Product Applications

Optical Comparators in Medical Device Manufacturing

by User Not Found | Sep 20, 2011
New England Precision Grinding (NEPG) of Holliston, MA produces micro-ground wire from 304 stainless steel and super-elastic nitinol to meet the precise demands of medical devices. Established in 1987, NEPG has developed a proprietary centerless grinding process for ever-smaller devices to meet medical requirements.


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. Often, the wire is used for needles and catheter-placed stents so small that their features cannot be seen with the naked eye. With material this small, inspection is critical, to say the least.

“Our quality control requires an inspection system that is fast and accurate,” said Bill Robinson, Quality Assurance Manager for NEPG. “The Starrett Optical Comparators on our shop floor and in our metrology lab that 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 of each part on the Comparator.”

Optical comparators are used at the very beginning of the process --  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 Starrett Optical Comparators to check their in-process work. In addition, roaming QC inspectors randomly select pieces and view them on the optical comparators.

The optical comparators are used with 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 the work is checked, in process at each workstation, the inspectors check every dimension and angle of workpieces selected at random. 

A broad range of Starrett gages, micrometers, slide calipers, and other precision measuring tools are used, but the workhorses of the inspection room are the two Starrett Optical Comparators. 

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. Alternatives such as computer vision systems would 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
NEPG 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. The Company 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 with plans for aggressive growth. It all depends on NEPG's core expertise of working and inspecting at the micro-level -- operations that require the ability to view and measure precisely what the human eye cannot see.