Geometric Dimensioning and Tolerancing Standards

GD&T stands for Geometric Dimensioning & Tolerancing

• Dimension: Is a geometric characteristic the size of which is specified such as length, angle, location, or center distance

• Tolerance: On a dimension is the total permissible variation in its size, which is equal to the difference between the limits of size.

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DIMENSIONS OF A PART 2

GEOMETRIC DIMENSIONING AND TOLERANCING

TOLERANCE SIZES 3


TYPE OF TOLERANCE 4


SIZES OF MATING PARTS

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• The difference between the basic size and the maximum and minimum sizes are called the UPPER & LOWER deviations.

• The BASIC DIMENSION represents the theoretical exact size or location of a feature.

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Deviations

DEVIATIONS

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BASIC (EXACT) DIMENSIONS Are shown in a rectangle to indicate that the tolerances in the general tolerance note do not apply

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• A Datum is a theoretical exact feature from which dimensions may be taken.

• A Datum is generally chosen as an edge or feature which has the greatest influence in a specific measurement.

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Datum

EXAMPLES OF DEVIATION OF FORM WHEN PERFECT FORM AT THE MAXIMUM MATERIAL CONDITION IS REQUIRED 10


EXAMPLES OF DEVIATION OF FORM WHEN PERFECT FORM AT THE MAXIMUM MATERIAL CONDITION IS REQUIRED 11


• A geometric tolerance is the maximum permissible variation of form, profile, orientation, location, and run-out from that indicated or specified on a drawing.

• Feature control frame for an individual feature is divided into compartments containing, at the least, the geometric tolerance symbol and the geometric tolerance value

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Geometric Tolerance

• Geometric characteristic symbol

• Geometric tolerance

• Datums (if used)

• Modifiers as required (*MMC for example)

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Feature Control Frame

GEOMETRIC CHARACTERISTIC SYMBOLS

STRAIGHTNESS FORM

INDIVIDUAL

FEATURES

SYMBOL CHARACTERISTIC

TYPE OF

TOLERANCE FEATURE

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MAXIMUM

MATERIAL

CONDITION

SUPPLEMENTARY

SYMBOLS

SYMBOL CHARACTERISTIC TYPE OF

TOLERANCE

FEATURE

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LEAST

MATERIAL

CONDITION

SUPPLEMENTARY

SYMBOLS


TOLERANCE

FEATURE

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PROJECTED TOLERANCE

ZONE

SUPPLEMENTARY

SYMBOLS


TOLERANCE

FEATURE

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BASIC DIMENSION

SUPPLEMENTARY

SYMBOLS


TOLERANCE

FEATURE

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DATUM

FEATURE

SUPPLEMENTARY

SYMBOLS


TOLERANCE

FEATURE

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DATUM

TARGET

SUPPLEMENTARY

SYMBOLS


TOLERANCE

FEATURE

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• Form tolerances control the straightness, flatness, circularity and cylindricity.

• Form tolerances apply to individual features and as such do not require locating dimensions.

• Orientation tolerances control angularity, parallelism, and perpendicularity

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Form Tolerances

• Straightness a condition in which the element of a surface or a median line is a straight line.

• Flatness of a surface is a condition in which all surface elements are in one plane.

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More on Form Tolerances

FEATURE CONTROL FRAME

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APPLICATION OF FEATURE CONTROL FRAME

ATTACHED USING

A LEADER

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• MMC (Maximum Material Condition) is when a feature or part is at the limit of size, which results in its containing the maximum amount of material.

• LMC (Least Material Condition) refers to the size of a feature that results in the part containing the minimum amount of material. Thus it is the minimum limit of size for an external feature.

• RFS (Regardless of feature size) indicates that a geometric tolerance applies to any size of a feature that lies within its size tolerance.

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MMC, RFS, LMC: Applies to features subject to variations in size

• Datum is a theoretical point, line, plane, or other geometric surface from which dimensions are measured when so specified or to which geometric tolerances are referenced

• Datum feature is a feature of a part, such as a surface, that forms the basis for a datum or is used to establish its location

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Datum's and the 3-plane Concept

• The three planes constitute a datum system from which measurements can be taken. They will appear on the drawing.

• Datum feature symbol is used on the drawing to identify these surfaces

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3-plane System

THE DATUM PLANES 28


THE DATUM PLANES 29


THE DATUM PLANES 30


• Angularity, parallelism and perpendicularity are orientation tolerances applicable to related features.

• Angularity is the condition of a surface or axis at a specified angle (other than 90⁰or 0⁰) from a datum plane or axis.

• Perpendicularity is the condition of a surface at 90⁰ to a datum plane or axis. *A perpendicularity tolerance for a flat surface specifies a tolerance zone defined by two parallel planes perpendicular to a datum plane or axis.

• Parallelism is the condition of a surface equidistant at all points from a datum plane. Parallelism for a flat surface specifies a tolerance zone defined by two planes or lines parallel to a datum plane or axis. 31

Orientation Tolerancing

• Datum target symbols are used to locate specific reference positions to be used on large datum surfaces.

• The surface of a feature may be so large that a gage designed to make contact with the full surface may be too expensive or cumbersome to use.

• Functional requirements of the part may necessitate the use of only a portion of a surface as a datum feature.

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Datum Targets

Documents

Geometric Dimensioning and Tolerancing Standards