MEDICAL METROLOGY AS A PART OF THE WHOLE

Compared to many other industries, medical device manufacturing has unique concerns about patient welfare, with lingering thoughts of liability. "These concerns drive extreme process control" explains Frank Powell, product manager for Marposs (Auburn Hills, MI), a provider of in-process gaging and measurement equipment. “We have installed systems that use a 2-D bar code on each part to relate gaging data, what operation it was measured on, what the measurements were, and even which machine it was made on and who was the operator at the time,” he says.

Another example of the extreme care sometimes needed in medical device manufacturing was the company’s experience in developing a mark-free measuring system for titanium hip joints. The ball and socket have to mate perfectly, requiring a superfinish for a nearly friction-free range of motion. Specialized machine tools create the superfinish. In-process gages monitor and control these machines.
The problem is that titanium is a highly reactive metal that attaches readily to carbon-based materials, such as the diamond commonly used for precision gage contacts. The buildup on the carbonbased contacts led to inaccuracies. Modern ceramics provide the best wear characteristics.
Machining processes using contact gages can leave a mark on the workpiece. In most cases, it would not affect the functional performance of the artificial joint. As Powell points out, however: “Surgeons are not metrologists.” If they see a mark, concerns over patient welfare will lead them to reject and scrap the part. In response, Marposs works with process engineers and machine builders to fine-tune the grinding process to prevent visible marks. Powell concludes that more inspection systems will be needed to control quality in the future.

Read the Medical Metrology article, that first appeared in the March 2012 issue of Manufacturing Engineering® MEDICAL Manufacturing 2012 Yearbook.

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