Geometric Dimensioning and Tolerancing

BySaravanan Kulasekaran

ME., LLB.,Technical Manager

What is Meant by GD&T

• Geometric Dimensioning and Tolerancing (GD&T) is a system for defining and communicating engineering tolerances.

• It uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models that explicitly describes nominal geometry and its allowable variation.

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ISO vs. ASME

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Comparing the ISO and the ASME Approaches to GD&T

Issue or Topic ISO ASME

Approach

Explanation

Cost of Standards

Number of Standards

Theoretical Functional

Graphical, Few Words

700 – 1000 USD

10 - 16

Comprehensive

< 100 USD

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ASME Y14.5M – 2009 Structure Section 1 Scope, Definitions, and General Dimensioning

Section 2 General Tolerancing and Related Principles

Section 3 Symbology

Section 4 Datum Reference Frames

Section 5 Tolerances of Form

Section 6 Tolerances of Orientation

Section 7Tolerances of Location

Section 8 Tolerances of Profile

Section 9 Tolerances of Runout

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5

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Fundamental Rules• Each dimension shall have a tolerance.

(except for those dimensions specifically identified as reference, maximum, minimum, or stock)

• Ensure full understanding of each feature.

• Show the detail needed and no more.

• Serve function needs, no misinterpretation.

• Manufacturing methods are not specified.

• Non-mandatory dimensions are OK.

• Designed of optimal readability.

• Tolerances apply for full size of feature.

• Dimensions and tolerances only apply at the drawing level where they were specified.

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Angular Units

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Decimal Inch Dimensions

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Millimetre Dimensions

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Application of Dimensions

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Grouping of Dimensions

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Spacing of Dimension Lines

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Staggered Dimensions

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Point Locations

Breaks in Extension Lines

Oblique Extension Lines

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Leader-Directed Dimensions

Minimizing Leaders

Leaders

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Overall Reference Dimension

Intermediate Reference Dimension

Reading Direction

Diameters

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Radii With Unlocated Centers

Radius With Located Center

Radii

Foreshortened Radii

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Round Holes

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Round Holes

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Round Holes

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Countersunk and Counter drilled Holes

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Internal Chamfers

Chamfers

Chamfers Between Surfaces at OtherThan 90°

Key seats

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KNURLING

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Rectangular Coordinate Dimensioning

Rectangular Coordinate Dimensioning Without Dimension Lines

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Rectangular Coordinate Dimensioning in Tabular Form

Polar Coordinate Dimensioning Repetitive Features and Dimensions

Repetitive Features and Dimensions

Repetitive Features

Section-2 General Tolerancing and Related

Principles

Key Definitions• Tolerance – The total permissible variation in size for a

specified dimension.

• Bilateral Tolerance – A tolerance zone where the boundary conditions contain the specified dimension.

• Geometric Tolerance – A general term that refers any of the 14 symbols used to control form, orientation, profile, runout, or location.

• Unilateral Tolerance – A tolerance zone that only exists on one side of the specified dimension.

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Limit Dimensioning

Plus and Minus Tolerancing

Tolerance Accumulation

Tolerance Accumulation

Section-03 Symbology

Geometric Characteristic Symbols

BEZELCASE

CLAMP

PROBE

DIAL INDICATOR

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810 12 10

8

6

422

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Dial Indicator

Verification of Flatness

Z

DATUMREFERENCEFRAME

SURFACEPLATE

GRANITE

PROBE

COORDINATE MEASURING MACHINEBRIDGE DESIGN

Coordinate Measuring Machine

STRAIGHTNESS

Straightness is a condition where an element of a surface,or derived median line, is a straight line. A straightnesstolerance specifies a tolerance zone within whichthe considered element of a surface or derived medianline must lie. A straightness tolerance is applied in theview where the elements to be controlled are representedby a straight line.

STRAIGHTNESS

FLATNESSFlatness is the condition of a surface or derived median plane having all elements in one plane. A flatness tolerance specifies a tolerance zone defined bytwo parallel planes within which the surface or derived median plane must lie.

Checking for Flatness

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CIRCULARITY

Definition Circularity exists when all of the points on a perpendicular cross section of a cylinder or a cone are equidistant to its axis.

Tolerance Zone Two concentric circles that contain each circular element of the surface.

Note: Circularity also applies to spheres

Checking for Circularity

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CYLINDRICITY

Definition Cylindricity exists when all of the points on the surface of a cylinder are equidistant to a common axis.

Tolerance Zone Two concentric cylinders that contain the entire cylindrical surface.

Checking for Cylindricity

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PROFILE OF A LINE Profile of a line is of course closely related to profile of a surface. The difference being that profile of a line takes only the measurement at a specific cross section, and does not take into account the variance of one cross section to the next.

PROFILE OF A SURFACEProfile of a surface describes a 3-Dimensional tolerance zone around a surface, usually which is an advanced curve or shape. If it is called out on a curved surface, like a fillet on a welded part, the entire surface where the radius is has to fall within the tolerance zone.

ANGULARITY

Two parallel planes or lines which are oriented at the specified angle in relation to a datum. All points on the referenced surface must fall into this tolerance zone.

Checking for Angularity

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ANGULARITY• is the condition of a surface, axis, or median plane which

is at a specific angle (other than 90°) from a datum plane or axis.

• Can be applied to an axis at MMC.• Typically must have a Basic dimension.

The surface is at a 45º angle with a .005 tolerance zone relative to datum A.

PERPENDICULARITY

• is the condition of a surface, center plane, or axis at a right angle (90°) to a datum plane or axis.Ex:

The tolerance zone is the space between the 2 parallel lines. They are perpendicular to the datum plane and spaced .005 apart.

The perpendicularity of this surface must be within a .005 tolerance zone relative to datum A.

PERPENDICULARITY

• Location of hole (axis)

This means ‘the hole (axis) must be perpendicular within a diametrical tolerance zone of .010 relative to datum A’

Checking for Perpendicularity

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±0.01

PARALLELISM:• The condition of a surface or center plane equidistant at all points

from a datum plane, or an axis.• The distance between the parallel lines, or surfaces, is specified by

the geometric tolerance.

Checking for Parallelism

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Position Tolerance

• A position tolerance is the total permissible variation in the location of a feature about its exact true position.

• For cylindrical features, the position tolerance zone is typically a cylinder within which the axis of the feature must lie.

• For other features, the center plane of the feature must fit in the space between two parallel planes.

• The exact position of the feature is located with basic dimensions.

• The position tolerance is typically associated with the size tolerance of the feature.

• Datums are required.

Concentricity

Definition Concentricity defines the location of cylindrical features about an axis of rotation.

Tolerance Zone The tolerance zone is defined as a cylinder about the datum axis that must contain the median points of diametrically opposed elements of a feature.

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A

Checking for Concentricity

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Runout Tolerances

• Circular Runout

• Total Runout

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2-D Application

3-D Application

Runout

Definition Runout is a composite control used to specify functional relationships between part features and a datum axis.

Tolerance Zone Circular runout is a 2-D application that evaluates full indicator movement on a perpendicular cross section rotating about a datum axis. Total runout evaluates full indicator movement of the full surface rotating about a datum axis.

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3-D Application 2-D Application

Checking for Runout

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Symmetry

Definition Symmetry defines the location of non-cylindrical features about a derived median plane.

Tolerance Zone The tolerance zone is defined by two planes, equidistant to a datum center plane. The derived median points must fall within these two planes.

A

Set Up for Symmetry

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Geometric Characteristic Symbols

TO BE CONTINUED ON SESSION 02

Material Source: Google Internet