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.
Dimensioning specifications define the nominal, as-modeled or as-intended geometry. One example is a basic dimension.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. Two examples are linear dimensions and feature control frames using a datum reference (both shown above).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
According to the ASME Y14.5-2009 standard, the purpose of geometric dimensioning and tolerancing (GD&T) is to describe the engineering intent of parts and assemblies. The datum reference frame can describe how the part fits or functions. 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.
There are some fundamental rules that need to be applied (these can be found on page 7 of the 2009 edition of the standard):
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.Dimensions 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. Additionally, dimensions should not be subject to more than one interpretation.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 / 101.3 kPa unless stated otherwise.Unless explicitly stated, all dimensions and tolerances are only valid when the item is in a free state.Dimensions and tolerances apply to the length, width, and depth of a feature including form variation.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 Y14.5-2009 standard.)
Tolerances: 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.
Notes:
- When applied to a feature-of-size.
- Can also be used as a form control without a datum reference.
- When a datum feature-of-size is referenced with the MMC modifier.
- When an MMC modifier is used.
- Automatic per rule #3.
- 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.
Datums and datum references
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.,
◻ A − − ◂ | These are then referred to by one or more or many 'datum references' which indicate measurements that should be made with respect to the corresponding datum feature .
Exchange of geometric dimensioning and tolerancing (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 display them on the screen or print them out, but only a human could interpret them.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 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 deals 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 to a programming language) which also has built-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.ISO 128 Technical drawings – Indication of dimensions and tolerancesISO 7083 Symbols for geometrical tolerancing – Proportions and dimensionsISO 13715 Technical drawings – Edges of undefined shape – Vocabulary and indicationsISO 15786 Simplified representation and dimensioning of holesISO 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)ISO 2010ISOISO/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.
Fundamental GPS standardsISO 8015 Concepts, principles and rulesGlobal GPS standardsISO 14660-1 Geometrical featuresISO/TS 17, orientation and locationISO 1101 Geometrical tolerancing – Tolerances of form, orientation, location and run-outAmendment 1 Representation of specifications in the form of a 3D modelISO 1119 Series of conical tapers and taper anglesISO 2692 Geometrical tolerancing – Maximum material requirement (MMR), least material requirement (LMR) and reciprocity requirement (RPR)ISO 3040 Dimensioning and tolerancing – ConesISO 5458 Geometrical tolerancing – Positional tolerancingISO 5459 Geometrical tolerancing – Datums and datum systemsISO 10578 Tolerancing of orientation and location – Projected tolerance zoneISO 10579 Dimensioning and tolerancing – Non-rigid partsISO 14406 ExtractionISO 22432 Features used in specification and verificationGeneral GPS standards: Areal and profile surface textureISO 1302 Indication of surface texture in technical product documentationISO 3274 Surface texture: Profile method – Nominal characteristics of contact (stylus) instrumentsISO 4287 Surface texture: Profile method – Terms, definitions and surface texture parametersISO 4288 Surface texture: Profile method – Rules and procedures for the assessment of surface textureISO 8785 Surface imperfections – Terms, definitions and parametersForm of a surface independent of a datum or datum system. Each of them has a part 1 for the Vocabulary and parameters and a part 2 for the Specification operators:ISO 12180 CylindricityISO 12181 RoundnessISO 12780 StraightnessISO 12781 FlatnessISO 25178 Surface texture: ArealGeneral GPS standards: Extraction and filtration techniquesISO/TS 1661 FiltrationISO 11562 Surface texture: Profile method – Metrological characteristics of phase correct filtersISO 12085 Surface texture: Profile method – Motif parametersISO 13565 Profile method; Surfaces having stratified functional propertiesASME standards
ASME Y14.41-2009 Digital Product Definition Data PracticesASME Y14.5 – Dimensioning and TolerancingASME Y14.5M-1994 Dimensioning and TolerancingASME Y14.5.1M-1994 Mathematical Definition of Dimensioning and Tolerancing PrinciplesGD&T standards for data exchange and integration
ISO 10303 Industrial automation systems and integration — Product data representation and exchangeISO 10303-47 Integrated generic resource: Shape variation tolerancesISO/TS 10303-1130 Application module: Derived shape elementISO/TS 10303-1050 Application module: Dimension toleranceISO/TS 10303-1051 Application module: Geometric toleranceISO/TS 10303-1052 Application module: Default toleranceISO/TS 10303-1666 Application module: Extended geometric toleranceISO 10303-203 Application protocol: Configuration controlled 3D design of mechanical parts and assembliesISO 10303-210 Application protocol: Electronic assembly, interconnection, and packaging designISO 10303-214 Application protocol: Core data for automotive mechanical design processesISO 10303-224 Application protocol: Mechanical product definition for process planning using machining featuresISO 10303-238 Application protocol: Application interpreted model for computerized numerical controllers (STEP-NC)
