This information provided by Bryan R. Fischer, of TDP360 LLC.
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The Purpose of GD&T
In the early 1900s, as mass production, assembly lines, and interchangeable parts became more prominent, industry recognized that a better way to define product geometry and its allowable variation was needed. Geometric Dimensioning and Tolerancing (GD&T) was developed in the mid-1900s to satisfy that need.
The primary purpose of GD&T is to define unambiguous requirements for product geometry. This can be separated into three beneficial purposes. GD&T allows us to:
Unambiguously define nominal (perfect) product geometry.
Unambiguously define allowable variation for product geometry.
Unambiguously define the setup to evaluate geometry of an actual manufactured product.
GD&T allows us to define requirements that can be rigorously and repeatably evaluated on actual imperfect parts. All manufactured parts are imperfect – e.g., manufactured surfaces are not perfectly flat, and holes are not perfectly round or in the perfect location. GD&T defines how tolerances apply to imperfect parts and what conformance means. If GD&T is used properly, how a part should be set up for inspection, the geometric requirements defined by the specifications, and which geometry must conform to the requirements are clearly defined. Plus and minus tolerancing alone does not and cannot define unambiguous geometric requirements.
Note that if GD&T is applied properly, understood, and used properly, GD&T leads to less expensive parts. This occurs because properly applied GD&T specifications are unambiguous and lead to greater understanding of requirements and conformance criteria, less confusion, less scrap and rework, and faster overall production.
Geometric CharacteristicsThere are four geometric characteristics used to describe geometry: Form Size Orientation Location Form & Size are characteristics of individual features. Every feature has form. Form describes the shape of a feature. Some features have size, some don’t. For example, a cylinder has size, but a single flat surface does not. (Size is defined in another part of this material.) Form and size describe geometry of individual features. Orientation & Location are relationships between features. The relationship between features can be defined as the orientation of features (inclination of features), and the location of features (distance between them). |
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GD&T provides geometric tolerances that allow us to define the allowable variation for the form and size of features, and geometric tolerances that allow us to define the orientation and location between features.
Dimension and Tolerance Formats
Metric and inch dimensions and tolerances as shown below have different rules for leading and trailing zeroes.
Inches – Defaults Metric – Defaults
Don’t use leading zeroes, use trailing zeroes Use leading zeroes, don’t use trailing zeroes in most cases
Dimensions with ± Tolerances and Limit Dimensions
Inch and metric dimensions and tolerances have different rules for the number of decimal places.
Inch dimensions and tolerances have the same number of decimal places (see preceding figure).
Metric dimensions and tolerances may have a different number of decimal places (see preceding figure).
Geometric Tolerances and Basic Dimensions
In inch and metric, basic dimensions and geometric tolerances may have a different number of decimal places.
Limits in GD&T
In GD&T all limits are considered as absolute. Dimensions and tolerances define limits, upper and lower limits which are directly specified or may be calculated. For example, a limit dimension of 10.2 - 10.4 directly specifies limits of 10.2mm and 10.4mm. If a dimension and tolerance is defined as 10.3 ±0.1, we get the same limits of 10.2 and 10.4. All limits in GD&T are treated as being followed by an infinite number of zeroes. So, 10.2 means 10.200000000… and 10.4 means 10.400000000…
For a measured value to conform to specified limits, the value must be on or between the limits. Any value outside the limits does not conform to the specification. For example, given a limit dimension of 10.2 - 10.4, 10.1999999999 and 10.4000000001 are outside the limits do not conform to the specification.