04 Form Tolerances

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    Interpret the f latness contro l .

    Interpret the straigh tness con trol .

    Interpret the circular ity con trol .

    Interpret the cyl ind r ic i ty contro l .

    FORM CONTROLS

    Flatness

    Straightness

    Circularity

    Cylindricity

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    FLATNESS

    ZONE OF TOLERANCE :- TWO PARALLEL PLANES

    SYMBOL :-

    STRAIGHTNESS

    ZONE OF TOLERANCE :- CYLINDER

    SYMBOL :-

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    CIRCULARITY

    ZONE OF TOLERANCE :- TWO COPLANAR CONCENTRIC CIRCLES

    SYMBOL :-

    CYLINDRICITY

    ZONE OF TOLERANCE :- TWO COAXIAL CYLINDERS

    SYMBOL :-

    t=0.3

    t=0.3

    3

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    Definition : Flatnessis the condition of a surface having all of itselements in one plane. The tolerance zone for aflatness control is three-dimensional.

    General representation

    FLATNESS

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    Interpretation of Flatness tolerance :

    It consists of two parallel planes within whichall the surface elements must lie. The distancebetween the parallel planes is equal to theflatness control tolerance value.

    FLATNESS

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    Rule #1 Effect on Flatness

    Whenever Rule #1 applies to a feature of size that consists of twoparallel planes, an automatic indirect flatness control exists for bothsurfaces.

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    Rule #1 Effect on Flatness

    When the feature of size is at MMC, both surfaces must be perfectlyflat.

    As the feature departs from MMC, a flatness error equal to the amountof the departure is allowed.

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    Flatness Control Application

    Some examples of when a designer uses flatness

    control on a drawing are to provide a flat surface: For a gasket or seal.

    To attach a mating part.

    For better contact with a datum plane.

    When these types of applications are involved, the

    indirect flatness control that results from Rule #1

    is often not sufficient to satisfy the functional

    requirements of the part surface.

    This is when a flatness control is specified on a

    drawing

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    Each two point

    measurement must

    be within the

    dimensional limits

    The size requirement

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    Rule #1 boundary requirement

    The part mustbe within a 10.6

    boundary

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    The Flatness requirement

    All of the surfaceelements must be

    within two parallel

    planes 0.2 apart

    Two parallel

    plane 0.2

    apart

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    STRAIGHTNESSDefinition :

    Straightness of a line elementis the

    condition where each line element (or axis or centerplane) is a straight line.

    The tolerance zone for a straightness control(as a surface line element control is two-dimensional).

    General Representation :

    0.05

    10.6

    10.4

    1. When applied to a surface

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    2. When applied to an axis

    3. When applied to a center plane

    10.610.4

    0.1

    10.610.4

    0.1

    13

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    Interpretation (Straightness applied to the surface element)

    The straightness controldoes not affect the formof the line element in this

    view

    Tolerance zone2 parallel lines 0.05apart

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    R l #1 Eff t S f St i ht

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    Rule#1s Effects on Surface Straightness

    Whenever Rule #1 is in effect, an automatic indirectstraightens control exists for the surface line elements.

    When the feature of size is at MMC, the line elements must beperfectly straight. As the FOS departs from MMC a straightness

    error equal to the amount of the departure is allowed.

    10.610.4

    10.6max

    Tolerance zone 2 parallel lines 0.2 apart

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    Straightness at MMC Application

    An importantbenefit becomesavailable whenstraightness isapplied at MMC:

    extra tolerance ispermissible.

    As the FOS

    departs from MMCtowards LMC, abonus tolerancebecomes available.

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    Definition: Circularity is a condition where all points ofa surface of revolution, at any Section perpendicular

    to a common axis, are equidistant from that axis.

    General representation:

    CIRCULARITY

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    A circularity controlis a geometric tolerancethat limits the amount of circularity on a part

    surface.

    It specifies that each circular element of a

    features surface must lie within a tolerance

    zone of two coaxial circles.

    It also applies independently at each cross

    section element and at a right angle to thefeature axis.

    The radial distance between the circles is equal

    to the circularity control tolerance value.

    Circularity control :

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    Circularity application:

    1. Is to limit thelobing (out of

    round) of a shaftdiameter.

    2. In certain cases,

    lobing of a shaftdiameter willcause bearingsor bushings tofail prematurely.

    3. The diametermust be within itssize tolerance.

    Tolerance zone is twocoaxial circles

    OuterBoundary

    0.3 tolerance zoneradial distance equal

    to the circularitytolerance value 19

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    Circularity application:

    4. The circularitycontrol does not

    override Rule #1.

    5. The circularitycontrol tolerancemust be lessthan the sizetolerance.

    6. The circularity

    control does notaffect the outerboundary of theFOS.

    Tolerance zone is twocoaxial circles

    OuterBoundary

    0.3 tolerance zoneradial distance equal

    to the circularitytolerance value 20

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    Cylindricity

    Definition :Cylindricityis a condition of a surface of revolution inwhich all points of the surface are equidistant from a common axis.

    General Representation :

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    A cy l indr ic i ty contro lis a geometric tolerance that limits theamount of cylindricity error permitted on a part surface.

    It specifies a tolerance zone of two coaxial cylinders within whichall points of the surface must lie. A cylindricity control appliessimultaneously to the entire surface.

    The radial distance between the two coaxialcylinders isequal tothe cylindricity control tolerance value.

    A cylindricity control is a composite control that limits thecircularity, straightness, and taper of a diameter simultaneously.

    Cylindricity control :

    Cylindricity application :

    is to limit the surface conditions (out of round, taper, and

    straightness) of a shaft diameter.

    In certain cases, surface conditions of a shaft diameter willcause bearings or bushings to fail prematurely.

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    Cylindricity application :

    The diameter must also be within its size tolerance.

    The cylindricity control does not override Rule #1.

    The cylindricity control tolerance must be less than the total size tolerance.

    The cylindricity control does not affect the outer boundary of the FOS. 23