5dimensioning and tolerancing

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    Dimensioning and Tolerancing

    Ahmad Mirabadi

    School of Railway Engineering

    Iran University of Science andTechnology

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    ANSI Y14.5-1994 Standard

    This standard establishes uniform practices for

    defining and interpreting dimensions, and

    tolerances, and related requirements for use on

    engineering drawings.

    The figures in this presentation

    are taken from Bruce Wilsons

    Design Dimensioning and Tolerancing.

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    What is a good level of tolerance?

    Designer:

    tight tolerance is better(less vibration, less wear, less noise)

    Machinist:

    large tolerances is better(easier to machine, faster to produce,

    easier to assemble)

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    The type offit between mating features

    Designer needs to specify:

    basic diameter and the tolerance of shaft: Ss/2

    basic diameter and the tolerance of hole:Hh/2

    Allowance: a = DhminDsmax.

    Tolerancing application: an example

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    Fit Between Parts

    Clearance Fit Interference Fit Transition Fit

    Clearance fit: The shaft maximum diameter is smaller than the hole minimum diameter.

    Interference fit: The shaft minimum diameter is larger than the hole maximum diameter. Transition fit: The shaft maximum diameter and hole minimum have an interference fit, while the shaft

    minimum diameter and hole maximum diameter have a clearance fit

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    Classes of Fit

    The limits to sizes for various types of fit of mating parts are defined by the standard ANSI B4.1

    There are five basic classes of fit:

    1. Running and sliding clearance (RC)--there are type of RC fits, RC1-RC9;

    2. Location clearance (LC)--there are eleven types of LC fits;

    3. Location transition (LT)--there are six types of LT fits;4. Location interference (LN)--there are three LN fits;

    5. Force fits (FN)--there are five FN fits.

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    Unilateral and Bilateral Tolerances:

    unilateral

    bilateral

    1.00 0.05+-

    nominal dimension

    tolerance

    0.95+ 0.10- 0.00 1.05

    + 0.00- 0.10

    1.00 0.05+-

    0.95 - 1.05means a range

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    Overview of Geometric

    Tolerances

    Geometric tolerances define the shape of a feature as opposed to its size.

    We will focus on three basic types of dimensional tolerances:

    1. Form tolerances: straightness, circularity, flatness, cylindricity;

    2. Orientation tolerances; perpendicularity, parallelism, angularity; and

    3. Position tolerances: position, symmetry, concentricity.

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    Feature Control Frame

    A geometric tolerance is prescribed using a feature control frame.It has three components:

    1. the tolerance symbol,

    2. the tolerance value,

    3. the datum labels for the reference frame.

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    DIMENSIONING

    Requirements

    1. Unambiguous

    2. Completeness

    3. No redundancy0.83 ' 0.95 ' 1.22 '

    3.03 '

    Redundant dimensioning

    0.83 ' 1.22 '

    3.03 '

    1.72 '

    0.86 '

    Adequate dimensioning

    Incompletedimensioning

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    TOLERANCING

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    "TOLERANCE IS ALWAYS ADDITIVE"

    What is the expected dimension and tolerances?

    d = 0.80 +1.00 + 1.20 = 3.00

    t = (0.01 + 0.01 + 0.01) = 0.03

    0.80 ' 0.01 1.20 ' 0.01

    1.00 ' 0.01

    ?

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    Maximum x length = 3.01 - 0.79 - 1.19 = 1.03Minimum x length = 2.99 - 0.81 - 1.21 = 0.97

    Therefore x = 1.00 0.03

    0.80 ' 0.01 1.20 ' 0.01

    3.00 ' 0.01

    ?x

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    Order of Precedence

    The part is aligned with the datum planes of a reference frameusing 3-2-1 contact alignment.

    3 points of contact align the part to the primary datum plane;

    2 points of contact align the part to the secondary datum plane;

    1 point of contact aligns the part with the tertiary datum plane

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    Straightness of a shaft

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    Straightness of a Shaft A shaft has a size tolerance defined for its fit into a hole. A shaft meets this tolerance if at every point

    along its length a diameter measurement fall within the specified values.

    This allows the shaft to be bent into any shape. A straightness tolerance on the shaft axis specifies the

    amount of bend allowed.

    Add the straightness tolerance to the maximum shaft size (MMC) to obtain a virtual

    condition Vc, or virtual hole, that the shaft must fit to be acceptable.

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    Straightness of a Hole

    The size tolerance for a hole defines the range of sizes of its

    diameter at each point along the centerline. This does not

    eliminate a curve to the hole.

    The straightness tolerance specifies the allowable curve to the

    hole.

    Subtract the straightness tolerance from the smallest hole size

    (MMC) to define the virtual condition Vc, or virtual shaft, that

    must fit the hole for it to be acceptable.

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    Straightness of a Center Plane

    The size dimension of a rectangular part defines the range of sizes at any cross-section.

    The straightness tolerance specifies the allowable curve to the entire side.

    Add the straightness tolerance to the maximum size (MMC) to define a virtual condition Vc that

    the part must fit into in order to meet the tolerance.

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    Flatness

    1.000 ' 0.002

    0.001

    0.001

    parallel

    planes

    Tolerance zone defined by two parallel planes.

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    Flatness

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    Flatness, Circularity and

    CylindricityFlatness Circularity Cylindricity

    The flatness tolerance defines a distance between parallel planes that must contain the

    highest and lowest points on a face.

    The circularity tolerance defines a pair of concentric circles that must contain the

    maximum and minimum radius points of a circle.

    The cylindricity tolerance defines a pair of concentric cylinders that much contain the

    maximum and minimum radius points along a cylinder.

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    Circularity (Roundness)

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    CYLINDRICITY

    1.00 ' 0.05

    0.01

    0.01

    Rotate in a V

    Rotate between points

    Tolerance zone bounded by two concentric cylinders

    within which the cylinder must lie.

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    Parallelism

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    Parallelism ToleranceA parallelism tolerance is measured relative to a datum specified in the control frame.

    If there is no material condition (ie. regardless of feature size), then the tolerance defines parallel planes thatmust contain the maximum and minimum points on the face.

    If MMC is specified for the tolerance value:

    If it is an external feature, then the tolerance is added to the maximum dimension to define a virtual

    condition that the part must fit;

    If it is an internal feature, then the tolerance is subtracted to define the maximum dimension that must fit

    into the part.

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    Perpendicularity

    A perpendicular tolerance is

    measured relative to a datum plane.

    It defines two planes that must

    contain all the points of the face.

    A second datum can be used to

    locate where the measurements aretaken.

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    Perpendicular Shaft, Hole, and

    Center PlaneShaft Hole Center Plane

    Shaft: The maximum shaft size plus the tolerance defines the virtual hole. Hole: The minimum hole size minus the tolerance defines the virtual shaft.

    Plane: The tolerance defines the variation of the location of the center plane.

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    Angularity

    An angularity tolerance is measured relative to a datum plane.

    It defines a pair planes that must

    1. contain all the points on the angled face of the part, or2. if specified, the plane tangent to the high points of the face.

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    Concentricity Tolerance Note

    XX

    A

    YY

    .007 A

    This cylinder (the right cylinder) must be concentric

    within .007 with the Datum A (the left cylinder)as measured on the diameter

    .007 ToleranceZone

    What It Means

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    TRUE POSITION

    1.20 0.01

    1.00 0.01

    1.20

    1.00

    Tolerance zone

    0.01dia

    O0.01MA B

    O.800.02

    Dimensionaltolerance

    True positiontolerance

    Hole center tolerance zone

    A

    B

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    Position Tolerance for a Hole

    The position tolerance for a hole defines a zone that has a defined shape, size, location and orientation. It has the diameter specified by the tolerance and extends the length of the hole.

    Basic dimensions locate the theoretically exact center of the hole and the center of the tolerance zone.

    Basic dimensions are measured from the datum reference frame.

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    Position Tolerance on a Hole

    PatternA composite control frame signals a tolerancefor a pattern of features, such as holes.

    The first line defines the position tolerance

    zone for the holes.

    The second line defines the tolerance zone for

    the pattern, which is generally smaller.

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    Virtual Condition Envelope

    All Required Tolerances20.06 MaximumEnvelope

    20.00MaximumAllowableDiameter

    0.06MaximumAllowableCurvature

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    SURFACE FINISH

    waviness width

    roughness width

    waviness

    roughness

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    PROFILEA uniform boundary along the true profile within whcih

    the elements of the surface must lie.

    A

    B

    0.005 A B

    0.001

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    RUNOUT

    0.361 " 0.002

    1.500 " 0.005A

    0.005 A

    A composite tolerance used to control the functional relationshipof one or more features of a part to a datum axis. Circular runout

    controls the circular elements of a surface. As the part rotates360 about the datum axis, the error must be within the tolerancelimit.

    Datum

    axis

    Deviation on each

    circular check ring

    is less than the

    tolerance.

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    TOTAL RUNOUT

    Datum

    axis

    Deviation on the

    total swept when

    the part is rotating

    is less than the

    tolerance.

    0.361 " 0.002

    1.500 " 0.005A

    0.005 A

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    Runout

    G i T l i

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    Geometric Tolerancing -

    Definitions Maximum Material Condition (MMC) The condition in

    which a feature of size contains the maximum amount ofmaterial with the stated limits of size, - fore example,minimum hole diameter and maximum shaft diameter

    Least Material Condition (LMC) Opposite of MMC, thefeature contains the least material. For example,maximum hole diameter and minimum shaft diameter

    Virtual Condition The envelope or boundary thatdescribes the collective effects of all tolerance

    requirements on a feature (See Figure 7-25 TG)

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    Material Condition ModifiersIf the tolerance zone is prescribed for the maximum

    material condition (smallest hole). Then the zone expands

    by the same amount that the hole is larger in size.

    Use MMC for holes used in clearance fits.

    MMC

    RFS

    No material condition modifier means the tolerance is

    regardless of feature size.

    Use RFS for holes used in interference or press fits.

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    MMC HOLE

    Given the same peg (MMC peg), when theproduced hole size is greater than the MMC hole,the hole axis true position tolerance zone can beenlarged by the amount of difference between theproduced hole size and the MMC hole size.

    hole axis tolerance zone

    MMC holeLMC hole

    MMC peg will fit in t he hole

    axis must be in the tolerance zone,

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    TOLERANCE VALUE MODIFICATION

    Produced True Pos tol

    hole size0.97 out of diametric tolerance

    0.98 0.01 0.05 0.01

    0.99 0.02 0.04 0.01

    1.00 0.03 0.03 0.01

    1.01 0.04 0.02 0.01

    1.02 0.05 0.01 0.01

    1.03 out of diametric tolerance

    1.20

    1.00

    O 0.0 1 M A B

    O 1.00 0.02

    M L S

    The default modifier fortrue position is MMC.

    MMC

    LMC

    For M the allowable tolerance = specified tolerance + (produced holesize - MMC hole size)

    A

    B