GD&T Improves Inspection

Excerpt from article in Quality Magazine, November 2002.

A working knowledge of geometric dimensioning and tolerancing (GD&T) is necessary if engineers want to truly understand the designer’s intent, and 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. 

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 does not know how to determine a reasonable, functional tolerance.

“Two people involved in every design need to understand GD&T—the designer and the inspector,” says consultant Alex Krulikowski of Effective Training Inc., (Westland, MI.) Krulikowski is the author of a self-study workbook based on the revised ASME standard, Y14.5M-1994.

“CAD (computer-aided design) 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,” says Krulikowski. He adds 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 says. Krulikowski adds that the drawing can 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?

Where to find out more
According to the experts, dimensioning rules are violated frequently. In fact, Effective Training has developed a list of the seven deadly sins of GD&T. These are: incorrect use of the word “thru,” incorrect use of the word “central,” unnecessarily tight titleblock tolerances, use of esoteric notes, imaginary dimensions, dimensions without tolerances and missing dimensions.

“These errors have appeared so often, and for so long, they are accepted without question by many drawing makers and users,” according to Effective Training’s Web-site. “Violations, however, are dangerous and expensive because they introduce ambiguity, multiple interpretations and guesswork into the manufacturing process.”  End of excerpt.

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.