# Gdt's

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17/3/2011[pic]

|RECOPILADO POR ING IVAN DE LA ROSA | Semestre Ene-May 2011 |

index

Geometric dimensioning & tolerancing 1

Symbols 2
Tolerances 3
Angularity 4
Perpendicularity 4
Cylindricity 5
Runout 6
Total Runout 7
Profile 8
Profile of a Line 9
Parallelism 10
Position11
Straightness 12
Concentricity 13
Flatness 14
Symmetry 15
Circularity 16
Rules 17
Concepts 18
Feature 18
Axis 19
Feature-of-Size (FOS) 19
Maximum Material Condition (MMC) 20
Bonus Tolerance 21
Tolerance Stack 22
Datum 23
Full Indicator Movement 24
Datum Target(s) 24
Control 26
Median Points 26

Geometric dimensioning &tolerancing

(GD&T) is a method of specifying geometry of physical objects on conventional or CAD drawings, along with allowable variations (tolerances) in form, size and orientation.

One of the most common standards for GD&T is ANSI Y14.5.

GD&T uses rectangular blocks:

[pic]

... which include:

A GD&T symbol

A tolerance

When needed, up to 3 letters that correspond to datums(from which measurement is to be made).

The above block would be read, "The position of the feature shall be according to the dimensions given, +/- 0.003, as measured from datums A and B." Datums are the lines or planes from which measurements are to be taken - often the lowest and leftmost part of a basically rectangular workpiece or the center of a basically round part.

GD&T

SymbolsTolerances

Rules

Concepts

Symbols

Following are some of the 14 primary symbols used in GD&T:

[pic]Angularity

[pic]Circularity

[pic]Concentricity

[pic]Cylindricity

[pic]Flatness

[pic]Parallelism

[pic]Perpendicularity

[pic]Position

[pic]Profile

[pic]Profile of a line

[pic]Runout

[pic]Straightness

[pic]Symmetry

[pic]Total runout

TolerancesTolerances

Angularity

Perpendicularity

Cylindricity

Runout

Total Runout

Profile

Profile of a Line

Parallelism

Position

Straightness

Concentricity

Flatness

Symmetry

Circularity

When using GD&T, tolerances are applied based on the type of geometric control required (e.g. Position, Concentricity, etc). These pages describe the 14 different tolerancesspecific to GD&T use.  The symbols are explained and an application of each one is shown.

Angularity

In a mechanical drawing of a part, angularity tolerance allows the designer to specify the degree to which the orientation of an angled part feature may vary. The angularity symbol is often used to insure that the part can properly mate with another.  In GD&T, the degree of permissible variationis not specified as a tolerance on the angle. Rather an indirect method is used where one specifies a tolerance zone at a specified angle from a datum, within which a part feature, axis, or center plane must lie.

[pic]

In the left figure above, the boxed angularity symbol, tolerance and datum are used to control the center axis of an angled hole. The boxed symbols can be read "This axis mustlie within two planes 0.5 apart , the planes inclined 60° to surface A".

In the right figure above, the tolerance zone created is indicated by the parallel lines.

This form of angularity tolerance applies only in the drawing view in which the tolerance is specified, and requires the permissible variation to be defined for other views. However, if a diameter symbol were placed in front ofthe boxed 0.5, this would create a cylindrical tolerance zone which would then apply to all drawing views.

Angularity is used in a tolerance stack when applied to a surface or line element. Angularity refines the orientation of the surface or line element, acting like a flatness control for the purposes of performing a tolerance stack.

Perpendicularity

In a mechanical part drawing,...