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Introduction to Geometric Dimensioning and Tolerancing

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  • Introduction to Geometric Dimensioning and Tolerancing

If you’re familiar with the CNC industry, you may have heard the term “geometric dimensioning and tolerancing,” commonly referred to as GD&T.

This language of symbols is essential for CNC machinists to understand, as it helps engineers and designers communicate how to bring a part design to life. In the CNC world, detail is everything—so processes like this are critical to creating parts efficiently and accurately.

GD&T can seem intimidating at first, but once learned, it makes the life of a CNC machinist much easier. Follow along as we walk through the basics of what GD&T is, how it works and how it’s used in the industry.

Geometric Dimensioning and Tolerancing: A Brief History

Before jumping into exactly what GD&T is, it can be helpful to understand its history. During WWII, a machinist named Stanley Parker created the first GD&T concepts while he was working at a torpedo factory in Scotland. The idea of GD&T is credited to Parker, who created the well-known concept, "true position."

Parker worked at the Royal Torpedo Factory in Alexandria, West Dunbartonshire, Scotland. While employed there, his work significantly increased the production of naval weapons by new contractors.

Parker published notes on Design and Inspection of Mass Production Engineering Work in 1940, which is the earliest work on geometric dimensioning and tolerancing. He published Drawings and Dimensions in 1956, which became the basic reference in the field.

Parker started questioning traditional tolerancing techniques after encountering a situation where torpedo parts did not meet inspection criteria, yet were actually functional parts. He dug deeper and found that two point tolerances result in a square shaped zone of acceptability, and parts can still function if the tolerance zone is expanded into a circle that includes the corners of the square.

Parker’s idea developed into a complete organized system of controls and a global standard by 1957. Today, GD&T has four core elements: size, location, form and orientation. These provide a basis for 14 primary feature control symbols such as perpendicularity, circularity, parallelism and circular runout. 

What is GD&T?

Geometric dimensioning and tolerancing (GD&T) is a system for specifying and communicating engineering tolerances and design intent. It aids engineers and manufacturers in optimally controlling variations in manufacturing processes.

GD&T uses a symbolic language on engineering drawings and computer-generated, three-dimensional solid models. Essentially, “geometrical product specifications” are related to the size, shape and positional relationship of a product, and “tolerance” refers to the allowable error. “Geometric tolerance” includes the size and allowable errors for the form and position.

GD&T answers the question, “How far off the CAD/perfect model can a part be and still function?” It also:

  • Provides an unambiguous reference coordinate system
  • Makes clear how a part is to be inspected and indicates how to constrain the part when making a measurement
  • Reduces the need for special notes and the use of qualitative requirements
  • Reduces the time and cost spent achieving tight tolerances on unimportant part features
  • Enables the use of pass/fail inspection blocks

How Geometric Dimensioning and Tolerancing Works

Engineering drawings show the dimensions for all features of a part. By the dimensions, a tolerance value must be specified with the maximum and minimum acceptable limit. The difference between the minimum and maximum limit is known as the tolerance.

For instance, if there was a table that would be acceptable with a height between 640 mm and 670 mm, 30mm would be the tolerance.

However, the tolerance for the table communicates that a table that is 640 mm high on one side and 670 mm on the other, or has a waved surface with 30mm variation, is acceptable. To appropriately tolerance the product, a symbol is needed to communicate the design intent of a flat top surface. As a result, in addition to the overall height tolerance, a flatness tolerance must be included.

In a similar way, a cylinder with a toleranced diameter will not fit into its hole if it becomes bent during the manufacturing process. So, it also needs a straightness control, which is tricky to communicate with basic plus-minus tolerancing.

In sum, parts with complex shapes and unpredictable variations require GD&T practices that go beyond basic plus-minus tolerancing.

GD&T Symbol Examples

GD&T uses a library of symbols to communicate design intents. There are several standards available worldwide that describe the symbols and define the rules used in GD&T, including the American Society of Mechanical Engineers (ASME) Y14.5 and standards from the International Organization for Standardization (ISO), which vary slightly.

The Y14.5 standard provides a complete set of standards for GD&T in one document. On the other hand, ISO standards typically only address one topic at a time. There are separate standards that provide the details for each of the major symbols. The below chart displays a handful of these symbols and what they mean.

GD&T chart

How GD&T is Used in the CNC Industry

GD&T is a large and complex topic, and diving into it is like learning a new language. It is a means of eliminating misinterpretation and is essential to machining, as it makes it easier to create acceptable mating parts.

“Essential to machining” and “easier to create acceptable mating parts” means that GD&T provides everything needed to ensure a part is manufactured to reflect feature relationships and ensure that mating parts will truly mate across thousands of parts and assemblies.

The drawing is the controlling document that ensures the vendor is creating precisely what the customer's design requires. When dealing with very small tolerances, such as, +/- .002", it becomes even more important to use GD&T. If you have a cylinder that needs to be "cylindrical" within .0003," for example, you learn to use it very quickly.

One of the most significant benefits of GD&T is that it describes the design intent rather than the resulting geometry. Similar to a formula or vector, it is not the actual object—but rather, a representation of it.

When used correctly, GD&T allows statistical process control (SPC), which helps to reduce assembly failures and the means needed for quality control. This saves organizations substantial resources. Thanks to GD&T, multiple departments can work simultaneously, as they have a shared language and vision for what they want to create.

GD&T Training

So how does one learn to utilize geometric dimensioning and tolerancing in a CNC environment?

As mentioned previously, GD&T is very complex. This means that it often requires the completion of a formal training program, like UTI’s CNC Machining Technology program, to learn.

Created in conjunction with Roush Yates, a leading brand in the performance industry, this 36-week program will teach you everything from reading blueprints and interpreting geometric dimensioning and tolerancing to the programming, setup and operation of CNC lathes and mills. UTI’s CNC program covers GD&T extensively in Course 130.

Throughout your training, you’ll have the chance to work with some of the same tools and technology used by machinists in the field today. This program combines classroom learning with hands-on application in order to prepare you for a successful career in the field.

The CNC program is offered at the NASCAR Technical Institute campus in Mooresville, NC. For students who need to relocate to complete their training, UTI offers housing assistance to help them make living arrangements near campus. Scholarships and grants may also be available to help lower the cost of your education.

Prepare for a Career in CNC Machining

Does a career in the CNC industry sound like the right fit for you?

UTI’s CNC Machinist Training program begins every six weeks, so you’re able to start training and prepare for your career sooner. To learn more, visit our CNC program page and request information to get in touch with an Admissions Representative today.

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