In today’s world, effective communications and consistent measurements across all engineering disciplines and processes are essential to the design and manufacture of the highest quality products. Geometric Dimensioning and Tolerancing (GD&T) is key to achieving these goals.
GD&T is a standard language used to communicate the allowable geometric variation on parts. The language includes symbols that are used on mechanical engineering drawings to quickly and accurately define design, manufacturing and inspection requirements for various 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 that is 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 in order 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 engineers where they need to begin measuring or manufacturing from. Additionally, the use of datums dramatically simplifies the design and specification processes.
There are two standard GD&T languages: ASME Y14.5-2009 within the United States, and ISO 1101-2004 outside the United States.
Advantages of GD&T
The key benefits of using GD&T include:
• Clarity and consistency in the design process
GD&T provides 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 that can result from a lack of clarity.
Before GD&T existed, there were no standards; many people interpreted each drawing differently. GD&T is standardized and mathematized which means that anyone who knows the language can read a drawing and interpret it as intended.
• Dramatic time savings
By using GD&T, engineers dramatically reduce their need for drawing notes to describe complex geometry requirements on components and assemblies.
• Fit with accepted design-for-manufacturing methods
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 3-dimensional tolerance analysis software like 3DCS
. When combined together, this set of tools can statistically predict whether or not a product or assembly will meet its fit, finish and function requirements well before any actual products are produced.
As a tool, GD&T has been used in the automotive, aerospace, electronics, commercial design, and other manufacturing industries for the last few decades. Its usage has grown in tandem with the industries’ move from mechanical drawings to digital design.
But is it being used to its fullest potential? Many experts, like GD&T pioneer and educator Bob Kaphengst, say it is not.
According to Kaphengst, the lack of formal education about GD&T is one of the main roadblocks.
“Most engineering schools and graduate programs do not teach GD&T as part of their curricula,” explained Kaphengst, “so when engineers get into the workplace, they simply apply the symbols they learn on the job to their design work without a true understanding of how to optimize the use of GD&T to improve product quality.”
GD&T Must Be a Team Effort
While GD&T must, first and foremost, capture design intent, 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 they are not involved, drawings often end up with overly tight tolerances or result in parts that cannot be produced at the quality level, cost and turnaround times expected by industry.
“In many businesses, there isn’t smooth, consistent coordination between the designers who apply GD&T and the manufacturing engineers who rely on the symbols for tooling and assembly,” notes Kaphengst. “There needs to 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.”
Kaphengst believes that an understanding of GD&T should extend even beyond the product and manufacturing engineers and that anyone who creates, approves or uses an engineering drawing should know how to read and apply GD&T..
GD&T is one of the most powerful tools available to improve quality, reduce cost and shorten delivery time. With a solid understanding of how to optimize this tool, companies around the globe can experience the highest levels of customer satisfaction and profitability.
What do you think?
Do you agree that GD&T isn't as widely used as it should be?
Should GD&T be added to engineering school curriculum? Do you know of programs that include it?
What do you think is the best way to teach GD&T across the organization, including manufacturing, purchasing, etc... not just designers?
More about Robert Kaphengst:
Robert Kaphengst serves as president and chief executive officer of Dimensional Control Systems, Inc., www.3dcs.com, which he co-founded in 1994. In addition, Kaphengst is a frequent industry speaker on the continuous improvement of product quality through processes including GD&T. Prior to co-founding DCS, he was with General Motors for nearly 30 years where he drove manufacturing process improvements. In his last 10 years at GM, he was responsible for the dimensional management process and the use of GD&T within the Truck Division. After leaving General Motors, Kaphengst continued to serve as a contractor, responsible for dimensional problem-solving at assembly plants. Kaphengst is a widely respected pioneer in the application of GD&T. including special applications on body sheet metal. He has taught several hundred classes on the subject at companies across North America, South America, Germany and England.