Effective communication and consistent measurement 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.
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GD&T defined
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 point,” 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 engineers where they need to begin measuring or manufacturing from. Additionally, using datums dramatically simplifies the design and specification processes.
There are two standard GD&T languages: ASME Y14.5—“Dimensioning and Tolerancing” within the United States, and ISO 1101—“Geometrical product specifications—Geometrical tolerancing—Tolerances of form, orientation, location and run-out” 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-D tolerance analysis software such as 3DCS.
When combined, this set of tools can statistically predict whether the engineering design and manufacturing process is robust and will meet its fit, finish, and function requirements. This is done early in the program, before committing to build piece parts, tools, or gauges.
Opportunity lost
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 use has grown in tandem with the industries’ move from mechanical drawings to digital design.
But is it being used to its fullest potential? I don’t think so. That’s because the lack of formal education about GD&T is one of the main roadblocks.
Most engineering schools and graduate programs don’t teach GD&T as part of their curricula, 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
Although 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’s why manufacturers, suppliers, and quality engineers should all be involved with the requirements on each drawing. When they aren’t 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. There needs to be a process where designers ask manufacturing engineers and quality inspectors whether the parts and tooling they’ve drawn will fit together and function as intended.
I believe 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.
Comments
GD&T
Some attention shoudl be given to making GD&T easier or more natural to learn. A language that is not well understood isn't going to be real effective. Requiring it in school is a good idea, but there will still be a lot of 'on the job learners'.
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