Geometric Dimensioning and Tolerancing
|
|
Geometric dimensioning and tolerancing is a method for explicitly describing a geometry and the allowable variation in the size and position of its features. It is often referred to by the abbreviation, GD&T.
GD&T is used most often on engineering drawings.
There are many 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 document will be based on that standard. Other standards may vary slightly.
Fundamental Rules
As the purpose of GD&T is to describe the engineering intent of the item, there are some fundamental rules that need to be applied:
- All dimensions must have a tolerance. Nothing can be made to perfection; therefore, an appropriate tolerance must be available. Tolerances may come in the form of limits of size applied to basic dimensions, with +- sytle tolerance applied directly to dimensions as a tolerance block or a general note. The only exception is when a dimension is marked as minimum, maximum, stock or reference.
- All dimensions necessary to exactly reproduce the shown geometry must by present. Measurement and scaling should not be required.
- In order to avoid ambiguities, only the minimum dimensions required should be present. If additional dimensions would be helpful but not required, they should be marked as reference.
- Dimension should be applied to features and arranged in such a way as to represent the function of the individual features.
- Descriptions of manufacturing methods should be avoided. The geometry should be described without expicitly defining the method of manufacture.
- If certain sizes are required during the processing, but are not required in the final geometry (due to shrinkage or other causes) they should be marked as NON-MANDATORY.
- All symbols should be arranged for maximum readability. They should be applied to visible lines in true profiles whenever possible.
- When geometry is normally controlled by gage sizes or by code, the dimensions should still be included but should with the gauge or code number in parentheses following or below the dimension.
- Angles of 90 degrees are assumed when basic dimensions or centerlines are used but no angular dimension is explicitly shown.
- Dimensions and tolerances are valid at 20 degrees Celsius unless stated otherwise.
- Unless explicitly stated, all dimensions and tolerances are valid when the item is in a free, unconstrained state.
- Dimensions and tolerances apply to the full length, width, and depth of a feature.
- Dimensions only apply at the level of the drawing where they are placed. If the intention is for them to apply at multiple levels, this must be stated.
Symbols
External References
- GD&T Glossary (http://etinews.com/gdt_glossary.html) A complete list of topics that are involved in geometric dimensioning and tolerancing with a short definition of each.
- GD&T Newsletter (http://www.etinews.com/etimail/) A free online publication devoted to the topic of geometric dimensioning and tolerancing.