Measurement & Inspection
“No measurement process is per- fect, but [it can be useful] as long as the uncertainty in the process is acceptable and balances out against the cost.” That is the key issue in dimensional measurement planning, converting design intent and GD&T specifications into a cost-effective measurement plan. Plenty of choices are available to metrologists. That includes hard- ware such as choosing a CMM over a micrometer or selecting a least squares fitting algorithm to measure hole diameter and circularity.
Practicalities and Point Clouds
For GD&T to be useful, applied correctly at the source during authoring of the origi- nal CAD model is vital. Importing GD&T that is attached to the CAD is possible today, but not widespread. Automatic error checking is possible in those situations.
“One of the toughest things with GD&T is interpretation,” agrees Josh Old, applications engineer with Capture 3D
(Farmington Hills, MI). “About 20% of the problems I deal with are GD&T issues.” When asked to look at a print that someone is having difficulty using, it is sometimes obvious that the originating engineer lacked experience and knowl- edge in GD&T. Issues include not properly setting datums or the more complicated functionalities.
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Some of these issues may also be the nature of the parts they work with. Capture 3D specializes in offering metrol- ogy equipment from German supplier GOM, including ATOS blue light scanning equipment and the TRITOP hand-held photogrammetry system. Blue light scanning is used for various part geometries from simple, organic, to complex— from the grooves on a fingerprint to the leading and trailing edges on an airfoil. “Nothing is square on them,” explained Old. “So, they have to manufacture something on them that is square. Or, they have to figure out a way with GD&T and setting datums to get the quality measurements they need out of it—that is sometimes very complex.” He notes that the profile feature is important when scanning is used for the measurement. Profile lines or surfaces are two of the thirteen specific characteristics in either GD&T standard. Disagreements are common. “I have been in rooms where two engineers from the same company have disparities on how to interpret just one call out,” said Old.
He also believes that GD&T as a language is especially im- portant for building assemblies. “For example, when aligning holes to ensure components will mate when assembled,” he said. “They use GD&T for inspection to ensure that mating.”
Image courtesy Metrologic
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