Geometric dimensioning and tolerancing

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Geometric dimensioning and tolerancing is a symbolic language used on engineering drawings and computer generated three-dimensional solid models for explicitly describing nominal geometry and its allowable variation. It is often referred to by the abbreviation, GD&T. Geometric dimensioning and tolerancing is used to define the nominal geometry of parts and assemblies, to define the allowable variation in form and possibly size of individual features, and to define the allowable variation between features.

Dimensioning and tolerancing and geometric dimensioning and tolerancing specifications are used as follows:

Dimensioning specifications define the nominal, as-modeled or as-intended geometry.

Tolerancing specifications define the allowable variation for the form and possibly the size of individual features, and the allowable variation in orientation and location between features.

There are several standards available world-wide that describe the symbols and define the rules used in GD&T. One such standard is ASME Y14.5M-1994. This article is based on that standard, but other standards such as those from the International Organization for Standardization may vary slightly.

Contents

[edit] Dimensioning and tolerancing philosophy

According the ASME Y14.5M-1994 standard, the purpose of 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.

The purpose of GD&T is more accurately defined as describing the geometric requirements for part and assembly geometry. 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:

  • All dimensions must have a tolerance. Every feature on every manufactured part is subject to variation, therefore, the limits of allowable variation must be specified. Plus and minus tolerances may be applied directly to dimensions or applied from a general tolerance block or general note. For basic dimensions, geometric tolerances are indirectly applied in a related Feature Control Frame. The only exceptions are for dimensions marked as minimum, maximum, stock or reference.
  • Dimensioning and tolerancing shall completely define the nominal geometry and allowable variation. Measurement and scaling of the drawing is not allowed except in certain cases.
  • Engineering drawings define the requirements of finished (complete) parts. Every dimension and tolerance required to define the finished part shall be shown on the drawing. If additional dimensions would be helpful, but are not required, they may be marked as reference.
  • Dimensions should be applied to features and arranged in such a way as to represent the function of the features.
  • Descriptions of manufacturing methods should be avoided. The geometry should be described without explicitly defining the method of manufacture.
  • If certain sizes are required during manufacturing but are not required in the final geometry (due to shrinkage or other causes) they should be marked as non-mandatory.
  • All dimensioning and tolerancing should be arranged for maximum readability and should be applied to visible lines in true profiles.
  • When geometry is normally controlled by gage sizes or by code (e.g. stock materials), the dimension(s) shall be included with the gage or code number in parentheses following or below the dimension.
  • Angles of 90° are assumed when lines (including center lines) are shown at right angles, but no angular dimension is explicitly shown. (This also applies to other orthogonal angles of 0°, 180°, 270°, etc.)
  • Dimensions and tolerances are valid at 20 °C unless stated otherwise.
  • Unless explicitly stated, all dimensions and tolerances are valid when the item is in a free state.
  • Dimensions and tolerances apply to the full length, width, and depth of a feature.
  • Dimensions and tolerances only apply at the level of the drawing where they are specified. It is not mandatory that they apply at other drawing levels, unless the specifications are repeated on the higher level drawing(s).

(Note: The rules above are not the exact rules stated in the ASME Y14.5M-1994 standard.)

Symbol Description Geometry
Image:Gd&t angularity.png ANGULARITY ORIENTATION
Image:Gd&t concentricity.png CONCENTRICITY LOCATION
Image:Gd&t cylindricity.png CYLINDRICITY FORM
Image:Gd&t flatness.png‎ FLATNESS FORM
Image:Gd&t parallelism.png PARALLELISM ORIENTATION
Image:Gd&t perpendicularity.png PERPENDICULARITY ORIENTATION
Image:Gd&t position.png‎ POSITION LOCATION
Image:Gd&t profile.png‎ PROFILE PROFILE
Image:Gd&t profileline.png PROFILE OF A LINE PROFILE
Image:Gd&t circularity.png‎ CIRCULARITY FORM
Image:Gd&t runout.png RUNOUT RUNOUT
Image:Gd&t straightness.png STRAIGHTNESS FORM
Image:Gd&t symmetry.png SYMMETRY LOCATION
Image:Gd&t totalrunout.png TOTAL RUNOUT RUNOUT
Symbol Modifier
Image:Gd&t freestate.png FREE STATE
Image:Gd&t leastmaterialcondition.png LEAST MATERIAL CONDITION
Image:Gd&t maximummaterialcondition.png MAXIMUM MATERIAL CONDITION
Image:Gd&t projectedtolerancezone.png‎ PROJECTED TOLERANCE ZONE
Image:Gd&t regardlessoffeaturesize.png REGARDLESS OF FEATURE SIZE
Image:Gd&t tangentplane.png TANGENT PLANE
Image:Gd&t unilateral.png UNILATERAL

[edit] Symbols

  • The |(U)| symbol is not part of the '1994 version. It is in 'Y14.41-' and may appear in a future version of 'Y14.5. It refers to unequal profile distribution.
  • |(S)| is not part of the '1994 version. See para. A5, bullet 3. Also para. D3. Also, Figure 3-8.
  • These symbols are used in a feature control frame to specify a feature's description, tolerance, modifier and datum references.

[edit] GD&T data exchange

Exchange of GD&T information between CAD systems is available on different levels of fidelity for different purposes:

  • In the early days of CAD exchange only lines, texts and symbols were written into the exchange file. A receiving system could only display them on the screen or print them out, but only a human could .
  • GD&T presentation: On a next higher level the presentation information is enhanced by grouping them together into callouts for a particular purpose, e.g. a datum feature callout and a datum reference frame. And there is also the information which of the curves in the exchange file are leader, projection or dimension curves and which are used to form the shape of a product.
  • GD&T representation: Unlike GD&T presentation, the GD&T representation does not deal with how the information is presented to the user but only deal with which element of a shape of a product has which GD&T characteristic. A system supporting GD&T representation may display the GD&T information in some tree and other dialogs and allow the user to directly select and highlight the corresponding feature on the shape of the product, 2D and 3D.
  • Ideally both GD&T presentation and representation are available in the exchange file and are associated with each other. Then a receiving system can allow a user to select a GD&T callout and get the corresponding feature highlighted on the shape of the product.
  • An enhancement of GD&T representation is defining a formal language for GD&T (similar like a programming language) which also has build in rules and restrictions for the proper GD&T usage. This is still a research area (see below reference to McCaleb and ISO 10303-1666).
  • GD&T validation: Based on GD&T representation data (but not on GD&T presentation) and the shape of a product in some useful format (e.g. a Boundary representation), it is possible to validate the completeness and consistency of the GD&T information. The software tool FBTol from the Kansas City Plant is probably the first one in this area.
  • GD&T representation information can also be used for the software assisted manufacturing planning and cost calculation of parts. See ISO 10303-224 and 238 below.

[edit] GD&T standards

[edit] GD&T standards for technical drawings (2D)

  • ASME Y14.5M-1994 Dimensioning and Tolerancing
  • ASME Y14.5.1M-1994 Mathematical Definition of Dimensioning and Tolerancing Principles
  • ISO 286-1:1988 ISO system of limits and fits — Part 1: Bases of tolerances, deviations and fits
  • ISO 286-2:1988 ISO system of limits and fits — Part 2: Tables of standard tolerance grades and limit deviations for holes and shafts
  • ISO 1101:2005 Geometrical Product Specifications (GPS) — Geometrical tolerancing — Tolerancing of form, orientation, location and run-out
  • ISO 5458:1998 Geometric Product Specifications (GPS) — Geometrical tolerancing — Positional tolerancing
  • ISO 5459:1981 Technical drawings — Geometrical tolerancing — Datums and datum-systems for geometrical tolerances

[edit] GD&T standards for CAD systems (3D)

  • ASME Y14.41-2003 Digital Product Definition Data Practices
  • ISO 16792:2006 Technical product documentation -- Digital product definition data practices

(Note: ISO 16792:2006 was derived from ASME Y14.41-2003 by permission of ASME.)

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

  • ISO 10303 Industrial automation systems and integration — Product data representation and exchange
    • ISO 10303-47:1997 Integrated generic resource: Shape variation tolerances
    • ISO/TS 10303-1130:2006 Application module: Derived shape element
    • ISO/TS 10303-1050:2006 Application module: Dimension tolerance
    • ISO/TS 10303-1051:2006 Application module: Geometric tolerance
    • ISO/TS 10303-1052:2005 Application module: Default tolerance
    • ISO/TS 10303-1666:2006 Application module: Extended geometric tolerance
    • ISO 10303-203:2007/8 Application protocol: Configuration controlled 3D design of mechanical parts and assemblies
    • ISO 10303-210:2001 Application protocol: Electronic assembly, interconnection, and packaging design
    • ISO 10303-214:2003 Application protocol: Core data for automotive mechanical design processes
    • ISO 10303-224:2006 Application protocol: Mechanical product definition for process planning using machining features
    • ISO 10303-238:2007 Application protocol: Application interpreted model for computerized numerical controllers

[edit] References

  • M. R. McCaleb, "A Conceptual Data Model of Datum Systems," J. Res., Natl. Inst. Stand. Technol. (U.S.), 104, No. 4, pp. 349-400 (Jul/Aug 1999).

[edit] External links

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