Geometric dimensioning and tolerancing (GD&T) is an international language used on drawings to accurately describe a part. The language consists of a well-defined set of symbols, rules, definitions and conventions that can be used to describe the size, form, orientation and location tolerances of part features. Geometric tolerancing is an exact language that enables designers to say what they mean on a drawing, thus improving product designs. Production uses the language to interpret the design intent, and inspection looks to the language to determine setup. By providing uniformity in drawing specifications and interpretation, GD&T reduces controversy, guesswork and assumptions throughout the manufacturing and inspection process.
This myth is perpetuated by a preference for coordinate dimensioning, which most designers have used since high school and have clinged to because they understand it better than GD&T. But, the 150-year-old coordinate dimensioning system lacks clear rules and a quality-oriented design philosophy. GD&T is used by about one-half of the world's manufacturers because it does have clear rules and is quality-oriented. In addition, it allows communication among all levels and functions within a company, as well as with their customers and suppliers. The philosophy behind GD&T is pro-profits and pro-growth.
This myth stems from the idea that GD&T prescribes tight tolerances. The tight-tolerance myth is essentially a misunderstanding of GD&T's use of basic dimensions, which have no tolerances, and a lack of knowledge of feature-control frames, which do provide tolerances. In fact, GD&T and its ASME Y14.5M-1994 standard call for the maximum possible tolerance within the bounds of part function. GD&T's round tolerance zones allow 57% more tolerance than coordinate dimensioning's square zones. And, GD&T provides additional tolerancing through the use of the maximum material condition modifier. Drawing errors also drop with GD&T, which further reduces product cost.
There is a great deal of truth to this myth. GD&T and the ASME standard are complex and can be confusing to anyone unfamiliar with GD&T and its standard or who has been poorly trained in it. But with proper instruction by a qualified instructor with excellent training skills, a person can understand GD&T and the confusion can be cleared up.
Proponents of this myth want speed at all costs. Ironically, while they do not have time to make drawings correctly the first time, they do have time-and money-to make corrections later. The clarity and precision of GD&T may require more time initially, but time is saved later.
This myth, again, stems from a fondness for coordinate dimensioning and from assuming that it is better because it has been around longer. The problem with using coordinate dimensioning is that it is not able to meet the level of precision demanded by technologies such as CAD/CAM and electronic gaging. GD&T, on the other hand, meets today's demands and performs even better when designers use it to thoroughly think through part function.
This myth assumes that some part features are non-critical. The problem becomes, then, who decides which features are critical and which are not critical. Limited use of GD&T may bring some benefit, but a drawing that is partly coordinate dimensioning and partly GD&T may also be confusing. However, when GD&T's philosophy and rules are applied to all part features, treating all of them as critical and to the entire process of creating drawings, the benefit of GD&T increases exponentially.
This myth is caused by overconfidence in inadequate training programs. Poor training also produces people who know their GD&T skills are inadequate and who, therefore, do not criticize the skills of others. The result is that training and competency get ignored, and a company ends up with confusing and misinterpreted drawings, mismanufactured parts, unhappy customers and high engineering drawing change costs.
The myth of doing the drawing now and adding the symbols later has developed from the need for speed as well as from a lack of knowledge about geometric tolerancing. A drawing with missing dimensions and insufficiently defined tolerances may produce artificially low cost estimates that can cause manufacturing to produce nonfunctional parts. GD&T allows a product to be tested on paper rather than in prototype form. Companies that train their designers to test on paper, and give them the time to do it, often eliminate the need for what are frequently costly prototype iterations.
Most people get only 16 hours of training, so they assume that is enough. Two days may be enough time to teach a person to read GD&T drawings, but it is not enough to teach someone how to make drawings. And it is certainly not enough to train a person to become fluent in GD&T. Fluency requires a minimum of 120 hours of interactive classroom sessions, testing and on-the-job training. The only people who can be said to have learned and know GD&T are those who have completed this rigorous course of training, testing and work experience and who have become certified by the American Society of Mechanical Engineers as geometric dimensioning and tolerancing professionals-and even then the learning never stops.
To eliminate the myths of GD&T, a company must develop a thorough understanding of GD&T's scope and make the time available for the education of engineering drafters and engineering drawing users. Management must not reward speed over quality and tight tolerances over the largest possible functional tolerances. When GD&T is thoroughly learned, fully understood and correctly used, the myths of GD&T and their consequences will be eliminated.