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Geometric dimensioning and tolerancing is a symbolic language used on engineering drawings and computer generated three-dimensional solid models (CAD) for explicitly describing nominal geometry and its allowable variation. It is often referred to by the abbreviation, GD&T. Table of Geometry, from the 1728 Cyclopaedia. ...
Geometric Dimensioning and Tolerancing is used to define the nominal geometry of parts and assemblies, to define the allowable variation in form and possibly size of individual features, and to define the allowable variation between features.
Dimensioning and Tolerancing and Geometric Dimensioning and Tolerancing specifications are used as follows:
Dimensioning specifications define the nominal, as-modeled or as-intended geometry.
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.
GD&T is used most often on engineering drawings. It has been suggested that this article or section be merged into Technical drawing. ...
There are several standards available world-wide that describe the symbols and define the rules used in GD&T. One such standard is ASME Y14.5M-1994. This article is based on that standard, but other standards may vary slightly. The American Society of Mechanical Engineers (ASME) is a professional body, specifically an engineering society, focused on mechanical engineering. ...
Dimensioning and Tolerancing Philosophy According the ASME Y14.5M-1994 standard, the purpose of GD&T 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 (within limits) 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 related Feature Control Frame. The only exceptions are for dimensions marked as minimum, maximum, stock or reference.
- Dimensioning and tolerancing shall completely 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.
- 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° Celsius unless stated otherwise.
- Unless explicitly stated, all dimensions and tolerances are valid when the item is in a free state.
- Dimensions and tolerances apply to the full length, width, and depth of a feature.
- 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 Y14.5M-1994 standard.) Celsius is, or relates to, the Celsius temperature scale. ...
Image File history File links Gd&t. ...
Symbols - The |(U)| symbol is not part of the '1994 version. It is in 'Y14.41-' and may appear in a future version of 'Y14.5. It refers to unequal profile distribution.
- |(S)| is not part of the '1994 version. See para. A5, bullet 3. Also para. D3. Also, Figure 3-8.
- These symbols are used in a feature control frame to specify a feature's description, tolerance, modifier and datum(s).
It has been suggested that this article or section be merged with Geodetic system. ...
External references - GD&T Information Information on GD&T, its history, how it is used and why it is important.
- GD&T Skills Test A free tool that measures how well an individual understands the correct application and interpretation of GD&T.
- GD&T Symbols and Definitions GD&T symbols and definitions list.
- ASME Y14.5M(1994) A standard reference for Dimensioning and Tolerancing.
- Training Resources A list of GD&T trainers who have passed the ASME Professional GD&T Certification Test.
- Justification for GD&T Reasons to use GD&T.
- GD&T Glossary A complete list of topics that are involved in geometric dimensioning and tolerancing with a short definition of each.
- What are the Advantages of using GD&T? Artical (sp) discusses why gd&t is important within industry.
- Mechanical Design Tolerance Calculators Ultilizing (sp) GD&T Free engineering and design on-line tolerance calculators. Note: These are only valid in very limited circumstances.
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