16) Geometric Tolerances

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ME 216: Engineering Metrology

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Fundamentals of Geometric Tolerances

Dr. Suhas S. Joshi, Department of Mechanical Engineering, IndianInstitute of Technology, Bombay, Powai, MUMBAI 400 076 (India)

Phone: 91 22 2576 7527 (O) / 2576 8527 ; [email protected]


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Tolerance of Size and Geometry

The tolerance on Size is also called as Dimensional tolerance.

Example

But these tolerances may not be sufficient to manufacture the desired fitperfectly. So we require additional tolerances, called GeometricalTolerances. Example

0.020

0.00025 +

0.020 0.020

2

.+ .

0.040+

.+ .

0.040+

Fit is acceptable, assembly possibleParts in Correct Orientation

Fit is acceptable, assembly Not possibleParts not in correct Orientation


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Tolerance of Size and Geometry0.020

0.00025

+

0.020

0.00026 +

+

0.020

0.04025 +

+

Fit is acceptable, assembly Notpossible Parts not in correct

Shape

3

0.020

0.00025

+

0.020

0.04025 +

+

Fit is acceptable, assembly Not possibleParts not in correct Position

XY


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Tolerance of Size and Geometry

Therefore, in addition to selecting FIT, it is also important that we specify

additional tolerances so that the desired FIT is achieved. These types of

tolerances which help specify the functional requirements more clearly, arecalled as Geometrical Tolerances. These are of following three types

1. Tolerances on Shape or Form

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2. Tolerances on Orientation3. Tolerances on Position

Symbol Tolerance

Value

Reference

datum

Geometric Tolerance Representation


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Geometric Tolerances

Sr.No.

Characteristics Symbol

1. Straightness

A. Characteristics of Form/Shape

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2. Flatness3. Circularity

4. Cylindricity

5. Profile of a Line

6. Profile of a Surface


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S. N. Characteristics Symbol

1. Parallelism

2. Perpendicularity

3. An ularit

B. Characteristics of Orientation

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S.N. Characteristics Symbol

1. Position2. Concentricity/Co-axiality

3. Symmetry

4. Run out

C. Characteristics of Position


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Straightness

It is the characteristic of a line where all the elements of a line are co-linear. In general, there could be two lines, within which, all thepoints on a line lie.

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Uppermost point

Lowermost point

Tolerance onstraightness

Definition of Straightness

Two parallel planes

enclosing the surface

Surface


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25 0.020 0.01

25.030

25 0.020

Size of Tol.zone

Feature size

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FeatureSize

Tolerance Size of zone

25.020 0.01 25.030

25.010 0.02 25.030

25.000 0.03 25.030

24.990 0.04 25.030

0.01 MSymbol of Maximummaterial condition

As the feature size reducesbelow its MMC, the tolerancegoes on increasing as can beseen from the adjoining table.


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Geometric TolerancesFlatnessIt is defined as minimum distance between two planes within which all

the points on a surface lie. A surface along which all the points lie

along single plane is called as perfectly flat surface.

0.01

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Tolerance 0.01

Lowermost plane

Uppermost plane

The maximum material condition is not applicable to Flatness

Tolerance zone bound by two parallel

planes


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Geometric TolerancesCircularityIt is defined for a cylindrical or conical surfaces. It defines the distance

between the surface and its axis.

Ideally, all points on a surface (at a cross-section), should be equidistant fromthe axis for the cross-section to be perfectly circular.The tolerance on circularity is defined by two concentric circles within which a

surface can lie. The distance between two the concentric circles is calledtolerance.

10

25 0.020 0.01

0.01

25.030Maximum Limit

of Tol. zone

25.010

The maximum material condition is not applicable to Circularity

Givensurface

Tolerancezone

Min. limit of Tol. Zone.


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Geometric TolerancesProfile of Line or SurfaceIt is defined for a line of any shape of surface of any shape. It defines the

distance between two lines or surfaces of the same shape as that of the lineor surface in question, separated by a distance equal to tolerance.

0.010.01

12

Profile of a line

Profile of a surface


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Geometric TolerancesPerpendicularityIt is defined for a feature (like surface or line) with reference to another

feature called reference. It defines the distance between two lines or

surfaces that are parallel to each other and perpendicular to the datumsurface and encompass the line or surface in question.

Perpendicularity of a Line with Surface as datum

Given axis25 0.020

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0.01

Datum

Tolerance zone

A

0.01 A

Maximum size of the part = Maximum size permitted by the dimensionaltolerance (25.020) + geometrical tolerance (0.01) = 25.03 mm

Minimum size of the part = Minimum size permitted by the dimensionaltolerance (24.98) - Geometrical tolerance (0.01) = 24.97 mm


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A

Perpendicularity of a Surface with Line as a datum

0.01Datum

Given plane


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0.01 A Tolerance zone: Twoparallel planes


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Perpendicularity of a Surface with Surface as a datum

0.01 A

0.01

Given plane0.040.04

15


15

A

Tolerance zone: Twoparallel planes

Datum plane

Maximum size of the part = Maximum size permitted by the dimensional tolerance(15.040) + geometrical tolerance (0.01) = 15.05 mm

Minimum size of the part = Minimum size permitted by the dimensional tolerance

(14.96) - Geometrical tolerance (0.01) = 14.95 mm


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ParallelismIt is defined for a feature (like surface or line) with reference to another

feature called reference. It defines the distance between two lines orsurfaces that are parallel to each other and parallel to the datum surface

and encompass the line or surface in question.

Parallelism of a Line with Line as datum

0.01 A0.01


16A

0.040.04

10

Datum line

Tol. zone


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0.01 A

0.04

0.01

Parallelism of a Line with Line as a datum


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Max. distance between holes = Max distance permitted by the dimensional tolerance(70.040) + (1/2) Geometrical tolerance (0.005) = 70.045 mm

Min. distance between holes = Min. distance permitted by the dimensional tolerance

(69.96) (1/2) Geometrical tolerance (0.005) = 69.955 mm

0.04

0.0410

A

.

0.04

0.04 ..


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Parallelism of a Line with Surface as a datum

0.01 A

0.01


18A

0.01 A

Datum Surface

0.01

ADatum Surface


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0.01 A

Parallelism of a Surface with Surface as a datum

0.01

Given axis


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A

Datum Surface

Tolerance zone


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AngularityIt is defined for a feature (like surface or line) with reference to another

feature called reference. It defines the distance between two lines orsurfaces that are at an angle to the datum surface and encompass the

line or surface in question.

0.01 A

10

0.01


20

600

A

600

Given axis of hole

Tolerance zone


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PositionIt defines the perfect (exact) location of a point, line or a surface in relation to

the other datum

Position of a line

15

0.01 0.01


21

20

15

20

0.01

A

A

Position of a line with surface as a datum Given axis

0.01

Datum plane


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ConcentricityIt defines the position of an axis in relation to the other datum axis. It defines

a cylinder which coincides with the datum axis and of diameter given bythe geometrical tolerance.


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Cylinder concentricwith the datum axis

A

0.01 A

DatumAxis

0.01

Given axis


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Run outIt defines the deviation from the desired form and orientation during one full

rotation of the part on the datum axis.

0.010.01 A

DatumAxis

Datum axis0


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Givensurface

Tolerance zone bound by two

concentric cylinders

A

16) Geometric Tolerances

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