Effective communication and consistent measurement across all engineering disciplines and processes are essential for designing and manufacturing the highest quality products. Geometric dimensioning and tolerancing (GD&T) is the key to achieving these goals.
Geometric Dimensioning and Tolerancing (GD&T) was developed to address the many problems that had been encountered over the years as organizations tried to describe their part geometry. It is a “language” composed of symbols used to efficiently and accurately communicate geometry requirements for related features on components and assemblies. GD&T is very important to understand detailed engineering drawings and has been successfully used by engineers, designers, and manufacturers in the automotive, aerospace, electronics, commercial design and manufacturing industries. It helps to specify and control the form, location and orientation of the features of components and manufactured parts.
The GD&T symbols for dimensions on a part are defined in relationship to a datum—a theoretically perfect point, line, or plane on a part that is used as a reference point for tolerance calculations and dimensional measurements. The datum(s) on each part is considered the “zero point,” and calculations are built from that point to all other dimensions to ensure the consistency of the part, providing manufacturing and measurement clarity to design, manufacturing, and quality engineers. Additionally, using datums drastically simplifies the design and specification processes.
The currently used standards of ASME Y14.5M-2009 within the US and ISO 1101:2012 abroad have replaced previously complex and long descriptions with simple symbols. These symbols should be clearly and unanimously understood by all who must read and interpret an engineering drawing.
Some advantages of geometric dimensioning and tolerancing include:
- Provides a clear and concise technique for defining a reference coordinate system (datums) on a component or assembly to be used throughout the manufacturing and inspection processes. This system reduces misinterpretations and the need for costly engineering changes and rework that can result from a lack of clarity.
- Proper application of geometric dimensioning closely dovetails accepted and logical mechanical design processes and design for manufacturing considerations.
- Geometric dimensioning dramatically reduces the need for drawing notes to describe complex geometry requirements on a component or assembly by the use of standard symbols that accurately and quickly define design, manufacturing, and inspection requirements.
- GD&T concepts such as MMC (maximum material condition) when applied properly will facilitate and simplify the design of cost saving functional check gages and manufacturing fixtures and jigs.
To reap all of the benefits GD&T has to offer, designers must properly specify GD&T on their engineering drawings. If prints are rushed and not enough time is spent on specifying GD&T correctly, the time saved pales in comparison to the potential savings from tolerancing and orienting a part with GD&T. It is important, therefore, for everyone involved in the manufacturing process to have a thorough understanding of GD&T so that cost-effective manufacturing can produce high-quality products within turnaround times that benefit both the manufacturer and their customer. The GD&T methodology was created to standardize the “language” of engineering drawings, so that no matter what role you play, or where you are in the world, you can read an engineering drawing and understand exactly what is required for that component.
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