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Dimensioning and TolerancingMaheneder Kumar

ANSI Y14.5-1994 StandardThis 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.

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)

Tolerancing application: an example

The type of fit 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 = Dhmin Dsmax.

Fit Between Parts 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 Clearance Fit Interference Fit Transition Fit

Classes of FitThe 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.

Unilateral and Bilateral Tolerances:nominal dimension 1.00 + 0.05 tolerance means a range 0.95 - 1.05

unilateral bilateral

0.95

+ 0.10 - 0.00

1.05

+ 0.00 - 0.10

1.00 + 0.05 -

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

We will focus on three basic types of dimensional tolerances: 1. 2. 3. Form tolerances: straightness, circularity, flatness, cylindricity; Orientation tolerances; perpendicularity, parallelism, angularity; and Position tolerances: position, symmetry, concentricity.

Feature Control FrameA 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.

DIMENSIONINGRequirements 1. Unambiguous 2. Completeness 3. No redundancy0.83 ' 0.95 ' 3.03 ' 1.22 '

Incomplete dimensioning

1.72 '

Redundant dimensioning

0.86 '

0.83 ' 3.03 '

1.22 '

Adequate dimensioning

TOLERANCING

"TOLERANCE IS ALWAYS ADDITIVE"

0.80 '

0.01 1.00 ' 0.01 ?

1.20 '

0.01

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

x0.80 ' 0.01

?3.00 ' 0.01

1.20 '

0.01

Maximum x length = 3.01 - 0.79 - 1.19 = 1.03 Minimum x length = 2.99 - 0.81 - 1.21 = 0.97 Therefore x = 1.00 0.03

Order of PrecedenceThe part is aligned with the datum planes of a reference frame using 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

Straightness of a shaft

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.

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.

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.

FlatnessTolerance zone defined by two parallel planes.0.0 01

1.000 '

0.002

p ar al le l p lanes 0.0 01

Flatness

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.

Circularity (Roundness)

CYLINDRICITY Tolerance zone bounded by two concentric cylinderswithin which the cylinder must lie.0.01

1.00 ' 0.05

Rotate in a V

0.01

Rotate between points

Parallelism

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 that must 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.

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 are taken.

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.

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, or 2. if specified, the plane tangent to the high points of the face.

Concentricity Tolerance Note.007

A

A

.007 Tolerance Zone

XX

YY

This cylinder (the right cylinder) must be concentric within .007 with the Datum A (the left cylinder) as measured on the diameter

What It Means

TRUE POSITIONDimensional tolerance1 .0 0 0 .0 1

1 .2 0 0 .0 1

O .8 0 0 .0 2 O 0 .0 1 M A B

Hole center tolerance zone

True position tolerance1 .0 0

Tolerance zone 0 .0 1 dia

B A 1.2 0

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.

Position Tolerance on a Hole PatternA composite control frame signals a tolerance for 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.

Virtual Condition Envelope All Required Tolerances20.06 Maximum Envelope

0.06 Maximum Allowable Curvature

20.00 Maximum Allowable Diameter

SURFACE FINISHwaviness roughness

roughness widt h

waviness

widt h

PROFILEA uniform boundary along the true profile within whcih the elements of the surface must lie.0 .0 0 5 A B

B

A

0.0 01

A composite tolerance used to control the functional relationship of one or more features of a part to a datum axis. Circular runout controls the circular elements of a surface. As the part rotates 360 about the datum axis, the error must be within the tolerance limit.A 1.500 " 0.005 0 .0 0 5 A 0.361 " 0.002

RUNOUT

Dat um ax is

Deviat ion on each circular check ring is less t han t he t olerance.

TOTAL RUNOUTA 1.500 " 0.005 0.0 0 5 A 0.361 " 0.002

Dat um ax is

Deviat ion on t he t ot al swept when t he part is rot at ing is less t han t he t olerance.

Runout

Geometric Tolerancing Definitions Maximum Material Condition (MMC) The condition in which a feature of size contains the maximum amount of material with the stated limits of size, - fore example, minimum hole diameter and maximum shaft diameter Least Material Condition (LMC) Opposite of MMC, the feature contains the least material. For example, maximum hole diameter and minimum shaft diameter Virtual Condition The envelope or boundary that describes the collective effects of all tolerance requirements on a feature (See Figure 7-25 TG)

Material Condition ModifiersRFSIf 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

No material condition modifier means the tolerance is regardless of feature size. Use RFS for holes used in interference or press fits.

MMC HOLELMC hole MMC hole hole axis t olerance zone

MMC peg will f it in t he hole , axis must be in t he t olerance zone

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

TOLERANCE VALUE MODIFICATIONO 1 .0 0 0 .0 2 O 0 .0 1 M A B

Produced1 .0 0

hole size 0.971 .2 0

True Pos tol M L 0.01 0.02 0.03 0.04 0.05 0.05 0.04 0.03 0.02 0.01

S 0.01 0.01 0.01 0.01 0.01

B A

out of diametric tolerance

MMC

0.98 0.99 1.00

The default modifier for true position is MMC.

1.01 LMC 1.02 1.03

out of diametric tolerance

For M

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