Geometric Tolerances and Dimensions

8/17/2019 Geometric Tolerances and Dimensions

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GTDGeometric Tolerances

and Dimensions


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TOLERANCE

• Tolerance is the total amount adimension may vary and is thediference between the upper(maximum and lower (minimumlimits!

• Tolerances are used to control the

amount of variation inherent in allmanufactured parts. In particular,tolerances are assigned to mating

parts in an assembly.


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"#EN $OE% TOLERANCE%&ECO'E ')ORTANT

Assemblies: Parts will often not ttogether if their dimensions do not fallwith in a certain range of values.

Interchangeability: If a replacement part isused it must be a duplicate of the originalpart within certain limits of deviation.

The relationship between functionality andsi!e or shape of an ob"ect varies from partto part


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TOLERANCE N RELATON "T#CO%T

#ost generally increases with smaller tolerance

$ %mall tolerances cause an e&ponential increasein cost

$ Therefore your duty as an engineer have toconsider : Do you need '(.)))(in or is (.)(ingood enough*

Parts with small tolerances often re+uire special

methods of manufacturing.Parts with small tolerances often re+uire greater

inspection and call for the re"ection of parts Greater -uality Inspection Greater cost.


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#O" ARE TOLERANCE%%)EC*E$

%i!e

imits specifying the allowed variation in each

dimension /length, width, height, diameter,

etc.0 are given on the drawingGeometry

11Geometric Tolerancing

(. Allows for specication of tolerance for the

geometry of a part separate from its si!e2. GDT /Geometric Dimensioning and Tolerancing0

uses special symbols to control di3erentgeometric features of a part


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+AL,E O* TOLERANCE

• The tolerance for a single dimensionmay be specied with the dimensionand then the tolerance.

• The tolerance is total variationbetween the upper and lower limits.


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TOLERANCE% %)EC*E$*OR %-E

imit Tolerances $ /(2.456(2.25 0

Plus67inus Tolerances

$ 8nilateral Tolerances 1 /(2.)) 9 or 1&&&0

$ ilateral Tolerances 1 /(2.)) 9&&&61&&&0

These tolerance values indicate the:

77#: 7a&imum 7aterial #ondition

7#: east 7aterial #ondition


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L'T TOLERANCE%


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)L,%.'N,%TOLERANCE%


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'A/',' 'ATERAL CON$TON(''C

• It is the condition of a part when itcontains the most amount ofmaterial.

• The 77# of an e&ternal feature suchas a shaft is the upper limit.

• The 77# of an internal feature suchas a hole is the lower limit.


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LEA%T 'ATERAL CON$TON(L'C

• It is the condition of a part when itcontains the least amount of materialpossible.

• The 7# of an e&ternal feature is thelower limit of the part.

• The 7# of an internal feature is theupper limit of the part.


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TOLERANCN0

;it types

#learance t occurs when two tolerancedmating parts will always leave a space or

clearance when assembled.Interference t occurs when two toleranced

mating parts will always interfere whenassembled.

Transition t occurs when two tolerancedmating parts will sometimes be an interferencet and sometimes be a clearance t whenassembled


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0EO'ETRC $'EN%ONN0 AN$TOLERANCN0

• GDT is a method of dening parts based onhow they function, using standard A%7


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0$T

• 77#67#

• Datums

•

Geometric #ontrols• ;orm

• >rientation

•

Position


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)RNC)LE% O* $AT,' %)EC*CATON

Three perfect plans used tolocate an imperfect part.

a. Three point contact is usedon the primary plane.

b. Two point contact is used onthe secondary plane.

c. >ne point contact is used onthe tertiary plane


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$AT,' %)EC*CATON


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ONE1)LANE $AT,' RE*ERENCE *RA'E


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T"O1)LANE $AT,' RE*ERENCE *RA'E


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T#REE1)LANE $AT,' RE*ERENCE *RA'E


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%2'&OL% A$+ANTA0E%3

(. The symbol has uniform meaning.

2. %ymbols are compact, +uic@ly drawn, and canbe placed on the drawing where the controlapplies.

. %ymbols are the international language andsurmount individual language barriers.

B. Geometric tolerance symbols follow theestablished precedent of other well @nownsymbol systems, e.g., electrical and electronic,welding, surface te&ture.


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+RT,AL CON$TON

• Cirtual condition, based on 77# or 7 #

is a features e&treme boundaryE itrepresents the Fworse case

• ;or 77#, Fworse case concerns tsand6or clearances with mating parts ;or

7#, Fworst case is concerned withstrength, alignment, wall thic@ness, etc.with reference to mating parts


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+RT,AL CON$TON (''C1 )N

• Cirtual #ondition for a Pin

/ased on 7a&imum 7aterial #ondition0

4 7a&imum 7aterial #ondition 5 the%tated Position or >rientation Tolerance

•

C# 4 77# 5 Tolerance


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+RT,AL CON$TON (''C1 )N

+RT,AL CON$TON (''C


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+RT,AL CON$TON (''C1#ole0

• Cirtual #ondition for a Hole

/ased on 7a&imum 7aterial #ondition0

7a&imum 7aterial #ondition 1 the%tated Position or >rientation Tolerance

•C# 77# 1 Tolerance


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+RT,AL CON$TON (''C1 #ole


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+RT,AL CON$TON (L'C1 )N

• +irtual Condition 6or a )in

/ased on east 7aterial #ondition0 4

east 7aterial #ondition 1 the %tatedPosition or >rientation Tolerance

J C# 4 7# 1 Tolerance


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+RT,AL CON$TON (L'C1 )N


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+RT,AL CON$TON (L'C1#ole

• +irtual Condition 6or a #ole

/ased on east 7aterial #ondition0 4

east7aterial #ondition 5 the %tated Position

or >rientation Tolerance

• C# 4 7# 5 Tolerance

+RT,AL CON$TON (L'C #ole


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+RT,AL CON$TON (L'C1 #ole


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TOLERANCE CALC,LATON

• *loatin7 6astener tolerancin7 is used toconrm that loose bolts, screws or otherfasteners have the standard clearance in theirholes.

• *ixed 6astener tolerancin7 is measured thesame as with Koating fasteners e&cept that thefastener is already &ed6located on one of themating parts and the tolerance is now dividedbetween the parts.

• #ole diameter tolerancin7 is used tocalculate the 77# of the hole.


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$'EN%ON 0,$ELNE%

A minimum spacing between the ob"ect anddimensions and between dimensions must bemaintained.

A visible gap shall be placed between the end ofe&tension lines and the feature to which theyrefer.

7anufacturing methods should not be speciedas part of the dimension unless no othermethod of manufacturing is acceptable.

Placing dimensions within the boundaries of aview should be avoided whenever practicable.


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$'EN%ON 0,$ELNE%

Dimensions for materials typically manufacturedto gages or code numbers shall be specied bynumerical values.

8nless otherwise specied, angles shown ondrawings are assumed to be ?) degrees.

Dimensioning to hidden lines should be avoidedwhenever possible. Hidden lines are less clearthan visible lines.

The depth of blind, counterbored, orcountersun@ holes may be specied in a notealong with the diameter.


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$'EN%ON 0,$ELNE%

Diameters, radii, s+uares, counterbores, spot faces, countersin@s, anddepth should be specied with the

appropriate symbol preceding thenumerical value.

eader lines for diameters and radii

should be radial lines.


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THA=L

M>8.....

Geometric Tolerances and Dimensions

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