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Optical Comparators in Medical Device Manufacturing
| Quality Control at Near-Nano Dimensions Starrett Optical Measure Comparators Micro-ground Medical Parts | | Imagine working with wire diameters as small as .005 inch. Consider meeting tolerances within .0001 inch and maintaining angles within one degree. And then add in a fabrication process more commonly associated with large work pieces: centerless grinding.
That's all in a day's work at New England Precision Grinding (NEPG) of Holliston, MA where they are producing micro-ground wire from 304 stainless steel and super-elastic nitinol to meet the ever-more precise demands of medical devices. The wire is used for needles and catheter-placed stents so small that their micro-miniature features cannot be seen with the naked eye.
NEPG, established in 1987, has developed a proprietary centerless grinding process to meet medical requirements for ever-smaller devices. They begin with wire, in diameters as small as .003 inch and as large as .062 inch. The work pieces can be as small as a human hair--.002 inch. With material this small, inspection is critical, to say the least.
Quality Control through Precision Vision
“Our quality control requires an inspection system that is fast and accurate,” states Bill Robinson, Quality Assurance Manager for NEPG. “The Starrett Optical Comparators on our shop floor and in our metrology lab help us maintain a high standard based on associated quality. We use a three-pan system with each pan containing multiple parts. The operator verifies the first piece on the optical comparator and runs the parts. The first part of pan two is checked. However the process is running, any variation from tolerance prompts a sort of the preceding pan, a process that calls for the visual inspection on each part on the Starrett Optical Comparator.” |  |
The first use of the optical comparators is the incoming inspection of material, especially if it has been worked by a supplier.
“We visually check such processes as laser cutting, electro polishing, welding, EDM work and laser marking,” explains Robinson.
From inspection, the raw material goes to the production floor for centerless grinding. Operators use seven conveniently located Starrett Optical Comparators to check their in-process work. Roaming Quality Control Inspectors randomly select pieces and view them on the optical comparators.
|  | The optical comparators use 50 or 100-power lenses, depending on the size of the micro parts. For radius checks, overlays are used. When a sorting operation is required, a template helps complete the full inspection quickly.
Micro-inspection Metrology Lab
NEPG uses two Starrett HB400 Measuring Projectors in their climate-controlled Inspection Room. While each grinding operation checks its work, the Metrology Inspectors check every dimension and angle of work selected at random, depending on L.S. Starrett for precision.
A broad range of Starrett gages, micrometers, slide calipers, and other precision measuring tools are used, but the workhorses are the two Starrett Optical Comparators.
The Starrett HB400, for example, is ideal for this type of intensive and varied inspection work. Its 16-inch screen diameter provides a wide field of view with cross lines and calibration marks. |  | It provides a built-in digital protractor with DD/DMS conversion and Heidenhain .00005-inch resolution scales. The centrally mounted vertical (Y-axis) scale is in-line with the lens and the work piece for a vertically correct image.
| While the dual fixturing slots can handle work pieces up to 110-pounds, NEPG's typical part weighs in at fractions of an ounce. Dual intensity tungsten halogen bulbs provide razor sharp profile illumination and twin-bundle, adjustable on-axis fiber optic surface illumination.
NEPG uses 50x and 100x magnifications in the lab to match the magnifications used on the production floor. The system enlarges part features to ensure the production line is holding its tolerances, ± 0.0001” and within 1 degree for angles.
A full inspection of a part by the Metrology Lab may involve checking up to eight dimensions. Each inspector performs 20 to 30 full checks of incoming material, in-process work and post-production finished goods.
“The Starrett Projectors have enabled us to reduce inspection times from as much as 4 to 4-1/2 hours to just 25 minutes,” stated Jim Phillips, Lead Inspector for NEPG. “We like the |  | way the Starrett system works, especially its ability to handle a wide variety of work sizes. We trust it.”
According to Bill Robinson, the Starrett Optical Comparators are essential equipment at NEPG.
“The Starrett Optical Comparators are critical in this work, “observed Robinson. “Our operations would be impossible without them. Yes, there are alternatives, such as computer vision systems, but they have to be programmed. Our optical comparators are the most efficient, fastest and accurate way to inspect parts without the need for complicated setups.”
Medical Trials and Tribulations
While NEPG is now producing as many as 50,000 pieces annually for some micro-ground wire products, the path to that success is as difficult as the tests and measurements to which the company subjects its work.
Before any medical device reaches the surgical suite, it must pass a gauntlet of tests, trials, inspections and approvals to be certified fit for use by the U.S. Federal Drug Administration (FDA). |  |
For example, one customer specified nitinol, a nickel-titanium alloy with shape memory properties that are ideal for surgical needles and stents. In the prototype stage, NEPG worked with the nitinol producer to address a major problem: the nitinol needles were causing sutures to break.
After material and design changes, not to mention additional prototypes, the product entered medical trials. This professional evaluation period may include animal trials, data collection and analyses, product modifications and additional trials. After documenting its performance and potential, the new product, and its substantial dossier, are submitted to the FDA for clinical trials, pre-market testing and, finally, approval. In some cases the approval period may last two to three years. |
Serving a Fast Growing Market
New England Precision Grinding has found ways to adapt its operations to the lengthy medical product development cycle. Its commitment to precision and quality has attracted business from some of the largest companies in the medical products field. NEPG is developing methods to make even smaller products, some involving the equivalent of grinding features on the point of a pin.
While their products seem to be getting smaller, their business is growing much larger. The NEPG five-year-plan calls for aggressive growth, which will depend on working and inspecting at the micro-level, operations that require the ability to view and measure precisely what the human eye cannot see.
Thanks to Starrett Precision Measuring equipment, NEPG is helping medical OEMs meet the demand for very “small” medical breakthroughs. |  | | | |
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