Geometric Dimensioning and Tolerancing Standards

GD&T stands for Geometric Dimensioning & Tolerancing •

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characteristic the Dimension: Is a geometric characteristic 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 diff d ifference erence between the limits of size.

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GEOMETRIC DIMENSIONING AND TOLERANCING

DIMENSIONS DIMENSION S OF A PART PART 2


TOLERANCE SIZES 3


TYPE OF TOLERANCE 4


SIZES OF MATING PARTS PARTS 5

Deviations

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The difference 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 7


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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Datum •

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A Datum is a theoretical exact feature 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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EXAMPLES OF DEVIATION OF FORM WHEN PERFECT FORM AT THE 10


EXAMPLES OF DEVIATION OF FORM WHEN PERFECT FORM AT THE 11

Geometric Tolerance

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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 symbol and the geometric tolerance value

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Feature Control Frame

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Geometric characteristic characteristic symbol

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Geometric tolerance tolerance

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Datums (if used)

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Modifiers as required (*MMC for example)

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TYPE OF FEATURE INDIVIDUAL

TOLERANCE CHARACTERISTIC

FORM

SYMBOL

STRAIGHTNESS

FEATURES

GEOMETRIC CHARACTERISTIC SYMBOLS 14


FEATURE

TYPE OF

CHARACTERISTIC

SYMBOL

TOLERANCE

SUPPLEMENTARY

MAXIMUM

SYMBOLS

MATERIAL CONDITION



FEATURE

TYPE OF

CHARACTERISTIC

SYMBOL

TOLERANCE

SUPPLEMENTARY

LEAST

SYMBOLS

MATERIAL CONDITION



FEATURE

TYPE OF

CHARACTERISTIC

SYMBOL

TOLERANCE SUPPLEMENTARY

PROJECTED

SYMBOLS

TOLERANCE ZONE



FEATURE

TYPE OF

CHARACTERISTIC

SYMBOL


BASIC

SYMBOLS

DIMENSION



FEATURE

TYPE OF

CHARACTERISTIC

SYMBOL


DATUM

SYMBOLS

FEATURE



FEATURE

TYPE OF

CHARACTERISTIC

SYMBOL

TOLERANCE

SUPPLEMENTARY

DATUM

SYMBOLS

TARGET


Form Tolerance olerancess

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Form tolerances control the straightness, flatness, circularity and cylindricity. cylindricity. Form tolerances apply to individual features features and as such do not require locating dimensions. ang ularity,, Orientation tolerances control angularity parallelism, and perpendicularity

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More on Form Tolerances

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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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FEATURE CONTROL FRAME 23


ATTACHED USING A LEADER

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

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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 for an external external feature. RFS (Regardless of feature size) indicates that a geometric tolerance applies to any size of a feature that lies within its it s size tolerance. 25

Datum's and the 3-plane Concept

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Datum is a theoretical point, line, plane, or other geometric surface from which dimensions are measured when so specified or to which geometric tolerances tolerances are referenced feature of a part, such as a Datum feature is a feature surface, that forms forms the basis basis for a datum or is used to establish its location 26

3-plane Sys System tem

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The three planes constitute constitute a datum system system from which measurements can be taken. taken. They will appear on the t he drawing. Datum feature feature symbol is used us ed on the t he drawing to identify these surfaces

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Orientation Tolerancing •

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

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Datum Targets •

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Datum target symbols are used to locate specific reference reference positions to be used on large datum surfaces. The surface of a feature feature may be so large that a gage designed to make contact 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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Geometric Dimensioning and Tolerancing Standards

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