Advantages of GD&T
Geometric dimensioning and tolerancing (GD&T) is an international engineering drawing system that offers a practical method for specifying 3-D design dimensions and tolerances on an engineering drawing. Based on a universally accepted graphic language, as published in national and international standards, it improves communication, product design, and quality. Some of the advantages of using GD&T on engineering drawings and product data sheets are that it:
- Removes ambiguity by applying universally accepted symbols and syntax.
- Uses datums and datum systems to define dimensional requirements with respect to part interfaces.
- Specifies dimensions and related tolerances based on functional relationships.
- Expresses dimensional tolerance requirements using methods that decrease tolerance accumulation.
- Provides information that can be used to control tooling and assembly interfaces.
The GD&T reduces the direct cost of products because it uses larger fields of tolerance eliminates the scraping of good parts and guarantees the zero-dimensional defect in the manufacture of components.
It is estimated that over 80% of the engineering documents generated in the United States are flawed in some way. Often the ideal geometry is well defined. It is the amount of acceptable variation or tolerance from the ideal or nominal geometry that is usually unclear. The coordinate system of dimensioning and tolerancing on drawings has following disadvantages:-
- Does not define geometric requirements and relationships adequately.
- Often results in rejection of good parts or acceptance of functionally unacceptable parts.
- Often does not provide adequate information for efficient and consistent inspection.
- Results in tolerances accumulating in unacceptable ways.
- Uses notes to define needed geometric controls that are often difficult to understand and interpret.
The GD&T system is a concise engineering language that helps overcome these short-comings. Defining the functional requirements and Datum Reference Frames for the parts requires more time than with the outdated system. Yet even the effort invested within the Engineering Department to implement GD&T can be paid for via the greatly reduced amount of expensive drawing revisions.
When GD&T is properly applied and understood, product costs go down because:
- Engineering can accurately define what is required. This can cut down time taken to interpret the requirements by 25%.
- Inspection setups are not subject to interpretation. The lack of trial-and-error may, in some cases, save as much as 40% in gages that often get scraped.
- Everyone can do their jobs right the first time, resulting in a reduction of a possible 25% less Material Review Board action.
- Datum systems speed up manufacturing and inspection and provide an effective basis for SPC. The savings vary and are difficult to determine, but they do exist.
- Accumulated benefits of GD&T often enable lower and more accurate price estimates for fabrication.
One of the most costly items for a manufacturing facility is to "shut down the production line." Since the entire GD&T system is based on the function of the mating parts, the use of pry bars, hammers, and expensive rework is no longer necessary. This alone could possibly save 20% in the overall costs of assembly.