Tolerancing and Fits

8/10/2019 Tolerancing and Fits

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Tolerances


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Tolerance The total amount a specific dimension can

vary. (ANSI Y14.5M1!!4"

Tolerances are assi#ned so that any t$omatin# parts $ill fit to#ether.

%i#hly accurate parts are e&tremelye&pensive' so tolerances should e as

#enerous as possile $hile still maintainin#proper function for the part.


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Limit Dimensions

)imit dimensions specify the upper and lo$er value.An acceptale part may e at the upper limit' lo$er limit' or

any value in et$een.


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Allowance & Fit

The minimum clearance space or ma&imum interference is

the allowance. Fitis the ran#e of ti#htness or looseness

et$een parts for their function.


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Types of fit *learance fit+internal memer al$ays has space orclearance.

Interference fit+internal memer is al$ays lar#er and has to

e forced inside e&ternal memer.

Transition fit+either clearance or interference.

)ine fit+clearance or surface contact results.


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Basic Hole System %ole $ill e machined

$ith a standard si,edtool.

-etermine type of fitnecessary. se fit taleor other$ise determineallo$ance.

Apply tolerances usin#hole si,e as asicdimension.


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Basic Shaft System )ess commonly

used than asic holesystem.

Most commonlyused $hen manyparts $ill fit on a

standard shaft. Shaft si,e is asic

dimension.


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Plus/minus dimensioning /asic si,e of dimension is #iven $ithtolerances e&pressed as a plus0minus ran#e.


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!ual Bi"lateral Tolerances


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Baseline #s$ %ontinued /aseline tolerances dont 2stac3up

*ontinued (chained" tolerances 2stac3

up


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etric Tolerance Terminology


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Basic Hole ethod' T()% The 1 InternationalTolerance (IT" #radesvary accordin# to theasic si,e.

% indicates tolerance isfor hole asis. 6 is forshaft asis.


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*eometric Dimensioning & Tolerancing

uses standard symols to indicate tolerancesthat are ased on the features #eometry.

sometimes called feature ased dimensionin#7 tolerancin# or true position dimensionin# 7tolerancin#

uses feature control frames to indicatetolerance


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* D & T Sym+ols


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,nderstanding Tolerance -ones Traditional plus0minus type of tolerancin# descries a

s8uare ,one for acceptale locations.

9-7T descries a ,one around the theoretically e&act

location for the feature.


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Basic Dimension A theoretically e&act

dimension used to

locate features in9-7T

/asic dimensions

are untoleranced.


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Datums -atums are features

on the o:ect that are

used as reference

surfaces from $hichother measurements

are made.

Not every 9-7Tfeature re8uires a

datum.


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%ylindrical Tolerance -one


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a.imum aterial %ondition Ma&imum material condition for a hole feature isthe smallest si,ed hole.

Ma&imum material condition for a shaft feature is

the lar#est shaft. The ma&imum material condition modifier in a

feature control frame means that the toleranceapplies at ma&. material condition. If the actual

hole is lar#er than the smallest allo$ale hole' aonus tolerance can e added to the statedtolerance value.


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a.imum aterial %ondition


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Traditional methods for tolerancin# an#les re8uire that an#led surfacese very accurate near the verte& of the an#le' ut can vary morealon# the len#th of the an#led feature.

Angular Tolerances


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*D&T Angular Tolerance -ones


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%oncentricity


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Summary There are several methods you can use to

specify tolerances in a dra$in#.

)imit tolerances #ive the upper and lo$erran#e for a dimension

;lus0minus tolerances #ive the nominal orasic si,e and a plus and0or minus ran#e for

the tolerance. 9-7T uses symols to descrie tolerance

,ones ased on #eometric features.

Documents

Tolerancing and Fits