Geometric dimensioning and tolerancing

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]

According to the ASME Y14.5-2009[2] standard, the purpose of 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 dimension and tolerance 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 (68 °F) and 101.3 kPa (14.69 psi) 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.

Geometric tolerancing reference chart (per ASME Y14.5 M-1982)

Type of control	Geometric characteristics	Symbol	Unicode character	Relevant feature		Virtual condition affected	References datum	Modified by		Affected by
				Surface	Of size			Ⓜ	Ⓢ	Bonus	Shift
Form	Straightness[3]		⏤ U+23E4	Yes	Yes	Of size[lower-alpha 1]	No	Of size[lower-alpha 1]	No[lower-alpha 3]	Ⓜ[lower-alpha 4]	No
Form	Flatness[4]		⏥ U+23E5	Yes	No	No	No	No	No[lower-alpha 3]	No	No
Form	Circularity[4]		○ U+25CB	Yes	No	No	No	No	No[lower-alpha 3]	No	No
Form	Cylindricity		⌭ U+232D	Yes	No	No	No	No	No[lower-alpha 3]	No	No
Profile	Profile of a line		⌒ U+2312	Yes	No	No	Yes[lower-alpha 5]	No	No[lower-alpha 3]	No	Datum, Ⓜ[lower-alpha 2]
Profile	Profile of a surface		⌓ U+2313	Yes	No	No	Yes[lower-alpha 5]	No	No[lower-alpha 3]	No	Datum, Ⓜ[lower-alpha 2]
Orientation	Perpendicularity		⟂ U+27C2	Yes	Yes	Of size[lower-alpha 1]	Yes	Of size[lower-alpha 1]	No[lower-alpha 3]	Ⓜ[lower-alpha 4]	Datum, Ⓜ[lower-alpha 2]
Orientation	Angularity		∠ U+2220	Yes	Yes	Of size[lower-alpha 1]	Yes	Of size[lower-alpha 1]	No[lower-alpha 3]	Ⓜ[lower-alpha 4]	Datum, Ⓜ[lower-alpha 2]
Orientation	Parallelism		∥ U+2225	Yes	Yes	Of size[lower-alpha 1]	Yes	Of size[lower-alpha 1]	No[lower-alpha 3]	Ⓜ[lower-alpha 4]	Datum, Ⓜ[lower-alpha 2]
Location	Symmetry[lower-alpha 6][lower-alpha 7]		⌯ U+232F	No	Yes	Yes	Yes	No	No	No	No
Location	Position		⌖ U+2316	No	Yes	Yes	Yes	Yes	Yes	Ⓜ[lower-alpha 4]	Datum, Ⓜ[lower-alpha 2]
Location	Concentricity[lower-alpha 6]		◎ U+25CE	No	Yes	Yes	Yes	No	No[lower-alpha 3]	No	No
Run-out	Circular run-out		↗ U+2197	Yes	Yes	Of size[lower-alpha 1]	Yes	No	No[lower-alpha 3]	No	No
Run-out	Total run-out		⌰ U+2330	Yes	Yes	Of size[lower-alpha 1]	Yes	No	No[lower-alpha 3]	No	No

Symbols used in a "feature control frame" to specify a feature's description, tolerance, modifier and datum references

Symbol	Unicode character	Modifier	Notes
	Ⓕ U+24BB	Free state	Applies only when part is otherwise restrained
	Ⓛ U+24C1	Least material condition (LMC)	Useful to maintain minimum wall thickness
	Ⓜ U+24C2	Maximum material condition (MMC)	Provides bonus tolerance only for a feature of size
	Ⓟ U+24C5	Projected tolerance zone	Useful on threaded holes for long studs
	Ⓢ U+24C8	Regardless of feature size (RFS)	Not part of the 1994 version. See para. A5, bullet 3. Also para. D3. Also, Figure 3-8.
	Ⓣ U+24C9	Tangent plane	Useful for interfaces where form is not required
		Continuous feature	Identifies a group of features that should be treated geometrically as a single feature
		Statistical tolerance	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.

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 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.

• Dimensional instruments
• Engineering fit
• Engineering tolerance
• Gauge (instrument)
• Geometrical Product Specification and Verification
• Position sensor
• Specification of surface finish

• McCaleb, Michael R. (1999). "A Conceptual Data Model of Datum Systems" (PDF). Journal of Research of the National Institute of Standards and Technology. 104 (4): 349–400. doi:10.6028/jres.104.024. Archived from the original (PDF) on 2011-10-18. Retrieved 2011-09-13.
• Henzold, Georg (2006). Geometrical Dimensioning and Tolerancing for Design, Manufacturing and Inspection (2nd ed.). Oxford, UK: Elsevier. ISBN 978-0750667388.
• Srinivasan, Vijay (2008). "Standardizing the specification, verification, and exchange of product geometry: Research, status and trends". Computer-Aided Design. 40 (7): 738–49. doi:10.1016/j.cad.2007.06.006.
• Drake, Jr., Paul J. (1999). Dimensioning and Tolerancing Handbook. New York: McGraw-Hill. ISBN 978-0070181311.
• Neumann, Scott; Neumann, Al (2009). GeoTol Pro: A Practical Guide to Geometric Tolerancing per ASME Y14.5-2009. Dearborn, MI: Society of Manufacturing Engineers. ISBN 978-0-87263-865-5.
• Bramble, Kelly L. (2009). Geometric Boundaries II, Practical Guide to Interpretation and Application ASME Y14.5-2009,. Engineers Edge.
• Wilson, Bruce A. (2005). Design Dimensioning and Tolerancing. US: Goodheart-Wilcox. p. 275. ISBN 978-1-59070-328-1.

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