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Geometric Dimensioning and Tolerancing (GD&T)

Dimensioning can be divided into three categories:

• general dimensioning, • geometric dimensioning, and • surface texture.

The following provides information necessary to begin to understand geometric dimensioning and tolerancing (GD&T)

Three Categories of Dimensioning

Limit Tolerancing Applied To An Angle Block

Geometric Tolerancing Applied To An Angle Block

Geometric Dimensioning & Tolerancing (GD&T)

 GD&T is a means of dimensioning & tolerancing a drawing which considers the function of the part and how this part functions with related parts.

– This allows a drawing to contain a more defined feature more accurately, without increasing tolerances.

GD&T cont’d

  GD&T has increased in practice in last 15 years because of ISO 9000. –  ISO 9000 requires not only that something

be required, but how it is to be controlled. For example, how round does a round feature have to be?

  GD&T is a system that uses standard symbols to indicate tolerances that are based on the feature’s geometry. –  Sometimes called feature based

dimensioning & tolerancing or true position dimensioning & tolerancing

  GD&T practices are specified in ANSI Y14.5M-1994.

For Example   Given Table Height

  However, all surfaces have a degree of waviness, or smoothness. For example, the surface of a 2 x 4 is much wavier (rough) than the surface of a piece of glass. –  As the table height is dimensioned, the

following table would pass inspection.

  If top must be flatter, you could tighten the tolerance to ± 1/32. –  However, now the height is restricted to

26.97 to 27.03 meaning good tables would be rejected.

Assume all 4 legs will be cut to length at the same time.

or

Example cont’d.

 You can have both, by using GD&T. – The table height may any height

between 26 and 28 inches. – The table top must be flat within

1/16. (±1/32)

27

.06

26

.06 28

.06

WHY IS GD&T IMPORTANT

  Saves money – For example, if large number

of parts are being made – GD&T can reduce or eliminate inspection of some features.

– Provides “bonus” tolerance   Ensures design, dimension, and

tolerance requirements as they relate to the actual function

  Ensures interchangeability of mating parts at the assembly

  Provides uniformity   It is a universal understanding of

the symbols instead of words

WHEN TO USE GD&T

  When part features are critical to a function or interchangeability

  When functional gaging is desirable

  When datum references are desirable to insure consistency between design

  When standard interpretation or tolerance is not already implied

  When it allows a better choice of machining processes to be made for production of a part

TERMINOLOGY REVIEW   Maximum Material Condition

(MMC): The condition where a size feature contains the maximum amount of material within the stated limits of size. I.e., largest shaft and smallest hole.

  Least Material Condition (LMC): The condition where a size feature contains the least amount of material within the stated limits of size. I.e., smallest shaft and largest hole.

  Tolerance: Difference between MMC and LMC limits of a single dimension.

  Allowance: Difference between the MMC of two mating parts. (Minimum clearance and maximum interference)

  Basic Dimension: Nominal dimension from which tolerances are derived.

LIMITS OF SIZE

LIMITS OF SIZE

A variation in form is allowed between the least material condition (LMC) and the maximum material condition (MMC).

Envelop Principle defines the size and form relationships between mating parts.

ENVELOPE PRINCIPLE

LMC CLEARANCE

MMC ALLOWANCE

LIMITS OF SIZE

LIMITS OF SIZE

The actual size of the feature at any cross section must be within the size boundary.

ØMMC

ØLMC

No portion of the feature may be outside a perfect form barrier at maximum material condition (MMC).

LIMITS OF SIZE

Other Factors I.e., Parallel Line Tolerance Zones

INDIVIDUAL (No Datum Reference)

INDIVIDUAL or RELATED FEATURES

RELATED FEATURES (Datum Reference Required)

GEOMETRIC CHARACTERISTIC CONTROLS

TYPE OF FEATURE TYPE OF

TOLERANCE CHARACTERISTIC SYMBOL

SYMMETRY

FLATNESS

STRAIGHTNESS

CIRCULARITY

CYLINDRICITY

LINE PROFILE

SURFACE PROFILE

PERPENDICULARITY

ANGULARITY

PARALLELISM

CIRCULAR RUNOUT

TOTAL RUNOUT

CONCENTRICITY

POSITION

FORM

PROFILE

ORIENTATION

RUNOUT

LOCATION

14 characteristics that may be controlled

Characteristics & Symbols cont’d.

–  Maximum Material Condition MMC –  Regardless of Feature Size RFS –  Least Material Condition LMC –  Projected Tolerance Zone –  Diametrical (Cylindrical) Tolerance

Zone or Feature –  Basic, or Exact, Dimension –  Datum Feature Symbol

–  Feature Control Frame

THE

GEOMETRIC SYMBOL TOLERANCE INFORMATION

DATUM REFERENCES

FEATURE CONTROL FRAME

COMPARTMENT VARIABLES

CONNECTING WORDS MUST BE WITHIN OF THE FEATURE

RELATIVE TO

Feature Control Frame

Feature Control Frame   Uses feature control frames to

indicate tolerance

  Reads as: The position of the feature must be within a .003 diametrical tolerance zone at maximum material condition relative to datums A, B, and C.

Feature Control Frame

  Uses feature control frames to indicate tolerance

  Reads as: The position of the feature must be within a .003 diametrical tolerance zone at maximum material condition relative to datums A at maximum material condition and B.

  The of the feature must be within a tolerance zone.

  The of the feature must be within a tolerance zone at relative to Datum .

  The of the feature must be within a tolerance zone relative to Datum .

  The of the feature must be within a zone at relative to Datum .

  The of the feature must be within a tolerance zone relative to datums .

Reading Feature Control Frames

Placement of Feature Control Frames

  May be attached to a side, end or corner of the symbol box to an extension line.

  Applied to surface.

  Applied to axis

Placement of Feature Control Frames Cont’d.

  May be below or closely adjacent to the dimension or note pertaining to that feature.

Ø .500±.005

Basic Dimension   A theoretically exact size, profile,

orientation, or location of a feature or datum target, therefore, a basic dimension is untoleranced.

  Most often used with position, angularity, and profile)

  Basic dimensions have a rectangle surrounding it.

1.000

Basic Dimension cont’d.

Form Features   Individual Features   No Datum Reference

Flatness Straightness

Cylindricity Circularity

Form Features Examples

Flatness as stated on drawing: The flatness of the feature must be within .06 tolerance zone.

.003

0.500 ±.005

.003 0.500 ±.005

Straightness applied to a flat surface: The straightness of the feature must be within .003 tolerance zone.

Form Features Examples

Straightness applied to the surface of a diameter: The straightness of the feature must be within .003 tolerance zone.

.003

0.500 0.505 ∅

Straightness of an Axis at MMC: The derived median line straightness of the feature must be within a diametric zone of .030 at MMC.

.030 0.500 0.505 ∅ M ∅

1.010 0.990

Dial Indicator

Verification of Flatness

Features that Require Datum Reference

  Orientation –  Perpendicularity –  Angularity –  Parallelism

  Runout –  Circular Runout

–  Total Runout

  Location –  Position –  Concentricity

–  Symmetry