GD&T is a language used to communicate design requirements. Usually engineering and drafting come up with the requirements and put it on the drawing. Manufacturing takes the requirements and selects processes capable of producing the product within the requirements, and inspection uses the drawing to verify that the requirements have been met.
Throughout this whole process, a single document is used. It is important that everyone understands the GD&T language.
Why use this language instead of traditional methods?
GD&T uses symbols to convey information. These symbols have clear definitions in a single document called ASME Y14.5 Dimensioning and Tolerancing. The alternative is using words and sentences to convey information. The problem with words and sentences is that there can be multiple interpretations of text. The longer the sentence, the more likely confusion will result. The U.S. constitution is a good example. The sentences seem clear on first impression, but as the stakes get higher, the likelihood of differing opinions grows.
What are the “stakes” of making drawings correctly?
Engineering drawings are legal documents, like a lease contract or cell phone contract. When someone agrees to a contract to make a part to the drawings specifications, they are liable for everything on the drawing. If a mistake is made by the contractor in understanding the drawing requirements, they will be liable for not meeting the contract. If the entity makes a mistake in preparing the drawing, but the contractor makes the part to the specifications, the entity making the drawing may have useless parts that must be purchased by the terms of the contract.
Where do drawing standards come from?
Prior to 1949, there were no U.S. national standard for drawings. Best practices were known, and many textbooks and reference books were available. Large companies like Ford and General Motors published their own drafting manuals that set standards within their respective companies. Large companies at the time were vertically integrated, with much less subcontracting than is common today. This all changed quickly in World War II. Suddenly companies like Ford and General Motors were tasked with producing airplanes, tanks and guns. This resulted in small and large companies working together, and needing to understand drawings made to different company standards. The US Military identified the problem of each company having different standards, and the potential for friction in the supply chain. The first national drafting standard was published in 1949, and was called MIL-STD-8 Dimensioning and Tolerancing. The stated purpose was to “..establish uniform practices of stating and interpreting requirements shown on drawing”. Similar standards were published to cover all facets of drawing preparation.
All companies did not immediately switch to the national standard.
What is the current standard?
In the United States, the American Society of Mechanical Engineers “ASME” publishes a series of standards known as the Y series. These cover all facets of drawing preparation and interpretation. Several standards incorporate GD&T. The most important is ASME Y14.5 Dimensioning and Tolerancing. This is updated every decade or so, the latest version is ASME Y14.5-2018, followed by ASME Y14.5-2009 and ASME Y14.5M-1994. All of these are common in industry today. There is no requirement for companies to update if they do not choose to, as long as the drawing states which of the versions is to be used for interpretation.