Geometric dimensioning and tolerancing

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Example of geometric dimensioning and tolerancing

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. It tells the manufacturing staff and machines what degree of accuracy and precision is needed on each controlled feature of the part. GD&T is used to define the nominal (theoretically perfect) geometry of parts and assemblies, to define the allowable variation in form and possible size of individual features, and to define the allowable variation between features.

There are several standards available worldwide that describe the symbols and define the rules used in GD&T. One such standard is American Society of Mechanical Engineers (ASME) Y14.5-2009. This article is based on that standard, but other standards, such as those from the International Organization for Standardization (ISO), may vary slightly. The Y14.5 standard has the advantage of providing a fairly complete set of standards for GD&T in one document. The ISO standards, in comparison, typically only address a single topic at a time. There are separate standards that provide the details for each of the major symbols and topics below (e.g. position, flatness, profile, etc.).

Dimensioning and tolerancing philosophy[edit]

According to the ASME Y14.5-2009[1] standard, the purpose of geometric dimensioning and tolerancing (GD&T) is to describe the engineering intent of parts and assemblies. This is not a completely correct explanation of the purpose of GD&T or dimensioning and tolerancing in general.[citation needed]

The purpose of GD&T is more accurately defined as describing the geometric requirements for part and assembly geometry.[citation needed] Proper application of GD&T will ensure that the allowable part and assembly geometry defined on the drawing leads to parts that have the desired form and fit (within limits) and function as intended.

There are some fundamental rules that need to be applied (these can be found on page 7 of the 2009 edition of the standard):

(Note: The rules above are not the exact rules stated in the ASME Y14.5-2009 standard.)

Symbols[edit]

Tolerances: Type of Tolerances; 1) Unilateral 2) Bi- Lateral type

Geometric tolerancing reference chart
Per ASME Y14.5 M-1982
Type of toleranceGeometric characteristicsSymbolCharacter
(Unicode)
Can be applied to a surface?Can be applied to a feature of size?Can affect virtual condition?Datum reference used?Can use
Gd&t maximummaterialcondition.png
modifier?
Can use
Gd&t regardlessoffeaturesize.png
modifier?
Can be affected by a bonus tolerance?Can be affected by a shift tolerance?
FormStraightness
Gd&t straightness.png

U+23E4
YesYesYes
(note 1)
NoYes
(note 1)
No
(note 5)
Yes
(note 4)
No
FormPlanarity (flatness)
Gd&t flatness.png

U+23E5
YesNoNoNoNoNo
(note 5)
NoNo
FormCircularity
Gd&t circularity.png

U+25CB
YesNoNoNoNoNo
(note 5)
NoNo
FormCylindricity
Gd&t cylindricity.png

U+232D
YesNoNoNoNoNo
(note 5)
NoNo
ProfileProfile of a line
Gd&t line profile.png

U+2312
YesNoNoYes
(note 2)
NoNo
(note 5)
NoYes
(note 3)
ProfileProfile of a surface
Gd&t profile.png

U+2313
YesNoNoYes
(note 2)
NoNo
(note 5)
NoYes
(note 3)
OrientationPerpendicularity
Gd&t perpendicularity.png

U+27C2
YesYesYes
(note 1)
YesYes
(note 1)
No
(note 5)
Yes
(note 4)
Yes
(note 3)
OrientationAngularity
Gd&t angularity.png

U+2220
YesYesYes
(note 1)
YesYes
(note 1)
No
(note 5)
Yes
(note 4)
Yes
(note 3)
OrientationParallelism
Gd&t parallelism.png

U+2225
YesYesYes
(note 1)
YesYes
(note 1)
No
(note 5)
Yes
(note 4)
Yes
(note 3)
LocationSymmetry
Gd&t symmetry.png

U+232F
 ?
(note 6)
 ?
(note 6)
 ?
(note 6)
 ?
(note 6)
 ?
(note 6)
 ?
(note 6)
 ?
(note 6)
 ?
(note 6)
LocationPositional Tolerance
Gd&t position.png

U+2316
NoYesYesYesYesYesYes
(note 4)
Yes
(note 3)
LocationConcentricity
Gd&t concentricity.png

U+25CE
NoYesYesYesNoNo
(note 5)
NoNo
Run-outCircular run-out
Gd&t runout.png

U+2197
YesYesYes
(note 1)
YesNoNo
(note 5)
NoNo
Run-outTotal run-out
Gd&t totalrunout.png

U+2330
YesYesYes
(note 1)
YesNoNo
(note 5)
NoNo

Notes:

  1. When applied to a feature-of-size.
  2. Can also be used as a form control without a datum reference.
  3. When a datum feature-of-size is referenced with the MMC modifier.
  4. When an MMC modifier is used.
  5. Automatic per rule #3.
  6. The symmetry symbol's characteristics were not included in the version of the chart that this chart is derived from. The symmetry symbol was dropped from the Y14.5M standard around 1982 and re-added around 1994.
Symbols used in a "feature control frame" to specify a feature's description, tolerance, modifier and datum references
SymbolModifierNotes
Gd&t freestate.png
Free state
Gd&t leastmaterialcondition.png
Least material condition (LMC)
Gd&t maximummaterialcondition.png
Maximum material condition (MMC)
Gd&t projectedtolerancezone.png
Projected tolerance zone
Gd&t regardlessoffeaturesize.png
Regardless of feature size (RFS)Not part of the 1994 version. See para. A5, bullet 3. Also para. D3. Also, Figure 3-8.
Gd&t tangentplane.png
Tangent plane
Gd&t unilateral.png
UnilateralAppears in the 2009 version of the standard, and refers to unequal profile distribution.

Datums and datum references[edit]

A datum is a virtual ideal plane, line, point, or axis. A datum feature is a physical feature of a part identified by a datum feature symbol and corresponding datum feature triangle, e.g.,

{\displaystyle\Box}\!\!\!\!{\scriptstyle\mathsf{A}}\!-\!\!\!-\!\!\!\blacktriangleleft\!\!\!|

These are then referred to by one or more datum references which indicate measurements that should be made with respect to the corresponding datum feature.

GD&T data exchange[edit]

Exchange of geometric dimensioning and tolerancing (GD&T) information between CAD systems is available on different levels of fidelity for different purposes:

GD&T[edit]

ISO TC 10 Technical product documentation[edit]

ISO/TC 213 Dimensional and geometrical product specifications and verification[edit]

In ISO/TR 14638 GPS – Masterplan the distinction between fundamental, global, general and complementary GPS standards is made.

ASME standards American Society of Mechanical Engineers[edit]

ASME Y14.5.1M-2

GD&T standards for data exchange and integration[edit]

See also[edit]

References[edit]

  1. ^ Dimensioning and Tolerancing, ASME y14.5-2009. NY: American Society of Mechanical Engineers. 2009. ISBN 0-7918-3192-2. 

Further reading[edit]

External links[edit]