Geometric dimensioning and tolerancing explained

Geometric dimensioning and tolerancing (GD&T) is a system for defining and communicating engineering tolerances via a symbolic language on engineering drawings and computer-generated 3D models that describes a physical object's nominal geometry and the permissible variation thereof. GD&T is used to define the nominal (theoretically perfect) geometry of parts and assemblies, the allowable variation in size, form, orientation, and location of individual features, and how features may vary in relation to one another such that a component is considered satisfactory for its intended use. Dimensional specifications define the nominal, as-modeled or as-intended geometry, while tolerance specifications define the allowable physical variation of individual features of a part or assembly.

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. This article is based on that standard. Other standards, such as those from the International Organization for Standardization (ISO) describe a different system which has some nuanced differences in its interpretation and rules (see GPS&V). The Y14.5 standard provides a fairly complete set of rules 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.). BS 8888 provides a self-contained document taking into account a lot of GPS&V standards.

Origin

The origin of GD&T is credited to Stanley Parker, who developed the concept of "true position". While little is known about Parker's life, it is known that he worked at the Royal Torpedo Factory in Alexandria, West Dunbartonshire, Scotland. His work increased production of naval weapons by new contractors.

In 1940, Parker published Notes on Design and Inspection of Mass Production Engineering Work, the earliest work on geometric dimensioning and tolerancing.[1] In 1956, Parker published Drawings and Dimensions, which became the basic reference in the field.[1]

Fundamental concepts

Dimensions

A dimension is defined in ASME Y14.5 as "a numerical value(s) or mathematical expression in appropriate units of measure used to define the form, size, orientation, or location, of a part or feature." Special types of dimensions include basic dimensions (theoretically exact dimensions) and reference dimensions (dimensions used to inform, not define a feature or part).

Units of measure

The units of measure in a drawing that follows GD&T can be selected by the creator of the drawing. Most often drawings are standardized to either SI linear units, millimeters (denoted "mm"), or US customary linear units, decimal inches (denoted "IN"). Dimensions can contain only a number without units if all dimensions are the same units and there is a note on the drawing that clearly specifies what the units are.

Angular dimensions can be expressed in decimal degrees or degrees, minutes, and seconds.

Tolerances

Every feature on every manufactured part is subject to variation, therefore, the limits of allowable variation must be specified. Tolerances can be expressed directly on a dimension by limits, plus/minus tolerances, or geometric tolerances, or indirectly in tolerance blocks, notes, or tables.

Geometric tolerances are described by feature control frames, which are rectangular boxes on a drawing that indicate the type of geometric control, tolerance value, modifier(s) and/or datum(s) relevant to the feature. The type of tolerances used with symbols in feature control frames can be:

  1. equal bilateral
  2. unequal bilateral
  3. unilateral
  4. no particular distribution (a "floating" zone)

Tolerances for the profile symbols are equal bilateral unless otherwise specified, and for the position symbol tolerances are always equal bilateral. For example, the position of a hole has a tolerance of .020 inches. This means the hole can move ±.010 inches, which is an equal bilateral tolerance. It does not mean the hole can move +.015/−.005 inches, which is an unequal bilateral tolerance. Unequal bilateral and unilateral tolerances for profile are specified by adding further information to clearly show this is what is required.

Datums and datum references

See main article: Datum reference. A datum is a theoretically exact 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}{\scriptstyleA

}\!-\!\!\!-\!\!\!\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. The datum reference frame can describe how the part fits or functions.

Purpose & rules

The purpose of GD&T is to describe the engineering intent of parts and assemblies.[2] GD&T can more accurately define the dimensional requirements for a part, allowing over 50% more tolerance zone than coordinate (or linear) dimensioning in some cases. Proper application of GD&T will ensure that the part defined on the drawing has the desired form, fit (within limits) and function with the largest possible tolerances. GD&T can add quality and reduce cost at the same time through producibility.

According to ASME Y14.5, the fundamental rules of GD&T are as follows,

  1. All dimensions must have a tolerance. 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.
  2. Dimensions and tolerancing shall fully define each feature. Measurement directly from the drawing or assuming dimensions is not allowed except for special undimensioned drawings.
  3. A drawing should have the minimum number of dimensions required to fully define the end product. The use of reference dimensions should be minimized.
  4. Dimensions should be applied to features and arranged to represent the function and mating relationship of the part. There should only be one way to interpret dimensions.
  5. Part geometry should be defined without explicitly specifying manufacturing methods.
  6. If dimensions are required during manufacturing but not the final geometry (due to shrinkage or other causes) they should be marked as non-mandatory.
  7. Dimensions should be arranged for maximum readability and should be applied to visible lines in true profiles.
  8. 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 the dimension.
  9. Angles of 90° are assumed when lines (including center lines) are shown at right angles, but no angle is specified.
  10. Basic 90° angles are assumed where center lines of features in a pattern or surfaces shown at right angles on a 2D orthographic drawing are located or defined by basic dimensions and no angle is specified.
  11. A basic dimension of zero is assumed where axes, center planes, or surfaces are shown coincident on a drawing, and the relationship between features is defined by geometric tolerances.
  12. Dimensions and tolerances are valid at and unless stated otherwise.
  13. Unless explicitly stated, dimensions and tolerances only apply in a free-state condition.
  14. Unless explicitly stated, tolerances apply to the full length, width, and depth of a feature.
  15. Dimensions and tolerances only apply at the level of the drawing where specified. It is not mandatory that they apply at other levels (such as an assembly drawing).
  16. Coordinate systems shown on drawings should be right-handed. Each axis should be labeled and the positive direction should be shown.

Symbols

List of geometric characteristics

Geometric characteristic reference chart
ApplicationType of controlCharacteristicSymbolUnicode
character		Relevant featureVirtual condition affectedReferences datumModified byAffected by
Surface
Individual featuresFormStraightness⏤
U+23E4
Flatness[3] ⏥
U+23E5
Circularity○
U+25CB
Cylindricity⌭
U+232D
Individual or related featuresProfileProfile of a line⌒
U+2312
Profile of a surface⌓
U+2313
Related featuresOrientationPerpendicularity⟂
U+27C2
Angularity∠
U+2220
Parallelism∥
U+2225
LocationSymmetry⌯
U+232F
Position⌖
U+2316
Concentricity◎
U+25CE
Run-outCircular run-out↗
U+2197
Total run-out⌰
U+2330

List of modifiers

The following table shows only some of the more commonly used modifiers in GD&T. It is not an exhaustive list.

Symbols used in a "feature control frame" to specify a feature's description, tolerance, modifier and datum references
Symbol Unicode
character 		Modifier DefinitionNotes
Ⓕ
U+24BB 		Free state "The condition of a part free of applied forces"Applies only when part is otherwise restrained
Ⓛ
U+24C1 		Least material condition (LMC) "The condition in which a feature of size contains the least amount of material within the stated limits of size"Useful to maintain minimum wall thickness
Ⓜ
U+24C2 		Maximum material condition (MMC) "The condition in which a feature of size contains the maximum amount of material within the stated limits of size"Provides bonus tolerance only for a feature of size
Ⓟ
U+24C5 		Useful on threaded holes for long studs
Ⓢ
U+24C8 		Regardless of feature size (RFS) "Indicates a geometric tolerance applies at any increment of size of the actual mating envelope of the feature of size"t part of the 1994 version. See para. A5, bullet 3. Also para. D3. Also, Figure 3–8.
Ⓣ
U+24C9 		Tangent plane "A plane that contacts the high points of the specified feature surface"Useful for interfaces where form is not required
Continuous feature Identifies "a group of features of size where there is a requirement that they be treated geometrically as a single feature of size"Identifies a group of features that should be "treated geometrically as a single feature"
Statistical tolerance Indicates that features "shall be produced with statistical process controls".Appears in the 1994 version of the standard, assumes appropriate statistical process control.
Ⓤ
U+24CA 		Unequal bilateral Added in the 2009 version of the standard, and refers to unequal profile distribution. Number after this symbol indicates tolerance in the "plus material" direction.

Certification

The American Society of Mechanical Engineers (ASME) provides two levels of certification:[4]

Data exchange

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

Documents and standards

ISO TC 10 Technical product documentation

ISO/TC 213 Dimensional and geometrical product specifications and verification

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

ASME standards

ASME is also working on a Spanish translation for the ASME Y14.5 – Dimensioning and Tolerancing Standard.

GD&T standards for data exchange and integration

See also

Further reading

External links

Notes and References

  1. Web site: Bibliography for Dimensioning and Tolerancing . 2014 . David M. . MacMillan . Rollande . Krandall . Circuitous Root . October 24, 2018 . https://web.archive.org/web/20190327221816/https://www.circuitousroot.com/artifice/drafting/drawing-studies/dt/bibliography-for-dt/index.html . 27 March 2019 . live.
  2. Book: Dimensioning and Tolerancing, ASME Y14.5-2009 . American Society of Mechanical Engineers . 2009 . 978-0-7918-3192-2 . NY.
  3. Web site: GD&T, Geometric Dimensioning and Tolerancing, GD&T, Flatness, Circularity, Flatness Tolerance, Circularity Tolerance. cobanengineering.com. 2020-04-02.
  4. Web site: Resources. Technical Training Consultants. 2020-09-20 . 2020.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Geometric dimensioning and tolerancing".

Except where otherwise indicated, Everything.Explained.Today is © Copyright 2009-2024, A B Cryer, All Rights Reserved. Cookie policy.