GD&,T (per ASME Y14.5M-1994) is an international language that is used on engineering drawings to accurately describe the size, form, orientation, and location of part features. It is also a design-dimensioning philosophy that encourages designers to define a part based on how it functions in the final product or assembly.
GD&,T is an exact language that enables design engineers to ",say what they mean", on a drawing, thus improving product designs and lowering cost. Process engineers and manufacturing use the language to interpret the design intent and to determine the best manufacturing approach. Quality control and inspection use the GD&,T language to determine proper set-up and part verification.
By providing company-wide uniformity in the drawing specifications and interpretation, GD&,T reduces controversy, guesswork, and assumptions throughout the design, manufacturing and inspection process.
Understanding how to apply and interpret GD&,T correctly will help you:
- Create clear, concise drawings
- Improve product design
- Create drawings that reduce controversy, guesswork, and assumptions throughout the manufacturing process
- Effectively communicate or interpret design requirements for suppliers and manufacturing
However, because GD&T is such a precise language, it involves a great many symbols and terms. Here is a list of some of the topics involved in geometric dimensioning and tolerancing fundamentals and a short definition of each.
Actual Local Size - The value of any individual distance at any cross section of a feature of size.
Actual Mating Envelope of an External Feature of Size - A similar perfect feature counterpart of the smallest size that can be circumscribed about the feature so that it just contacts the surfaces at the highest points.
Actual Mating Envelope of an Internal Feature of Size - A similar perfect feature counterpart of the largest size that can be inscribed within the feature so that it just contacts the surfaces at their highest points.
All-Around Symbol - A circle placed on the bend of the leader line of a profile control.
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