Effective communication and consistent measurements across engineering disciplines and processes are essential in the design and manufacture of the highest quality products. Geometric dimensioning and tolerancing (GD&T) is key to reaching these goals.

GD&T is a standard language that communicates the allowable geometric variation on parts. The language includes symbols that are used on engineering drawings to quickly and accurately define design, manufacturing, and inspection specs for features on components and assemblies.

The GD&T symbols for each dimension on a part represent their relationship to a “datum,” the feature on that part used as a reference point for tolerance calculations and dimensional measurements. The datum on each part is considered “zero” and calculations are built from that point to all other dimensions to ensure the consistency of the part. A datum system, often referred to as a “zero reference” system, makes it clear to design, manufacturing, and quality where engineers need to begin measuring or manufacturing. Additionally, the use of datums dramatically simplifies the design and specification processes.

There are two standard GD&T languages: ASME Y14.5-2009 in the U.S. and ISO 1101-2004 everywhere else. Advantages of using GD&T:

It is a clear and concise method for defining a reference coordinate system on a component or assembly that can be used throughout manufacturing and inspection. This reduces misinterpretations, and the need for costly engineering changes and rework.

GD&T is standardized and mathematized, which means that anyone knowing the language can read a drawing and interpret it as intended.

GD&T lets engineers dramatically reduce their need for drawing notes to describe complex geometry requirements on components and assemblies.

The proper application of GD&T closely dovetails accepted and logical mechanical-design processes and design-for-manufacturing considerations. For example, the allowable variations as defined through GD&T can be directly read, or “imported,” into 3D tolerance-analysis software like 3DCS. When combined together, this set of tools can statistically predict whether a product or assembly will meet its fit, finish, and function requirements well before any actual products are produced.

GD&T has been used in the automotive, aerospace, electronics, commercial design, and other manufacturing industries for the last few decades. Its use has grown in tandem with the move from mechanical drawings to digital design. But many experts, such as GD&T pioneer and educator Bob Kaphengst say the language is not being used to its full potential.

The lack of formal education about GD&T is one of the main roadblocks, says Kaphengst. “Most engineering schools and graduate programs do not teach GD&T,” he says. “When engineers get into the workplace, they simply apply the symbols they learn on the job without a true understanding of how to best use GD&T to improve product quality.”

GD&T must capture design intent, but it must also focus on function, cost, and other business concerns. The best designs in the world are worthless if they cannot be produced. That is why manufacturers, suppliers, and quality engineers should all be involved with the requirements on each drawing. When this doesn’t happen, drawings have overly tight tolerances or result in parts that cannot be effectively produced.

“In many businesses, there isn’t smooth, consistent coordination between the designers applying GD&T and the manufacturing engineers relying on the symbols for tooling and assembly,” says Kaphengst. “There must be a process where designers ask manufacturing engineers and quality inspectors if the parts and tooling they’ve drawn will fit together and function as intended. An understanding of GD&T should extend to anyone creating, approving, or using engineering drawings.”

Contributed by Robert Kaphengst, president and CEO of Dimensional Control Systems, Inc., www.info.hotims.com/27267-500