tolerances - geometric tolerance

Geometric dimensioning and tolerancing (GD&T) is used to define the nominal (theoretically perfect) 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. One example is a basic dimension.
• 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. Two examples are linear dimensions and feature control frames using a datum reference (both shown above).

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 °C 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).

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 1988 and re-added around 1994.

Notes :

1. The symbol is not part of the 1994 version. 
2. The symbol appears in the 2009 version of the standard, and refers to unequal profile distribution.
3. All the symbols above are used in a feature control frame to specify a feature's description, tolerance, modifier and datum references.