Geometrical Dimensioning and Tolerancing

By:Mahender KumarMahender Kumar

ANSI Y14 5-1994 StandardANSI Y14.5 1994 Standard

This standard establishes uniform practices for defining and interpreting dimensions, and interpreting dimensions, and tolerances, and related requirements for use on engineering drawings.

What is a ‘good level of tolerance’?What is a good level of tolerance ?

Designer: i h l i btight tolerance is better

(less vibration, less wear, less noise)

Machinist:large tolerances is betterg(easier to machine, faster to produce,easier to assemble)

Tolerancing application: an example

The type of fit between mating featuresDesigner needs to specify:Designer needs to specify:

basic diameter and the tolerance of shaft: S±s/2

basic diameter and the tolerance of hole: H±h/2

Allowance: a = Dhmin – Dsmax.

TolerancingTolerancing• Definition: Allowance for specific variation in the

size and geometry of a part g y p• Need for Tolerancing

– It is IMPOSSIBLE to manufacture a part to an EXACT size or geometry

– Since variation from the drawing is inevitable we must specify the acceptable degree of variation p y p g

– Large variation may affect part functionality– Small allowed variation affects the part cost

i i f t i• requires precise manufacturing • requires inspection and potential rejection of parts

Tolerance Follows FunctionTolerance Follows Function• Assemblies:

– Parts will not fit together if their dimensions do not fall with in a certain range of values

I t h bl P t• Interchangeable Parts: – If a replacement part is used it must duplicate the

original part within certain limits of deviationg p• The relationship between functionality and

size/shape of an object varies with the part– Automobile Transmission is Very Sensitive to the Size

& Shape of the Gears – A Bicycle is NOT Too Sensitive to the Size & ShapeA Bicycle is NOT Too Sensitive to the Size & Shape

of the Gears (sprockets)

Two Forms of Physical ToleranceTolerance

• SizeSize– Limits specifying the allowed variation in each

dimension (length, width, height, diameter, etc.) are given on the drawing

• GeometryG t i Di i i & T l i (GD&T)– Geometric Dimensioning & Tolerancing (GD&T)• Allows for specification of tolerance for the geometry of a part

separate from its size• GD&T uses special symbols to control different geometric

features of a part

Cost Sensitivity• Cost generally increases with “tighter”

tolerancesTh i ll ili t thi l ti hi– There is generally a ceiling to this relationship where larger tolerances do not affect cost • e.g.; If the Fabricator ROUTINELY Holds to ±0.5 mm,

Th ±3 S ifi ti ill NOT d C tThen a ±3 mm Specification will NOT reduce Cost– Tolerances at the Limits of the Fabricator’s

Capability cause an exponential increase in cost– Parts with small tolerances often require special

methods of manufacturing– Parts with small tolerances often require greaterParts with small tolerances often require greater

inspection, and higher part-rejection rates • Do NOT specify a SMALLER Tolerance than

i NEEDEDis NEEDED

Tolerance Spec HierarchyTolerance Spec Hierarchy

• Generally Three Levels of TolerancesGenerally Three Levels of Tolerances– DEFAULT: Placed in the Drawing Title-Block

by The Engineering Firmby The Engineering Firm• Typically Conforms to Routine Tolerance Levels

– GENERAL: Placed on the Drawing By theGENERAL: Placed on the Drawing By the Design-Engineer as a NOTE• Applies to the Entire Drawing• Supercedes the DEFAULT Tolerance

– SPECIFIC: Associated with a SINGLE Dimension or Geometric Feature

Fit Between PartsFit 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

Clearance Fit Interference Fit Transition Fit

,minimum diameter and hole maximum diameter have a clearance fit

Classes of FitClasses 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)2 Location clearance (LC)2. Location clearance (LC)3. Location transition (LT)4. Location interference (LN)5. Force fits (FN)

Unilateral and Bilateral Tolerances:Unilateral and Bilateral Tolerances:

1.00 0.05+-

nominal dimension

0.95 - 1.05means a range

tolerance

+ 0 10 + 0 00unilateral

bilateral

0.95 + 0.10- 0.00 1.05 + 0.00

- 0.10

1.00 0.05+-

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;1. Form tolerances: straightness, circularity, flatness, cylindricity;2. Orientation tolerances; perpendicularity, parallelism, angularity; and3. Position tolerances: position, symmetry, concentricity.

COMMON TERMS AND DEFINITIONS

Basic DimensionA numerical value used to describe the theoretically exact size, profile, orientation, orlocation of a feature or datum target. It is the basis from which permissible variations areestablished by tolerances on other dimensions in notes or in feature control framesestablished by tolerances on other dimensions, in notes, or in feature control frames.

DatumA theoretically exact point, axis, or plane derived from the true geometric counterpart of a specified datum feature. A datum is the origin from which the location or geometriccharacteristics of features of a part are established.

Datum TargetA specific line, or area on a part used to establish a datum.

Maximum Material Condition (MMC)The condition in which a feature of size contains the maximum amount of material within thestated limits of size-for example, minimum hole diameter, maximum shaft diameter.

Least Material Condition (LMC)The condition in which a feature of size contains the least amount of material within the statedlimits of size-for example, maximum hole diameter, minimum shaft diameter.

Regardless of Feature Size (RFS):The term used to indicate that a geometric tolerance or datum reference applies at any increment g pp yof size of the feature within its size tolerance.

Full Indicator MovementThe total movement of an indicator when appropriately applied to a surface to measure its variations (formerly called total indicator reading TIR)(formerly called total indicator reading-TIR).Virtual Condition

The boundary generated by the collective effects of the specified MMC limit of size of a feature and any applicable geometric tolerances.

Feature Control FrameThe feature control frame consists of: A) type of control (geometric characteristic), B) tolerance zone, C) tolerance zone modifiers (i e MMC or RFS) D) datum references if applicable and any datum referencetolerance zone modifiers (i.e., MMC or RFS), D) datum references if applicable and any datum reference modifiers.

PROFILE TOLERANCES

Profile of a LineA uniform two dimensional zone limited by two parallel zone lines extending along the length of a feature.

Profile of a SurfaceA uniform three dimensional zone contained between two envelope surfaces separated by the tolerance zone across the entire length of a surface.

ORIENTATION TOLERANCES

A l itAngularityThe distance between two parallel planes, inclined at a specified basic angle in which the surface, axis, or center planeof the feature must lie.

Perpendicularity (squareness)The condition of a surface axis median plane or line which is exactly at 90 degrees with respect to a datum plane or axisThe condition of a surface, axis, median plane, or line which is exactly at 90 degrees with respect to a datum plane or axis.

ParallelismThe condition of a surface or axis which is equidistant at all points from a datum of reference.

LOCATIONAL TOLERANCES

Tr e PositionTrue PositionA zone within which the center, axis, or center plane of a feature of size is permitted to vary from its true(theoretically exact) position.

ConcentricityConcentricityA cylindrical tolerance zone whose axis coincides with the datum axis and within which all cross-sectional axesof the feature being controlled must lie. (Note: Concentricity is very expensive and time-consuming to measure.Recommended that you try position or runout as an alternative tolerance.)

RUNOUT TOLERANCES

RunoutA composite tolerance used to control the relationship of one or more features of a part to a datum axisA composite tolerance used to control the relationship of one or more features of a part to a datum axisduring a full 360 degree rotation about the datum axis.

Circular RunoutEach circular element of the feature/part must be within the runout tolerance.

Total RunoutAll surface elements across the entire surface of the part must be within the runout tolerance.

FORM TOLERANCES

FlatnessA two dimensional tolerance zone defined by two parallel planes within which the entire surface must lie.

StraightnessA condition where an element of a surface or an axis is a straight line.

CircularityA condition on a surface of revolution (cylinder, cone, sphere) where all points of the surface intersected( y , , p ) pby any plane perpendicular to a common axis (cylinder, cone) or passing through a common center (sphere)are equidistant from the axis of the center.

CylindricityA condition on a surface of revolution in which all points of the surface are equidistant from a common axis.

Feature Control FrameFeature 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.

Order of PrecedenceOrder 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 ShaftStraightness 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 th t i ht t l t th i h ft i (MMC) t bt i “ i t l • 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 HoleStraightness 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 PlaneStraightness 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.

FlatnessFlatnessTolerance zone defined by two parallel planes.

0.001

1.000 ' ±0.002

p ar al le lp lanes

0.001

Flatness

Flatness, Circularity and Cylindricity

Flatness 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 • The cylindricity tolerance defines a pair of concentric cylinders that much contain the maximum and minimum radius points along a cylinder.

Circularity (Roundness)

CYLINDRICITYCYLINDRICITYTolerance zone bounded by two concentric cylinders

within which the cylinder must lie.

0.01

1.00 ' ±0.05 Rotate in a V

0.01

R t t b t i tRotate between points

Parallelism

Parallelism ToleranceParallelism 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.pIf 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 into the part.

PerpendicularityPerpendicularity• A perpendicular tolerance is measured relative to a datum plane 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 Plane

Shaft Hole Center Plane

• Shaft: The maximum shaft size plus the tolerance defines the virtual hole• 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.

AngularityAngularity

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.

Concentricit Tolerance NoteConcentricity Tolerance Note007 A .007 Tolerance

A.007 A Zone

XX YY

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

d th di t

What It Means

as measured on the diameter

TRUE POSITIONTRUE POSITIONDimensional

1.00 ± 0.01

Dimensionaltolerance

1.20± 0.01

O .80 ± 0.02 Hole center tolerance zone

Tolerance zone0 01dia

O 0.01 M A B

True positiont l

Hole center tolerance zone

1.00

0 .01diatolerance

1.20AB

Position Tolerance for a HolePosition 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 Pattern

A composite control frame signals a tolerance for a pattern of features, such as holes.p ,

• 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.t e patte , w c s ge e a y s a e .

Virtual Condition EnvelopeAll Required Tolerances

20.06 MaximumEnvelope

20 000 06 20.00MaximumAllowable Diameter

0.06MaximumAllowable Curvature

PROFILEPROFILEA uniform boundary along the true profile within whcih the elements of the surface must lie.

B

0.005 A B

A

B

0.001

RUNOUTA composite tolerance used to control the functional relationshipof one or more features of a part to a datum axis. Circular runoutcontrols the circular elements of a surface. As the part rotates360° about the datum axis, the error must be within the tolerance

1.500 " ±0.005A

,limit.

0.361 " ±0.0020.005 A

Dat umax is

Deviat ion on eachcircular check ringis less t han t het olerance.

TOTAL RUNOUTTOTAL RUNOUT

0 361 " ±0 002

1.500 " ±0.005A

0.005 A

D i t i t h

0.361 ±0.002

Dat umax is

Deviat ion on t het ot al swept whent he part is rot at ingis less t han t het 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 diametermaximum 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)requirements on a feature (See Figure 7-25 TG)

Material Condition ModifiersMaterial 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

RFSby 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 HOLEMMC HOLEMMC holeLMC hole

hole axis t olerance zone

MMC peg will f it in t he holeaxis must be in t he t olerance zone

,

Gi th (MMC ) h th

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 MODIFICATIONTOLERANCE VALUE MODIFICATIONO 0.01 M A B

O 1.00 ± 0.02

Produced True Pos tolhole size0.97 out of diametric tolerance0 98 0 01 0 05 0 01

1.20

1.00 M L S

MMCA

B

0.98 0.01 0.05 0.010.99 0.02 0.04 0.011.00 0.03 0.03 0.01

MMCA

1.01 0.04 0.02 0.011.02 0.05 0.01 0.011.03 out of diametric tolerance

The default modifier for true position is MMC. LMC

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

Thanks

Any question?Any question?