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 Y14.51994 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, RC1RC9;
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 321 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 crosssection.
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 725 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