The illustration shows the measurement of a cylindrical feature for its mating size and location of the axis of the mating envelope.
Select figure to enlarge.

A critical challenge facing medical OEMs and suppliers is validating the "analytical" software that determines compliance to Rule 21 CFR Part 11 regarding mechanical drawings for components and assemblies. In general, Part 11 requires medical device manufacturers, biotech companies, biologics developers, and other FDA-regulated industries to implement controls such as audit trails, electronic signatures, and documentation for software and systems that process electronic data according to FDA predicate rules. These rules dictate such things as what records must be maintained and for how long, the content of records, and whether signatures are required.

Medical components can have complex surface geometries and tight feature tolerances, so OEMs mandate the use of high-precision measurement devices and software that ensures traceability to ASME Y14.5M – 1994 for Geometric Dimensioning & Tolerancing (GD&T). The validation of this software is confusing at best. First, a discussion of GD&T should help further understanding.

GD&T is a symbolic design language that defines the nominal (theoretically perfect) geometry of parts and assemblies, allowable variation in form and size of individual features, and allowable variation between features. Its use moves the CAD model from a design to a manufacturing slant and lets software validate steps in manufacturing and inspection, thereby reducing human error.

In the past, major medical OEMs such as Medtronic, Minneapolis, analyzed a broad array of analytical software for determining compliance to Y14.5/Y14.5.1. This resulted in a push to standardize worldwide on analytical software called SmartProfile by Kotem Technologies Inc, in Canada, which successfully passed complex mathematical test cases.

Analytical software limitations

Most companies with advanced metrology needs have purchased a broad range of coordinate measuring machines (CMMs) over the years. Each machine comes with its own analytical software package that requires installation, upgrades, and licenses. Unfortunately, the different packages can evaluate the same data and derive completely different results. The real danger is that all of these results can be repeatable and reproducible — and incorrect. The majority of software does not have the ability to ensure full compliance to ASME Y14.5.1M-1994.

For a worst-case scenario, consider "Analysis of a cylindrical feature," shown above.

It shows the measurement of a cylindrical feature for its mating size and location of the axis of the mating envelope. The majority of software defaults to a least-squares fitting to determine the features size and location of its axis in the X and Y directions. Problem is, using least-squares analysis, commonly referred to as best-fit analysis, generates incorrect results.

In this case, correct results come from analyzing the largest inscribed cylinder or the smallest circumscribed cylinder, depending on whether the feature is internal or external. Most software can apply maximum inscribed and smallest circumscribed algorithms but, in most cases, OEM and supplier metrologists do not use the algorithms.