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7/31/2019 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

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