epm212 - chapter 9 slides gd t handout

8/16/2019 EPM212 - Chapter 9 Slides GD T Handout

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Chapter 9. Geometric Dimensioning and

Tolerancing (GD&T) – An Introduction

Learning Outcomes

At the end of this topic you should be able to:

• Recognize various symbols used in GD&T

• Explain the terms maximum material condition

(MMC) and least material condition (LMC)

• Explain the advantages of GD&T


Tolerancing (GD&T)

9.1 Introduction

Engineering

DesignProduct

Part

Inspection

What is to be

manufactured

What has beenmanufacturedCompare product

with design


Tolerancing (GD&T)

9.1 Introduction

Engineering

DesignProduct

Part

Inspection

GD&T

• GD&T binds the three elements (engineeringdesign, the product and part inspection together)

Activity

List the main factors that determine the

manufacturing cost of the stepped shaft shown:

9.2 Meaning of tolerance

• Tolerance is the amount by which a dimension is

allowed to vary

• Tolerances are applied to both position and size:

50 ± 0.5

25 ± 0.1

Tolerance on size

Tolerance on position


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9.3 GD&T

• GD&T is a method of dimensioning and tolerancing

a drawing with respect to the actual function or

relationship of part features that can be most

economically produced

9.3 GD&T

• GD&T is a method of dimensioning and tolerancing

a drawing with respect to the actual function or

relationship of part features that can be most

economically produced

• GD&T is used when:

(a) Features are critical to functionality of part

(b) Datum references are required to ensure

consistency between design, manufacturing

and inspection

“Proper application of GD&T willensure that the allowable part andassembly geometry defined on thedrawing leads to parts that have thedesired form and fit (within limits) andfunction as intended.”

From Wikipedia, the free encyclopedia

9.4 Benefits of GD&T

• GD&T adds clarity to conventional coordinate

dimensioning

• Universal symbols are used to:

i) convey design intent to remote

manufacturing or assembly sites

ii) provides a common standard for

dimensioning practices

iii) enhance repeatability of part orientation

iv) increases interchangeability of parts

• Traditional Cartesian coordinate system creates

square tolerance zone, e.g. tolerance for hole

center:

20 ±±±± 1

20 ±±±± 1

• Permissible machining limits:


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• Location tolerance of ±1 creates tolerance zone for

hole center of 2 mm:

2

2

20

20

Only hatched space is withintolerance

But, shaded areas are also

within same distance from

center

Unnecessary restriction becomes much greater

• Consider what happens when two holes are involved: • GD&T provides a diametrical (circular) tolerance

zone:2

2

9.5 Definitions

• Feature: Physical portion of a part, e.g. hole,

surface, slot etc.

• Datum: Theoretically exact plane, point or axis

from which a dimension is measured

• Datum feature: Part feature that contacts the

datum

• Datum reference frame: Set of three mutually

perpendicular datum planes:

First datum plane(primary plane)

Second datumplane

(secondaryplane)

Third

datum

plane(tertiary

plane)


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• Feature of size: One cylindrical or spherical surface

or a set of two opposed elements or opposed parallel

surfaces associated with a size dimension

9.5 Definitions

• Language of GD&T is a set of symbols, divided into

five types of dimensioning control:

- form tolerance- profile tolerance

- orientation tolerance

- location tolerance

- runout tolerance

• Form tolerance: States how far an actual surface or

feature is allowed to vary from the desired form on

the drawing

18 mm 17.89 mm

??0.055 mm


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Example where

flatness control can

be applied:

• Profile tolerance: States how far an actual surface

or feature is allowed to vary from the desired form on

the drawing or vary relative to a datum

• Form tolerance for lines: Profile

0.02

0.02


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• Orientation tolerance: States how far an actual

surface or feature is permitted to vary relative to a

datum

• Location tolerance: States how far an actual size

feature is permitted to vary from the perfect location

implied by the drawing as related to a datum or other

feature

• Runout tolerance: States how far an actual surface

or feature is permitted to vary from the desired form

implied by the drawing during full 360° rotation of

the part on a datum axis


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9.6 Material conditions

• Maximum material condition (MMC): Condition in

which a feature of size contains the material within

its stated tolerance limits (Symbol )

e.g. maximum material condition for pin:

MMC for pin is φ6.15

e.g. maximum material condition for hole:

MMC for hole is φ6.25

• Least material condition (LMC): Condition in which

a feature of size contains the least amount of

material within its permissible limits (Symbol ), e.g.

minimum shaft diameter or maximum hole diameter.

LMC for hole is

φ6.35

• Regardless of Feature Size (RFS): Geometric

tolerance that applies at any increment of size of

feature within its permissible limits

- RFS is implied on all geometric tolerances, unless

indicated by the presence of a modifier

9.7 Feature control frame

• A feature control frame is used to specify

geometric tolerances on a drawing

• The feature control frame is a rectangular box that

contains the geometric symbols, modifiers and

datum references


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• Examples of different feature control frames:

• Examples of different feature control frames (ctd.): • Form tolerance for lines: Circularity

0.02

0.02

• Form tolerance for surfaces: Flatness

0.05

0.05

• Form tolerance for surfaces: Cylindricity


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• Form tolerance for surfaces profiles

• Orientation tolerance: Parallelism

• Orientation tolerance: Perpendicularity

Describe the type of tolerance control shown in

the figure

ACTIVITY 1


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Describe the type of tolerance control shown in the

figure

ACTIVITY 2

• Orientation tolerance: Angularity

ACTIVITY 3

The language of GD&T is a set of symbols

divided into five types of dimensioning control.

Name the five types of dimensioning control.


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STRAIGHTNESS

‘Straightness is the condition where

all the points on a surface or an axis

are in a straight line. A straightness

tolerance specifies a zone within

which the surface or axis must lie. Inthe example the zone is bounded by

two parallel lines 0.03 mm apart.

When a diameter symbol is added to

the tolerance the derived axis of thefeature must lie within a cylindrical

tolerance zone of 0.03 mm diameter.

When a MMC modifier is added, the

tolerance zone is 0.03 diameter at

18mm diameter and the zone

increases as the feature decreases

from MMC.’

http://www.actphx.com/gd_and_t/gd_and_t.html FLATNESS

‘Flatness is the condition

of a surface having all

points in one plane.

A flatness tolerancespecifies a zone defined

by 2 parallel planes. In the

example shown thesurface must lie between 2

parallel planes 0.18mm

apart and the surface must

be within the specified size

limits.’

(http://www.actphx.com/gd_and_t/gd_and_t.html)

CIRCULARITY (ROUNDNESS)

‘Circularity or roundness is the

condition where all the points

on a surface are in a circle.

Circularity tolerance specifies a

zone bounded by 2 concentric

circles within which the

measured surface must lie.

In the example shown each

circular element must lie

between 2 concentric circles,

one having a radius 0.05mm

larger than the other. Each

circular element of the surface

must also be within the

specified limits of size.’


CYLINDRICITY

‘Cylindricity is the condition

where all points of a surface

of revolution are equidistant

from a common axis.

Cylindricity tolerance specifies

a zone bounded by 2

concentric cylinders within

which the measured surface

must lie.

The tolerance applies

simultaneously to both circular

and longitudinal elements.’


OPEN PROFILE (LINE)

‘A profile is the outline of an

object in a given plane. The

tolerance zone established by the

profile of a line tolerance is two

dimensional extending along the

length of the considered feature.

In this example each line elementat any cross section must lie

between 2 profile boundaries

0.6mm apart in relation to the

datum plane A. Additionally the

surface must be within any

specified limits of size. The

tolerance zone may be specified

to be unilaterally disposed either

inside or outside the true profile’


CLOSED PROFILE (SURFACE)

‘The tolerance zone established by

the profile of a surface tolerance is

three dimensional extending along

the length and width (or

circumference) of the considered

feature.

In the example all points on thesurface must lie between 2 profile

boundaries 0.6mm apart in relation

to datum plane A. Additionally the

surface must be within and

specified size limits.’



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ANGULARITY

‘Angularity is the condition of

a surface or axis at a specified

angle (other than 90º) from a

datum plane or axis.

The tolerance zone is defined

be 2 parallel planes at the

specified basic angle from a

datum plane or axis. The

surface or axis of the

considered feature must lie

within this zone. In the

example all points of the

surface must lie within the 0.5

mm wide tolerance zone.’


PERPENDICULARITY

Perpendicularity is the

condition of a surface or axis at

a right angle to a datum plane

or axis.

1) A zone defined by 2 parallel

planes perpendicular to a

datum plane or axis. In the

example shown, the surface of

the feature must lie within this

zone which is 0.15 mm wideand at right angles to datum A.


2) A zone defined by 2 parallel

planes perpendicular to the

datum axis. In this example,

the axis of the hole must lie

within the zone which is 0.3

mm wide and at right angles to

datum axis A. The feature axis

must also be within the

tolerance of location.

PERPENDICULARITY (ctd.)


3) A cylindrical tolerance zone

perpendicular to a datum

plane. In this example the axis

of the part must lie within a

cylindrical zone of 0.05 mm

diameter at right angles to

datum A.

This tolerance applies at the

maximum diameter of 14.984

mm (MMC). As the feature size

decreases from MMC, the

perpendicularity tolerance zone

is increased a corresponding

amount. The feature axis must

also be within the tolerance of

location.





4) A zone defined by 2 parallel lines

perpendicular to a datum plane or

axis. In the example shown, each

radial element of the surface must lie

within this zone 0.05mm wide and at

right angles to datum A.

The surface must also be within thespecified limits of size.




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PARALLELISM

The surface of the feature must lie within 2 planes 0.25

mm apart which are parallel to datum A.


figure

ACTIVITY 4


figure

ACTIVITY 5

The tolerance applies at the minimum diameter of 9.000

(MMC). As the feature size increases from MMC, the

parallelism tolerance zone is increased a correspondingamount.

TRUE POSITION

If position tolerances are to be modified as features depart from

maximum material condition, the MMC modifier must be specified on the

drawing.

A positional tolerance defines a zone within which the center, axis or

center plane of a feature of size is permitted to vary from the true (exact)

position. Basic dimensions establish the true position.

In the example shown, the center of the holes must lie within circles of

0.5 mm diameter when the holes are at 10.25 mm diameter.

As the diameter of the holes increases to 10.5 mm diameter, the

tolerance zones increase proportionately to 0.75 mm diameter.



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CONCENTRICITY CONCENTRICITY

CONCENTRICITY

Concentricity is the condition

where the axes of all cross

sectional elements of a surface

of revolution are common to the

axis of a datum feature.

Concentricity tolerance

specifies a cylindrical tolerance

zone whose axis coincides with

the datum axis.

In this example, the zone has a

diameter of 0.2mm and the

feature axis must lie within thiszone.


CIRCULAR RUNOUT

Runout is a composite tolerance used to control the

relationship of one or more features to a datum axis. The

illustration shows the types of features that can be

controlled by runout tolerances.


Circular runout provides control of circular elements of a

surface. It can be used to control the cumulative variations

of circularity (roundness) and coaxiality.


In the example shown, each circular element of the surfaces

toleranced must fall within 0.04mm (Full Indicator Movement)

when the part is rotated 360º about the datum axis.



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TOTAL RUNOUT

Total runout provides composite

control of all surface elements.

For surfaces around a datum

axis, including:

• Circularity (Roundness)

• Straightness

• Coaxiality

• Angularity

• Taper

• Profile of a Surface


For surfaces perpendicular to a datum axis it includes:

• Perpendicularity

• Flatness

In the example shown, the entire surface must lie within the 0.04mm

wide (Full Indicator Movement) tolerance zone when the part isrotated 360º about the datum axis.


Activity 5Name the following symbols:

epm212 - chapter 9 slides gd t handout

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