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.