Geometric Dimensioning and Tolerancing - Dimensioning and Tolerancing Philosophy

Dimensioning and Tolerancing Philosophy

According to the ASME Y14.5-2009 standard, the purpose of geometric dimensioning and tolerancing (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 (these can be found on page 6 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.
• 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 °C / 101.3 kPa 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 full 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.)