Geometric dimensioning and tolerancing

Example of geometric dimensioning and tolerancing

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.

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.).

History

The origin of GD&T has been credited to a man named Stanley Parker, who developed the concept of "true position" in 1938.[1] While very little is known about the life of Stanley Parker, it is recorded that he worked at the Royal Torpedo Factory in Alexandria, Scotland. Parker's work was used to increase production of naval weapons by new contractors.[2]

Dimensioning and tolerancing philosophy

According to the ASME Y14.5-2009[3] 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):

(Note: The rules above are not the exact rules stated in the ASME Y14.5-2009 standard.)

Symbols

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 Character
(Unicode)		 Can be applied to a surface? Can be applied to a feature of size? Can affect virtual condition? Datum reference used? Can use
modifier?		 Can use
modifier?		 Can be affected by a bonus tolerance? Can be affected by a shift tolerance?
Form Straightness
⏤
U+23E4
 Yes Yes Yes
(note 1)		 No Yes
(note 1)		 No
(note 5)		 Yes
(note 4)		 No
Form Flatness
⏥
U+23E5
 Yes No No No No No
(note 5)		 No No
Form Circularity
○
U+25CB
 Yes No No No No No
(note 5)		 No No
Form Cylindricity
⌭
U+232D
 Yes No No No No No
(note 5)		 No No
Profile Profile of a line
⌒
U+2312
 Yes No No Yes
(note 2)		 No No
(note 5)		 No Yes
(note 3)
Profile Profile of a surface
⌓
U+2313
 Yes No No Yes
(note 2)		 No No
(note 5)		 No Yes
(note 3)
Orientation Perpendicularity
⟂
U+27C2
 Yes Yes Yes
(note 1)		 Yes Yes
(note 1)		 No
(note 5)		 Yes
(note 4)		 Yes
(note 3)
Orientation Angularity
∠
U+2220
 Yes Yes Yes
(note 1)		 Yes Yes
(note 1)		 No
(note 5)		 Yes
(note 4)		 Yes
(note 3)
Orientation Parallelism
∥
U+2225
 Yes Yes Yes
(note 1)		 Yes Yes
(note 1)		 No
(note 5)		 Yes
(note 4)		 Yes
(note 3)
Location Symmetry
⌯
U+232F
 No
(note 6)		 Yes
(note 6)		 Yes
(note 6)		 Yes
(note 6)		 No
(note 6)		 No
(note 6)		 No
(note 6)		 No
(note 6)
Location Position
⌖
U+2316
 No Yes Yes Yes Yes Yes Yes
(note 4)		 Yes
(note 3)
Location Concentricity
◎
U+25CE
 No Yes Yes Yes No No
(note 5)		 No No
Run-out Circular run-out
↗
U+2197
 Yes Yes Yes
(note 1)		 Yes No No
(note 5)		 No No
Run-out Total run-out
⌰
U+2330
 Yes Yes Yes
(note 1)		 Yes No No
(note 5)		 No No

Notes:

  1. When applied to a feature-of-size.
  2. Can also be used as a form control without a datum reference.
  3. When a datum feature-of-size is referenced with the MMC modifier.
  4. When an MMC modifier is used.
  5. Automatic per rule #3.
  6. 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.
Symbols used in a "feature control frame" to specify a feature's description, tolerance, modifier and datum references
Symbol Modifier Notes
 Free state Applies only when part is otherwise restrained
 Least material condition (LMC) Useful to maintain minimum wall thickness
 Maximum material condition (MMC) Provides bonus tolerance only for a feature of size
 Projected tolerance zone Useful on threaded holes for long studs
 Regardless of feature size (RFS) Not part of the 1994 version. See para. A5, bullet 3. Also para. D3. Also, Figure 3-8.
 Tangent plane Useful for interfaces where form is not required
 Unequal Bilateral Appears in the 2009 version of the standard, and refers to unequal profile distribution.

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 .

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

References

  1. ↑ "GD&T | Geometric Dimensioning and Tolerancing | Quality-One". quality-one.com. Retrieved 2017-07-28.
  2. ↑ "Bibliography for Dimensioning and Tolerancing". www.circuitousroot.com. Retrieved 2017-07-28.
  3. ↑ Dimensioning and Tolerancing, ASME y14.5-2009. NY: American Society of Mechanical Engineers. 2009. ISBN 0-7918-3192-2.

Further reading

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