What is Geometrical Dimensioning and Tolerancing?
Geometric Dimensioning and Tolerancing (GD&T) is a global engineering language of symbols, used to efficiently and accurately communicate geometrical requirements for design features. It encourages designers to define a part based on its function (design intent) in the final product. ASME Y14.5M – 1994 is a globally accepted geometric dimensioning and tolerancing standard.
An example: Part dimensions using GD&T.
Types of tolerances
- Limit tolerancing
- Geometric tolerancing
Limit tolerancing –
- Tolerance can be specified only where a dimension is given.
- Gives an acceptable range of values of an individual dimension (limits of dimension).
- No provision to specify how flat a surface needs to be, or how much a hole can tilt relative to a surface.
- Induces problems related to ambiguity, guesswork and multiple interpretations of part drawing. Results in deviation from the design intent.
Geometric tolerancing –
- Separates the specification of tolerance from the dimensioning.
- Specifies a geometric region (tolerance zones), such as an area or a volume, in which the feature must lie in order to meet the design criteria.
- Communicates complex geometrical descriptions not possible otherwise in language. Allows more flexible and precise controls that relate directly to the form, fit and function and not just the size of the part, leading to a successful end product.
- Eliminates guesswork and enables mfg. according to design intent, thus reducing confusion, rejection, rework and loss of profits.
What is the feature control frame in GD&T?
- Basic sentences in GD&T are put in the form of a feature control frame panel.
- It captures the requirements for the feature to which it is aligned.
- Each feature control frame can state only one requirement/message. Only one set-up or gage for one FCF.
- Detail explanation of the Feature control frame is given below
Details of a feature control frame
- Geometric characteristic symbol: Specifies the character to which the tolerance is to be applied.
- Example Flatness, angularity, profile, parallelism etc.