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.
“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 title block 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.