaginary parallel lines to meet

straightness tolerance.• Distance between lines is

determined by size of specified

tolerance.

– Most rectangular parts have a straightnesstolerance.

– Edge or center axis of a cylinder may have a

straightness tolerance.

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Ci l it


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Circularity

• Circularity (or roundness) is atwo-dimensional tolerance.

– Demands that any two-dimensional

cross-section of a round feature

must stay within the tolerance zone

created by two concentric circles.

– Most often used on cylinders.

• Also applies to cones and spheres.

– Most inspectors check multiple

cross-sections.• Each section must meet the tolerance on

its own.

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C li d i it


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Cylindricity

• Cylindricity specifies the roundnessof a cylinder along its entire length.

– All cross-sections of the cylinder must

be measured together, so cylindricity

tolerance is only applied to cylinders.

• Circularity and cylindricity cannotbe checked by measuring various

diameters with a micrometer.

• Part must be rotated in a high-

precision spindle. – Best method would be to use a CMM.

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The thickness of the wall of a pipe represents

the cylindricity tolerance zone.

C lindricit (Cont’d)


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Cylindricity (Cont’d)

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Examples of Form Features


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Examples of Form Features

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Flatness as stated on drawing:

The flatness of the feature must

be within 0.06” tolerance zone.

Straightness applied to a flat

surface: The straightness of the

feature must be within 0.003” tolerance zone.

.003

0.500 ±.005

.0030.500 ±.005

Examples (Cont’d)


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Examples (Cont’d)

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Straightness applied to the

surface of a diameter: The

straightness of the feature must

be within .003 tolerance zone.

Straightness of an axis at MMC:

The derived median line

straightness of the feature

must be within a diametric

zone of .030 at MMC.

.003

0.5000.505

.0300.5000.505 M

1.0100.990

Features Requiring Reference


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Features Requiring Reference

• Unlike form features, the

followings necessitate datumreference:

– Orientation• Perpendicularity, Angularity, Paralellism

– Profile• Line (Curve), Surface

– Run-out

• Circular Run-out, Total Run-out

– Location• Position, Concentricity, Symmetry

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Perpendicularity


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Perpendicularity• Perpendicularity is the condition of a surface,

center plane, or axis at a right angle (90°) to adatum plane or axis.

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The perpendicularity of this

surface must be within a

.005 tolerance zone relative

to datum A.

The tolerance zone is the space

between the 2 parallel lines. They are

perpendicular to the datum plane

and spaced .005 apart.

Example Perpendicularity


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Example - Perpendicularity

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This means the hole (i.e. its axis)

must be perpendicular within a

diametrical tolerance zone of.010 relative to datum A

Angularity


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Angularity

• Angularity is a three-dimensional

tolerance.

• Shape of the tolerance zone

depends on the feature:

– If applied to flat surface, tolerancezone becomes two imaginary planes,

parallel to ideal angle.

– If applied to a hole, it is referenced

to an imaginary cylinder existing

around the ideal angle and center

of the hole must stay within that

cylinder.

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Parallelism


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Parallelism

• It is the condition of a surface or center plane

equidistant at all points from a datum plane, oran axis.

• The distance between the parallel lines, or

surfaces, is specified by the geometric tolerance.

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±0.01

Line (Curve) Profile


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Line (Curve) Profile

• A profile is an outline of the part

feature in one of the datum planes. – They control orientation, location, size

and form.

• The two versions of profile

tolerance.

– Both can be used to control features

such as cones, curves, flat or irregular

surfaces, or cylinders.

• The profile of a line is a two-

dimensional tolerance. – It requires the profile of a feature to fall

within two imaginary parallel lines that

follow the profile of the feature.

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Profile of a Surface


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Profile of a Surface

• Profile of a Surface is three-dimensional version of the

line profile.

– Often applied to complex and

curved contour surfaces suchas aircraft and automobile

exterior parts.

– The tolerance specifies that

the surface must remain

within two three-dimensional

shapes.

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Circular Runout


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Circular Runout

• Circular and Total Runout are three-

dimensional and apply only to cylindricalparts.

– Both tolerances reference a cylindrical

feature to a center datum-axis, and

simultaneously control the location, form

and orientation of the feature.

• Circular runout can only be inspected

when a part is rotated.

– Calibrated instrument is placed against the

surface of the rotating part to detect the

highest and lowest points.

– The surface must remain within twoimaginary circles, having their centers

located on the center axis.

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Total Runout


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Total Runout

• Total Runout is similar to circular

runout except that it involvestolerance control along the entire

length of, and between, two

imaginary cylinders, not just at

cross sections. – By default, parts that meet total

runout tolerance automatically satisfy

all of the circular runout tolerances.

– Runout tolerances, especially total

runout, are very demanding andpresent costly barriers to

manufacturing and inspection.

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Position Tolerance


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Position Tolerance

• Position is one of most common location tolerances:

– A three-dimensional, related tolerance.

– Ideal, exact location of feature is called true position.

– Actual location of a feature is compared to the ideal trueposition.

– Usually involves more than one datum to determine where

true position should be.

– Has nothing to do with size, shape, or angle, but rather“where it is.”

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Position Tolerance (Cont’d)


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Position Tolerance (Cont d)

• A position tolerance is the total permissible variation inthe location of a feature about its exact true position.

• For cylindrical features, the position tolerance zone istypically a cylinder within which the axis of the featuremust lie.

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

• The exact position of the feature is located with basicdimensions.

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

• Datums are required.

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Position (Cont’d)


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Position (Cont d)

• In the case of holes, the tolerance

involves the center axis of the hole andmust be within the imaginary cylinder

around the intended true position of

the hole.

• If toleranced feature is rectangular,

the zone involves two imaginaryplanes at a specified distance from the

ideal true position.

• Position tolerance is easy to inspect

and is often done with just a functional

gage (go / no-go gage).

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Concentricity


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Concentricity

• Concentricity is a three-dimensional tolerance.

– It relates a feature to one

or multiple datums.

– Difficult to measure!

– The shaft is measured in

multiple diameters to

ensure that they share a

common center-axis.

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Symmetry


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Symmetry

• Symmetry is much like

concentricity. – Difference is that it controls

rectangular features and involves

two imaginary flat planes, much like

parallelism. – Both symmetry and concentricity

are difficult to measure and

increase costs of inspection.

– When a certain characteristic, such

as balance, is important, thesetolerances are very effective.

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Example - Symmetry


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Example Symmetry

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Issues in Position Tolerancing


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Issues in Position Tolerancing

• Consider the following holedimensioned with coordinate

dimensions.

• The tolerance zone for the

location of the hole is as

shown.

• There exist several problems: – Two points, equidistant from

true position may not beaccepted.

– Total tolerance diagonally is.014, which may be more thanwas intended.

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2.000

. 7 5 0

Issues (Cont’d)


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Issues (Cont d)

• Consider the same hole, but add GD&T.

• Now, the actual center of the hole (axis) must lie in

the round tolerance zone. The same tolerance is

applied, regardless of the direction.

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MMC = .500 - .003 = .497

Bonus Tolerance


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Bonus Tolerance

• Material condition

modifiers giveinspectors a

powerful method of

checking shafts andholes that fit

together.

• Both MMC and LMCmodifiers allow for

bonus tolerance.

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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.

.010 M A

Example - Bonus Tolerance


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Example Bonus Tolerance

72

• 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 Tolerance 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 ? ?

Virtual Condition


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Virtual Condition

• Depending upon its intended purpose, afeature may be controlled by multiple

geometric tolerances.

• The combined effects of these factorsdetermine the clearances between mating

parts and they establish gage feature sizes.

• The collective effect of these factors is called“virtual condition.”

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Example - Virtual Condition


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Example Virtual Condition

• Regardless of its position (or

angle) the pin must lie withinthe .002 boundary.

• Tolerance for perpendicularity

allows a margin of .005.

• If the part were produced atMMC to .252 and it deviated

from perpendicularity by .005,

the total virtual size of the pin

would be.257 in relation todatum A.

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The size tolerance for the pin

(.250 ± .002) along with the location

and perpendicularity tolerances are listedin the Feature Control Frame.

Example (Cont’d)


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Example (Cont d)

• Position tolerance of .010

combined with the sizetolerance of .002 would

produce a virtual size of .262 in

relation to datums A, B and C.

• This means that an inspectiongage would have to have a hole

of .262 to allow for the

combined tolerances

• Therefore, three inspections

would be necessary in order to

check for size, perpendicularity,

and location.

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Exercise - Virtual Sizes


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Exercise Virtual Sizes

76

.192

.186

.387

.379

Calculate the virtual sizes for the indicated features.

(Answers are in red!)


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GD&T with SolidWorks


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GD&T with SolidWorks

• Among other things,

the dimension expert

(DimXpert ) tool allows

its users to work with

GD&T: – This feature has been

added to the SW after

2008.

• Lots videos areavailable on Youtube.

– Check it out!

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Summary


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Summary

• GD&T is an international design (and drafting)standard.

• Uses consistent approach and compact symbols

to define and control the features of

manufactured parts.

• Is derived from the two separate standards of

ASME Y14.5M and ISO 1101.

• Helps inspectors improve their methods byemphasizing fit, form, and function.

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ME 407

Summary (Cont d)

• Compares the physical, imperfect features of a

part to its perfect, imaginary form specified in the

design drawing.

• Controls flatness, straightness, circularity,cylindricity, and four form tolerances that

independently control a feature.

• Other tolerances, such as location, runout, and

orientation must be referenced to another datum.

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Summary (Cont d)

• The profile tolerances can define a featureindependently.

• A related datum can further define the orientation

and location.

• A series of internationally recognized symbols are

organized into a feature control frame.

• The control frame specifies the type of geometric

tolerance, the material condition modifier, andany datums that relate to the feature.

ME 407 - Introduction to GD&T (v1.3)

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Transcript of ME 407 - Introduction to GD&T (v1.3)