The Language of GD&T

Excerpt from article in Quality Magazine, May 1997.

Anyone who has marveled at Leonardo Da Vinci's stunning renderings of his own designs for flying machines and submarines knows that design drawings have existed for as long as man has had product ideas. While today's industrial part drawing may not have the artistic impact of a Da Vinci masterpiece, the fact remains that properly rendered engineering drawings represent the quality professional's incorruptible standard against which to measure the parts.

In 1935, with the publication of the American Standards Association's "American Drawing and Drafting Room Practices," the first recognized standard for engineering drawings was established. Steady progress was made, in the United States and abroad, throughout the World War II years to define and specify the symbols and terms used in functional dimensioning.

Fast-forward to 1997. In this era of tighter and tighter part tolerances, CAD-generated drawings, and digital transfer of parts data---and when many university engineering curricula no longer include any kind of drafting course—quality professionals could be forgiven for wondering if there was still a reason to learn this special international language of engineering drawings.

However, many are finding out that a working knowledge of geometric dimensioning and tolerancing (GD&T) is necessary if they want to truly understand the designer's intent, and therefore plan accurate and appropriate inspection of the product the drawing represents. As part tolerances get tighter and tighter, it's more important than ever to establish a realistic, agreed-upon part drawing that subsequent inspections can be measured against.

If that describes your inspection situation, experts suggest it's a smart idea to brush up on your knowledge of this precise language of terms and symbols, as well as ASME Y14.5M-1994 (American Society of Mechanical Engineers, New York), the applicable standard.

What GD&T is
GD&T is a precise mathematical language that describes the size, form, orientation, and location of part features. It’s also a design-dimensioning philosophy that encourages designers to define a part based on how it functions in the final product.

Through the use of functional dimensioning, tolerances are assigned to a part by the designer based on the part's functional requirements, often resulting in a larger tolerance for manufacturing. This eliminates problems that result when a designer assigns arbitrary, or too tight, tolerances to a part in a drawing because he or she doesn't know how to determine a reasonable, functional tolerance.

"Two people involved in every design need to understand GD&T—the designer and the inspector," said consultant Alex Krulikowski of Effective Training Inc., Westland, MI. Krulikowski is the author of a self-study workbook based on the revised ASME standard.

"CAD drawing systems do an outstanding job of displaying nominal geometries, but the designer still needs to put in allowable variation, and GD&T symbols are the way those tolerances are expressed on the drawing," said Krulikowski. He added that, ideally, there is a product team participating in the design process, including those involved in assembly, manufacturing, and quality. GD&T can provide uniformity in the specification and interpretation of the drawing, eliminating guesswork and erroneous assumptions, and ensuring that professionals in design, production, and inspection are all working in the same "language."

"There should be a quality/inspection plan for every CAD design, and inspection specifics should be notated in the drawing," he said. Krulikowski added that the drawing could specify such quality information as the following:

How will the part be inspected?
How frequently will the part be inspected?
What tools will be used? Hard (functional) gages? Coordinate-measuring machines (CMMs)?
If CMMs are to be used for inspection, how many data points will be taken?
What is the reliability and reproducibility of the gages to be used?
How much may the part deflect during inspection?
Will the part be clamped or fixtured during inspection? If so, where and with how much force?

Translation problem
Brushing up on your GD&T knowledge is important for another reason--the looming problem involving automated inspection equipment that does not properly measure parts in conformance with ASME Y14.5M-1994. Without adequate knowledge of functional dimensioning, it will be harder for the quality professional to pinpoint measurement discrepancies during inspection.

The basic discrepancy between functional dimensioning and automatic inspection is that the above-mentioned Standard assumes that parts are measured with hard, or functional, gages-simple go/no-go plug and ring gages, for example. When today's CMMs, which measure points rather than surfaces, are used to inspect parts that were dimensioned and documented to conform to ASME Y14.5M-1994, problems may result.

Most CMM software, with a couple of rare and expensive exceptions, does not compute basic measurements, such as hole size. Instead, the software uses least-squares/best-fit mathematical substitutions to perform inspections. This discrepancy could lead to CMM inspection passing bad parts and failing good ones, even when performing basic measurements such as flatness, perpendicularity, and parallelism.

According to Richard Jennings, vice president and general manager of software, supplier Icamp, Bolton, CT, this discrepancy between design and inspection methods didn't matter nearly so much a few years ago, when part tolerances were not so tight.

"With today's tighter tolerances, you can use the analogy of water-skiing in a manmade reservoir with a bunch of tree stumps at the bottom," Jennings said. "When the reservoir is full, you don't even know the stumps are there. During a drought, the falling water level can make the stumps a real hazard. Tighter tolerances are exposing these measurement/inspection problems."

Jennings' company provides gage-emulation software that can collect the data points generated by CMM inspection, and perform GD&T-based measurements. This type of software translation can solve many problems, or, as Jennings puts it, "referee the disagreement between measurement processes."

Several types of dedicated gage-emulation software packages are also available from inspection-equipment vendors. According to Krulikowski, a major thrust for CAD software developers in the next few years will probably be automatic tolerancing in solid modeling programs, which will also help designers and inspectors stay on the same page.

Still, it behooves the quality professional to know both the intricacies of inspection equipment, and the dimensional-tolerancing Philosophy followed by the designer and reflected in the part drawing--the better to help find a transition method when problems arise. End of excerpt.

Try the GD&T Potential Savings Calculator
The calculator is a tool that helps companies understand the amount of unnecessary expenditures each year due to employees not knowing how to correctly apply and interpret GD&T.

Where to find out more about GD&T
Effective Training Inc. is a world leader in the field of geometric tolerancing. ETI president, Alex Krulikowski is an expert on geometric tolerancing, with a degree in industrial vocational education and over 30 years of industry experience. He has taught GD&T to thousands of students through classroom seminars, and to countless others through his books, self-study workbooks, videos, and CD-ROMs.

ETI provides expert GD&T training with an emphasis on practical, on-the-job application. Onsite workshops include GD&T fundamentals and advanced concepts; tolerance stacks; statistical tolerance stacks; an ISO/ASME comparison; a GD&T overview; and solid model tolerancing.

Online training is also available at their ETI Learning Center. ETI's GD&T Trainer is a complete course in GD&T fundamentals available in single-user, multi-user or LAN software.

With proper training and implementation, GD&T will help your company reduce scrap, increase the percentage of usable parts, simplify inspection and assembly, replace fewer parts, avoid recalls, and increase efficiency. Geometric dimensioning and tolerancing can give your company the edge over the competition in today's cost competitive marketplace, and Effective Training can provide the training and materials you need to reap those benefits.

If you’d like to discuss how geometric tolerancing will benefit your company, call 800-886-0909 or email info@etinews.com today.

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