When Inspection’s a Matter of Life and Death

Originally published in Manufacturing Engineering Magazine.

Medical manufacturers seek and find inspection technology that’s appropriate to their special needs

By Michael Tolinski
Contributing Editor 

The US Midwest’s reliance on traditional manufacturing seems to be shifting toward the “life sciences” industries—consider the emergence of dozens of medical device manufacturers in Indiana alone. These manufacturers specialize in complex sculpted parts, custom parts in low volumes, and functional part assemblies that must be ultra-reliable. Accordingly, the unique requirements for measuring and inspecting medical device components, implants, and prosthetics are driving these companies toward more nontraditional measurement and inspection technologies.

One case is DePuy Orthopaedics Inc. (Warsaw, IN), a manufacturer of prosthetic devices made specifically to match a patient’s anatomy. DePuy’s products include knee, hip, ankle, and finger implants made from a variety of materials. “These products are manufactured from a CAD solid model using primarily surface-machining techniques,” explains DePuy manufacturing engineer Roger Erickson.

To compare the shapes of parts directly with 3-D CAD models, the company uses laser scanning. Laser scanners convert complex part surfaces into digital point clouds to reveal deviations from spec in detail. Erickson says the technology is a good fit for custom, one-time implant designs for which physical gaging would be cost and time-prohibitive.

More-traditional inspection alternatives remain for medical manufacturers. CMMs continue to be cost-effective options for a range of medical applications, says Jonathan O’Hare of Brown & Sharpe Inc. (North Kingstown, RI). “Medical components such as plastic valves and hard tubing are typically high-tolerance parts, and do not typically require CAD comparison.” These can be measured with basic CMM technology. “More complex geometry, such as that of medical prosthetic joint replacements, tends to have somewhat lower tolerances and require CAD comparison for sophisticated best-fit error calculations.” The tightest-tolerance medical components, requiring the highest-end CMM technologies, are used in precision surgical tools, he adds.

Given the various needs, Brown & Sharpe offers three levels of configuration packages for its Global CMMs. The Global Classic and Performance packages can handle basic and some prosthetics-level tolerance applications, respectively. More extreme tolerances for medical tools and prosthetics need the highest-precision Advantage configuration.

“High-accuracy scanning probes on Global Advantage offer both high accuracy and a sufficient quantity of information for effective process control,” says O’Hare. The high-end versions also use PC-DMIS CAD or CAD++ software that provides algorithms for complex analysis.

Touch probing can also effectively measure soft, easily deformed parts such as surgical tubing, urethane foams, seals, contact lenses (and their molds), and low-force switches. Here, the touch probe must use a measuring force that’s low and consistent enough to prevent major measurement errors. One touch indicator system, the Super Litematic VL-50AH from Mitutoyo America Corp. (Aurora, IL), uses only 0.01 N (about 1 g) of measuring force. “This cannot be achieved via manual means, for example, using calipers,” says product manager Wally Wardzala.

“The key lies in the fact that the Super Litematic can be calibrated to operate with an extremely consistent, very low force.” The indicator has a motor-driven spindle that drives downwards until the contact probe touches the workpiece, and maintains the force throughout the measurement cycle. “The manufacturer simply needs to take into account whatever effect that one gram will have on the workpiece and subsequent measurement,” Wardzala explains.

For calculating its digital measurements, the indicator uses interferometric laser light holographic diffraction and a low-expansion glass scale. In other words, the X-axis displacement of the probe is measured using the phenomenon of interfering diffracted laser beams. Two beams are diffracted by the glass scale while a detector measures their interaction and converts it into a displacement reading. The resulting accuracy over the 50-mm measurement range is reportedly less than half a micron.

But for complete inspection of its custom parts, DePuy Orthopaedics opted for the touchless approach of laser scanning. First, the company tried a laser probe system for inspecting its anatomical implants, says Roger Erickson. “It was very slow and had limited function, because it could only acquire one point at a time, and could only measure diameters.” Moving on to a faster and more versatile alternative, the company selected the AI300 laser scanner system from ShapeGrabber Inc. (Ottawa, ON, Canada).

“Laser scanning is a new technology which is displacing traditional methods due to [its] speed and completeness of coverage,” says ShapeGrabber executive vice-president Pierre Aubrey. It can measure very complex, compound curves, “which are very difficult to do quickly and accurately with traditional tools,” he adds. It provides acceptable dimensional tolerances of 0.002–0.010″ (0.05–0.25 mm) for parts from dime-size to 2′ (0.6 m) across. Moreover, the laser scanner can be reconfigured for different parts flexibly.

But new inspection systems do require some initial investment of money and time (not uncommon in zero-defect manufacturing industries). The newness of laser scanning is also resisted by the conservatism of some quality-control departments, says Aubrey. But he argues that its return on investment and potential benefits are high, although “it requires a willingness to invest in new technology, to learn new tools, and to change some internal processes.”

Operations making low-volume parts with sophisticated shapes and materials can benefit from other fast and flexible part inspection methods. For intricate machined parts, a major advantage would come from being able to measure a part CMM-style without having to remove it from the machine. On-machine part measurement allows key dimensions to be checked incrementally throughout the process, immediately indicating the need for reworking out-of-tolerance features (or for scrapping the part), without losing time sending the part to a CMM and then refixturing it after inspection.

With such on-machine measurement, the time savings can add up, says Michael Sterioff, product manager of Marposs Corp. (Auburn Hills, MI). “The time it takes to inspect a part for approval is typically reduced by amounts related to the current inspection process.” With the machining of large molds, the time saved might be measured in hours, versus minutes saved for small parts. “Think about unloading then reloading the part to re-machine, or just the time it takes to realign the part for measurement,” he adds.

To this end, Marposs developed the 3-D Shape Inspector software for on-machine measurements. Used with the company’s Mida touch probes mounted on the machine’s spindle, the new software reportedly enables precision measurements of both sculpted parts and geometrical features, essentially turning a machine tool into a CMM. For creating probing programs and inspection routines, the software is interfaced with a CNC-integrated or offline PC. Various automated features simplify calibration programming, part measuring, and GD&T reporting.

“Programs can be created just like a machine cutting program, so it’s very easy to change over from part to part.” Sterioff says that while CMM experience may be beneficial for the operator, it isn’t necessary, since the required programs are downloaded via a direct or RS-232 connection.

Value-added inspection checks not only the final part but also the machine tool and setup. For critical medical applications, proper tool alignment could be the difference between life and death. Here again, lasers play the key inspection role, even in the midst of cutting fluid and debris.

Particularly when manufacturing small components, extremely small cutting tools that are hard to see are correspondingly difficult to check—thus requiring a bit of specialized technology. Different-size lasers are offered for tool verification, says the company’s Sterioff. A standard Marposs laser can measure tools from 95-mm diam down to 0.1-mm diam. He says the smaller Mida 75 Pico laser can measure from about a half-inch (12.7 mm) to 0.05-mm diam.

The system overcomes interference from the fluid and dirt of machining using a shutter that protects the laser lens. “It opens and closes with an M-code activation/deactivation,” says Sterioff. To measure small tools with high accuracy, however, users need to purchase a tool blow-off option. The reason? “The laser is so accurate, it can even measure the small amount of coolant left on the tool.”

Manufacturing process monitoring goes hand-in-hand with dimensional checks for medical device assemblies. Critical devices must be put together just right to ensure proper functioning inside the body. This applies to stents, devices that deploy balloon catheters to open up clogged blood vessels to restore normal blood flow to the heart and reduce the incidence of heart attacks.

An effective stent system requires a uniformly folded balloon “charge” to be inserted inside a metal stent that is then crimped to size. Given the careful folding and crimping, the manual assembly process had required up to 45 min, until process automation specialist Machine Solutions Inc. (MSI; Flagstaff, AZ) introduced a system to pack and crimp the stents in seconds. MSI’s patented segmental crimping technology compresses the stents to a small enough diameter for insertion into a blood vessel. “Besides the enormous labor saving for stent manufacturers, automation also ensures a more precise and repeatable process,” says Scott Mickelson, MSI project manager.

To ensure stent reliability, complete crimping around the folded balloon must be monitored and confirmed every machine cycle. For this in-process monitoring, MSI machines use RGH24 optical encoders from Renishaw Inc. (Hoffman Estates, IL). The RGH24 continuously monitors the radial compression of the stent diameter via optical readheads riding above a flexible metal tape scale mounted in a partial arc. The 20-µm pitch scale reportedly provides an accuracy of ±3 µm/m.

For this kind of application, magnetic encoders could have been an option. However, the optical encoders allow users to check setup and scale alignment easily via a green/red LED go/no-go output, says Mickleson. The red LED also acts as a visual alert of low signal amplitude, signaling a loss of encoder count. “It’s critical for us and our customers to know that the unit is aligned. If it’s not, our machine is missing counts, which means that our diameter could be off.”

Magnetic encoders may seem to have an advantage in terms of being impervious to dust and fluids, but this isn’t an issue with this application, explains Howard Salt, Renishaw encoder products manager. “Magnetic encoders perform much better in dirty environments” (and especially around liquids), because optical encoders need to “see” the scale. “However and luckily, most medical applications are not in dirty environments.”

Even so, the optical encoders can tolerate some dirt; filtering optics in the readhead provide immunity to dust and scratches. The readhead averages the reflections from more than 80 scale facets, effectively filtering out signals that don’t match the scale period to ensure signal accuracy, despite contamination or minor damage.

Moreover, whereas magnetic encoders offer resolution down to 1 µm, optical encoders can offer resolutions down to nanometers, adds Salt. This is why they’re preferred in many precision motion applications.

At the extreme end, laser encoders offer the ultimate feedback solution. Mainly used in semiconductor applications, they offer resolutions far below a nanometer—a range that will eventually become more relevant for medical nanodevices of the future.

Hand Measurement Goes Wireless

Users of traditional measurement tools on the factory floor may now be able to simplify one of the hassles of manual measuring: recording the data. In a new wireless data-collection system, a miniature radio in each electronic measuring tool sends data through signal routers to a central PC for recording. Each radio has a range of 100′ (30 m), and routers can be added and arranged in a “mesh” network, effectively increasing the range to 3000′ (900 m) for complete plant-floor coverage. The system, called DataSure, is from L.S. Starrett Co. (Athol, MA).

Until recently, wireless data-collection systems had been unable to reliably capture and transmit data in manufacturing environments due to electrical interference, says Jeff Wilkinson, general manager of the company’s Advanced Technology Division. DataSure succeeds because of the mesh network architecture and other design features.

By automating and speeding up data collection, the system helps users “bring the gage to the work.” It reportedly interfaces with most major brands of electronic measuring tools and can handle up to 100 tools at once, sending each tool’s data to a central PC by the shortest or most robust connection path in the network. Once data are successfully received, a confirmation signal is sent back to the tool and when the host system is down or busy, readings can be stored at the end node (the miniature radio) and re-sent until they are safely recorded.