Geometric dimensioning and tolerancing 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 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. 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 Y14.5-2009. This article is based on that standard, but
other standards, such as those from the International Organization for Standardization, 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.
Dimensioning and tolerancing philosophy According to the ASME Y14.5-2009 standard,
the purpose of geometric dimensioning and tolerancing 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 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 relatedFeature 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 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
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, 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 are shown at right angles, but no angular dimension
is explicitly shown. 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 full 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.)
Symbols Tolerances: Type of Tolerances; 1) Unilateral
2) Bi- Lateral type 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.,
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 Exchange of geometric dimensioning and tolerancing
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 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 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 which also has built-in rules and restrictions for the proper
GD&T usage. This is still a research area. GD&T validation: Based on GD&T representation
data and the shape of a product in some useful format, 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. GD&T
ISO TC 10 Technical product documentation ISO 128 Technical drawings – Indication
of dimensions and tolerances ISO 7083 Symbols for geometrical tolerancing
– Proportions and dimensions ISO 13715 Technical drawings – Edges of
undefined shape – Vocabulary and indications ISO 15786 Simplified representation and dimensioning
of holes ISO 16792:2006 Technical product documentation—Digital
product definition data practices ISO 2010
ISO 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.
Fundamental GPS standards ISO 8015 Concepts, principles and rules
Global GPS standards ISO 14660-1 Geometrical features
ISO/TS 17, orientation and location ISO 1101 Geometrical tolerancing – Tolerances
of form, orientation, location and run-out Amendment 1 Representation of specifications
in the form of a 3D model
ISO 1119 Series of conical tapers and taper angles
ISO 2692 Geometrical tolerancing – Maximum material requirement, least material requirement
and reciprocity requirement ISO 3040 Dimensioning and tolerancing – Cones
ISO 5458 Geometrical tolerancing – Positional tolerancing
ISO 5459 Geometrical tolerancing – Datums and datum systems
ISO 10578 Tolerancing of orientation and location – Projected tolerance zone
ISO 10579 Dimensioning and tolerancing – Non-rigid parts
ISO 14406 Extraction ISO 22432 Features utilized in specification
and verification
General GPS standards: Areal and profile surface texture
ISO 1302 Indication of surface texture in technical product documentation
ISO 3274 Surface texture: Profile method – Nominal characteristics of contact instruments
ISO 4287 Surface texture: Profile method – Terms, definitions and surface texture parameters
ISO 4288 Surface texture: Profile method – Rules and procedures for the assessment of surface
texture ISO 8785 Surface imperfections – Terms,
definitions and parameters Form 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 Cylindricity
ISO 12181 Roundness ISO 12780 Straightness
ISO 12781 Flatness
ISO 25178 Surface texture: Areal
General GPS standards: Extraction and filtration techniques
ISO/TS 1661 Filtration ISO 11562 Surface texture: Profile method
– Metrological characteristics of phase correct filters
ISO 12085 Surface texture: Profile method – Motif parameters
ISO 13565 Profile method; Surfaces having stratified functional properties
ASME standards American Society of Mechanical Engineers
ASME Y14.41-2003 Digital Product Definition Data Practices
ASME Y14.5 - 2009 Dimensioning and Tolerancing ASME Y14.5M-1994 Dimensioning and Tolerancing
ASME Y14.5.1M-1994 Mathematical Definition of Dimensioning and Tolerancing Principles
ASME Y14.5.1M-2 GD&T standards for data exchange and integration
ISO 10303 Industrial automation systems and integration — Product data representation
and exchange ISO 10303-47 Integrated generic resource:
Shape variation tolerances ISO/TS 10303-1130 Application module: Derived
shape element ISO/TS 10303-1050 Application module: Dimension
tolerance ISO/TS 10303-1051 Application module: Geometric
tolerance ISO/TS 10303-1052 Application module: Default
tolerance ISO/TS 10303-1666 Application module: Extended
geometric tolerance ISO 10303-203 Application protocol: Configuration
controlled 3D design of mechanical parts and assemblies
ISO 10303-210 Application protocol: Electronic assembly, interconnection, and packaging design
ISO 10303-214 Application protocol: Core data for automotive mechanical design processes
ISO 10303-224 Application protocol: Mechanical product definition for process planning using
machining features ISO 10303-238 Application protocol: Application
interpreted model for computerized numerical controllers
See also Specification of surface finish
References
Further reading McCale, Michael R.. "A Conceptual Data Model
of Datum Systems". Journal of Research of the National Institute of Standards and Technology
104: 349–400. doi:10.6028/jres.104.024. HENZOLD, Georg. Geometrical Dimensioning and
Tolerancing for Design, Manufacturing and Inspection. 2nd Edition, Elsevier, Oxford,
UK, 2006. Srinivasan, Vijay. "Standardizing the specification,
verification, and exchange of product geometry: Research, status and trends". Computer-Aided
Design 40: 738–49. doi:10.1016/j.cad.2007.06.006. DRAKE JR, Paul J. Dimensioning and Tolerancing
Handbook. McGraw-Hill, New York, 1999 Neumann, Scott and Al Neumann. GeoTol Pro:
A Practical Guide to Geometric Tolerancing per ASME Y14.5-2009. Society of Manufacturing
Engineers, Dearborn, MI, 2009. ISBN # 978-0-8726-3865-5 Bramble, Kelly L. Geometric Boundaries II,
Practical Guide to Interpretation and Application ASME Y14.5-2009, Engineers Edge, 2009
Wilson, Bruce A.. Design Dimensioning and Tolerancing. US: Goodheart-Wilcox. p. 275.
ISBN 9781590703281. External links
General tolerances for linear and angular dimensions according to ISO 2768
Interactive map of GD&T. jj What is GD&T
The importance of GD&T GD&T Glossary of Terms and Definitions
Free GD&T Help and Tips Justification for GD&T
GDT: Introduction ASME Certification
Changes and Additions to ASME Y14.5M NIST MBE PMI Validation and Conformance Testing
Project Tests implementations of GD&T in CAD software