1.1

GEOMETRICAL DIMENSIONING AND TOLERANCE (GD&T)

5

1.2

1. INTRODUCTION

2. EFFECT OF GD&T

3. SPECIFY GD&T

4. BASIC DEFINITIONS

5. GEOMETRIC SYMBOLS

6. FORM TOLERANCE

7. PROFILE TOLERANCE

8. ORIENTATION TOLERANCE

9. RUN OUT TOLERANCE

10. MODIFY SYMBOLS

11. BONUS TOLERANCE

12. SOLVED EXAMPLE

CONTENT 5

1.3

DEFINITION

• Defined as the maximum permissible overall variation of form or

position of a feature.

• The objectives of GD & T are

– To specify the required accuracy in controlling the form of a feature

– To ensure correct functional positioning

– To ensure interchangeability of components

– To facilitate the assembly of mating parts.

5.1

1.4

WHY GD&T IS NECESSARY

Drawing that does not use GD&T Manufacturing part of the drawing

5.2

1.5

Drawing with GD&T

5.2

1.6

HOW DOES GD & T ACHIEVE

• A simple four steps to achieve GD & T is :

– Identify part surfaces to serve as origins and provide specific rules

explaining hoe these surfaces establish the starting point &

direction for measurement.

– Convey the nominal distances & orientations from origins to other

surfaces.

– Establish tolerance zones for specific attributes of each surface

along with specific rules for conformance

– Allow dynamic interaction b/n tolerances where appropriate to

minimize tolerance.

5.1

1.7

SPECIFYING GEOMETRICAL TOLERANCE 5.3

Feature Control Frame Placement :

(a) Place the frame below or attached to a leader-directed callout or dimension

pertaining to the feature.

(b) Run a leader from the frame to the feature.

(c) Attach either side or either end of the frame to an extension line from the

feature, provided it is a plane surface.

(d) Attach either side or either end of the frame to an extension of the dimension

line pertaining to a feature of size.

1.8

DEFINITIONS

• Datum

– It is a theoretically exact geometric reference to

which the tolerance features are related.• Datum feature

– A datum feature is a feature of a part, such as an edge, surface or a hole

which forms the basis for a datum or is used to establish its locations.• Datum triangle

– The datum are indicated by a leader line, terminated in a filled or open

triangle.• Datum letter

– To identify a datum for a reference purposes, a capital letter is enclosed in a

frame, connected to the datum triangle.

5.4

1.9

Datum Plane• Positioning the part with relation to three mutually perpendicular planes.

5.4

1.10

INDICATION OF FEATURE CONTROLLED

Correct

Surface only

Axis or median plane

5.4

CorrectCorrect

Correct In Correct

1.11

INDICATION OF DATUM FEATURE 5.4

1.12

• Maximum material condition (abbreviated MMC)

Is the condition in which a feature of size contains the maximum amount

of material within the stated limits of size.

MMC = smallest allowable hole or

the largest allowable shaft, relative to the stated size limits.

• Least material condition (abbreviated LMC)

Is the condition in which a feature of size contains the least amount of

material within the stated limits of size.

LMC = largest allowable hole or

the smallest allowable shaft , relative to the stated size limits.

THE TWO MATERIAL CONDITIONS 5.4

1.13

Symbols representing geometric characteristics

GDnT.exe

5.4

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1.14

CLASSIFICATION OF GD&T

1. Form tolerances

2. Profile tolerances

3. Orientation tolerances

4. Location tolerances

5. Run-out tolerances

5.4

1.15

FORM TOLERANCES

• A form (only) tolerance is specified on the drawing using a feature

control frame displaying one of the four form (only) characteristic

symbols, followed by the tolerance value for simple and planar features.

They are 4 characteristics :

1. Straightness Tolerance

2. Flatness Tolerance

3. Circularity Tolerance

4. Cylindricity Tolerance

5.5

1.16

STRAIGHTNESS TOLERANCE – line Elements

– The tolerance zone for controlling errors of straightness is the area b/n the

two parallel lines and the tolerance value is the distance b/n these lines.

– Tolerance feature control frame is placed as leader directed to a feature

surface or attached to an extension line of a feature surface. MMC/LMC

modifiers are prohibited.

5.5.1

1.17

STRAIGHTNESS TOLERANCE – Cyl Features

– The Straightness tolerance feature control frame placed accordingly

associated with a diameter extension and have the requirement for perfect

form at MMC with a separate tolerance controlling the overall straightness

of the cylindrical feature.

5.5.1

1.18

TOLERANCE OF FLATNESS

– The tolerance zone for controlling errors of flatness is the space b/n two

parallel planes and the value is the distance b/n these planes.GDnT.exe

5.5.2

1.19

TOLERANCE OF ROUNDNESS

The tolerance zone for controlling errors of roundness is the annular area

b/n two coplanar circles, and the tolerance value is the radial distance b/n

these circles.

GDnT.exe

5.5.3

1.20

TOLERANCE OF CYLINDRICITY

The tolerance zone for controlling errors of cylindricity is the annular

space b/n two perfect cylindrical surfaces lying on the same straight axis,

and the tolerance value is the radial distance b/n these surfaces.

GDnT.exe 5.5.4

1.21

PROFILE TOLERANCES

• A profile (only) tolerance is specified on the drawing using a feature

control frame displaying one of the profile characteristic symbols, followed

by the tolerance value for profile features such as automobiles, airplanes,

and ships are replete with parts having non planar, non cylindrical, non

spherical Features.

• The profile tolerance zone is generated by offsetting each point on the basic

profile in a direction normal to the basic profile at that point.

The 2 Characteristics are

1. Profile of a line

2. Profile of a surface

5.6

1.22

PROFILE FEATURE CONTROL FRAME 5.6.1

1.23

PROFILE OF A LINE 5.6.1

1.24

PROFILE OF A SURFACE

GDnT.exe

5.6.2

1.25

ORIENTATION TOLERANCES

• Orientation is a feature’s angular relationship to a DRF.

• Thus, an orientation tolerance is useful for relating one datum feature to

another and for refining the orientation of a feature already controlled with

a positional tolerance.

There are 3 orientation characteristics.

1. Parallelism

2. Perpendicularity

3. Angularity

5.7

1.26

The tolerance zone for controlling errors of parallelism is the area b/n two parallel lines or the space b/n the parallel planes which are parallel to the datum feature. The tolerance value is the distance b/n the lines or planes.

TOLERANCE OF PARALLELISM 5.7.1

1.27

TOLERANCE OF PERPENDICULARITY

The tolerance zone for controlling errors of parallelism is the area b/n two

parallel lines or the space b/n the parallel planes which are perpendicular to the

datum feature. The tolerance value is the distance b/n the lines or planes.

GDnT.exe

5.7.2

1.28

TOLERANCE OF ANGULARITY

The tolerance zone for controlling errors of angularity is the area b/n two

parallel straight lines, which are inclined to the datum feature at a specified

angle. The tolerance value is the distance, separating the lines or planes

GDnT.exe 5.7.3

1.29

APPLICATION OF ORIENTATION TOLERANCES 5.7.3pa

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1.30

Tolerance of location for related features

• It was customary to control the location of a feature on a part by specifying for

each direction a nominal dimension accompanied by plus and minus tolerances.

The 3 location tolerances are

1. Position Tolerance

2. Concentricity Tolerance

3. Symmetry Tolerance

5.8

1.31

POSITION TOLERANCE

• A positional tolerance is

specified using a feature control

frame displaying the “position”

characteristic symbol followed

by a compartment containing the

positional tolerance value

followed by the MMC or LMC

modifying symbol.

• The tolerance compartment

followed by 1 or 2 or 3 separate

compartments containing datum

reference letter.

5.8.1

1.32

TOLERANCE OF CONCENTRICITY

The tolerance zone for controlling errors of concentricity is a circle or cylinder

within which the centre of the controlled feature is to be contained. The tolerance

value is the diameter of the tolerance zone

5.8.2

1.33

TOLERANCE FOR SYMMETRY

• Symmetry is the correspondence in size, contour, and arrangement of part surface

elements on opposite sides of a plane, line, or point.

• A symmetry tolerance is specified using a feature control frame displaying the

characteristic symbol for symmetry about a plane. The feature control frame

includes the symmetry tolerance value followed by one, two, or three datum

planes or features.`

5.8.3

1.34

TOLERANCE OF RUN OUT

• Variation in the surface elements of the round feature to the axis is run out.

Run out tolerances are Two. They are :

1. Circular Run out

2. Total Run out

5.9

1.35

The tolerance zone is limited within any plane, perpendicular to the axis by two

concentric circles a distance apart.

Circularity + Concentricity = Circular Run out.

TOLERANCE OF CIRCULAR RUN OUT

Circular run out can be applied to any

feature that is nominally cylindrical,

spherical, to radial, conical, or any

revolute having round cross sections

5.9.1

1.36

TOLERANCE OF TOTAL RUN OUT

• The tolerance zone is limited for any measuring point, by two circles, a

distance apart.

Cylindricity + Concentricity = Total run out

GDnT.exe 5.9.2

1.37

MODIFYING SYMBOLS

The basic dimensions are denoted as 2.25

5.10

1.38

MODIFIER SYMBOLS FOR FEATURE OF SIZE

Each geometric tolerance for a feature of size applies in one of the

following three contexts:

1. Regardless of Feature Size (RFS), the default

2. Modified to Maximum Material Condition (MMC)

3. Modified to Least Material Condition (LMC)

5.10

1.39

REGARDLESS OF FEATURE SIZE (RFS) 5.10

A geometric tolerance applied to a feature of size with no modifying symbol

applies RFS. A few types of tolerances can only apply in an RFS context. RFS

establishes a central tolerance zone, within which a geometric element derived

from the feature shall be contained.

1.40

AT MAXIMUM MATERIAL CONDITION (MMC) 5.10

According to this modified symbol A

feature MMC size limit spline shall

be perfectly formed. That is invokes

a boundary to perfect form at MMC.

The feature does not require the

LMC boundary to have a perfect

form. This modified symbol we must

obtain a clearance Fit.

1.41

LEAST MATERIAL CONDITION (LMC) 5.10

This modified symbol is used where a geometry tolerance is necessary to ensure

an adequate skin of part material in or feature of size, rather than a clearance fit.

In such application the feature LMC spine shall be perfectly formed and the Mmc

boundary need not be perfect form.

1.42

AT BOTH MNC & LMC 5.10

There are some cases where a feature of size is associated with an MNC modifier

in one context and an LMC modifier in another Context.

1.43

BONUS TOLERANCE

• Bonus tolerance - a unit value to be added to the specified geometric tolerance.

• When True Position (positional location) is called out on a drawing, the

tolerance is applied at LMC (least material condition), MMC (maximum

material condition), or RFS (regardless of feature size).

• If LMC or MMC is listed, we can calculate a wider tolerance, or bonus,

depending on the actual size of the measured feature

5.11

1.44

In our example, we’ll use an inside feature. Our drawing states we have a diameter

tolerance of 1.5000" +0.0005" (so our MMC would be 1.5000").

In addition, we have a positional location tolerance of 0.001" applied at MMC.

1.5000" (diameter at MMC) = 0.001" (location)

But if we were to add

1.5000" + 0.0003" (diameter) = 0.001" + 0.0003" (location)

1.5003" (diameter) = 0.0013" (location)

5.11

1.45

STEPS TO DETERMINE THE BONUS TOLERANCE

AT RFS : No bonus to location tolerance zone.

AT MMC: Bonus tolerance equals the arithmetic difference between the feature’s

actual mating size and its specified MMC size limit.

AT LMC: Bonus tolerance equals the arithmetic difference between the feature’s

actual minimum material size and its specified LMC size limit.

Steps for determine Bonus Tolerance:

1. Determine the size of the hole at MMC or LMC.

2. Measure the feature on the machined part.

3. Subtract measured hole size from hole at MMC or LMC. i.e Bonus tolerance.

4. Determine the positional tolerance of feature.

5. Add the values for 3 and 4.This gives total tolerance.

5.11

1.46

EXAMPLE FOR BONUS TOLERANCE 5.11

1.47

SOLVED EXAMPLE FOR GD&T

A non GD&T Automobile wheel Rotor

5.12

1.48

Solution : Applying GD&T to the wheel as following.

.Mounting face as to be flat as 0.005 and considered as Datum feature A. The profile tolerance to the mounting face as given as 0.02 w.r.t datum A Another critical face i.e ¢ 11.00 is explicitly required to be parallel to A within .003 The profile tolerance for this face has to given w.r.t a as 0.02. as this can considered

as datum C from which profile of ¢ 7.0 bore is defined. The step diameter of ¢ 6.0 is required to be parallel to A with in 0.03. The perpendicularity of the ¢5.50 bore is directly controlled to the mounting face,

i.e datum A. Now the ¢ 5.50 bore can be labeled datum feature B and provide an unambiguous origin from which the¢.515 bolt holes and other rounded parts are located.

The profile tolerance to theφ 5.50 bore as given as 0.02 w.r.t datum A The ¢ 7.0 bore is with position with the datum B i.e ¢ 5.50 bore and datum A The position tolerance for ¢ 7.0 bore has to be given ie about datum C as 0.06. The ¢ o.550 holes ha to positioned w.r.t datum B and A. The depth ¢ 7.0 bore and ¢ 5.50 are denoted as basic dimensions.

5.12

1.49

Wheel after GD&T 5.12

1.50

ANY QURIES