Intricate, complex medical devices made in small lots are challenging to measure
By Bruce Morey
Machining parts makes money—everyone knows that. Measuring them, on the other hand, is not productive time. It is often considered a necessary evil, an overhead expense to be endured. Such an attitude is not always the case for those who think ahead. "Metrology can be a strategic tool to go after new markets or gain additional business," says Rob Marr, vice president of C&A Tool Engineering (Churubusco, IN). "It is especially important in medical—if you cannot measure the part accurately you cannot get the business." C&A, like others in the field, needs to comply with both the FDA 21 CFR Part 820 Quality System Regulation and the SO 13485 Medical Device Standard. The right metrology equipment used well is vital.
Investing in machines that deliver quality has been a cornerstone of the company’s strategy since its founding in a garage in 1969. It has grown since into four manufacturing locations totaling 750,000 ft² (69,675 m²), employing more than 500. Industries it serves includes aerospace, defense, and automotive as well as medical. Jobs range from prototype components to complete assemblies. "For medical, we specialize in surgical tools and implants," explains Marr. These include orthopedic chisels, tibia alignment guides, patella clamps, and acetabular shells and inserters. "We developed an important niche in spinal implants, such as replacements for discs," says Marr.
As a contract manufacturer serving many industries, they found they needed to organize their facilities differently than a dedicated manufacturer. Each portion of the plant, or pod, is dedicated to machine types—grinding, milling, or lathe/mill-turn. The company’s South Building, for example, is shaped like an "H"—180,000 ft² (16,722 m²) on 46 acres—with each leg containing four pods of over 20,000 ft² (1858 m²). "Our volume is not high enough in most cases in any one part to make dedicated workcells attractive for us," says Marr. This means a lot of material flowing around their plant as each job moves from operation to operation.
Each pod contains its own metrology laboratory, typically outfitted with one or two Contura CMMs from Carl Zeiss Industrial Metrology, LLC (Brighton, MI), and optical comparators from Optical Gaging Products (OGP, Rochester, NY). They use Zeiss CALYPSO software for programming their CMMs. Each CMM is equipped with touch probes and scanning analog probes. "We are looking into nonontact sensing, like laser scanning, for our CMMs, but have not made that decision yet," says Marr. Their noncontact sensing is in equipment such as Zygo Corporation (Middlefield, CT) surface profilers. Multiple quality labs scattered around the facility reduce the amount of time required to move pieces (and people) away from important machining operations to inspection. Cutting and grinding is still where the money is. In fact, metrology at the machine is the ideal. "Utopia is if our skilled associates do not have to walk away from the machine to measure a part," Marr says.
One such system C&A uses for this is an OASIS optical profile inspection system supplied by George Products Company (Middletown, DE). They program this 2-D digital shadowgraph to measure critical features for a particular part, which is stored in a library. "During production, we simply place the part on the machine and it automatically identifies the part from its library and gives a go/no go on each critical dimension," says Marr, requiring no fixturing for speedy, accurate measurements.
"This machine is sometimes described as an optical comparator on steroids," says Jeff Palmer, a manager with George Products. The system is accurate to ±0.0001" (2.54 µm), according to the company. "When a part program is created, the OASIS looks at the edges of the part using what we call Tool Windows," explains Palmer. "It searches through all the stored parts programs to see what lines up best with that part. If there are similar parts, it may offer up to three different files of part programs that are close for the user to choose." He reports the system as frequently used by medical device manufacturers, for parts with maximum dimensions of over 6" (152.4 mm) down to less than 0.5" (12.7 mm), such as dental implants.
In some respects, the GD&T requirements for medical devices are not very different from other industries, such as aerospace, according to Gary Meyer, C&A’s go-to-guy for medical quality. While each part requires a thorough first-article inspection, like many other industries, the difference is the depth and breadth of documentation. They tend to rely on the expertise of their customers, the medical device OEMs, to know the details of meeting regulatory requirements. "I just had one assembly submitted for first-article approval with 250 pages of documentation—for that one assembly. It only had eight or nine component level parts," he explains.
Joe Huelsenback, the medical machining go-to-guy for the company notes that tolerances for spinal implants are typically 20 µm. A 10:1 gage R&R means they need inspection capability down to 2 µm. A product they like to use near machinery on the shop floor is an IM-6500 Series image dimensions measurement system from Keyence (Itasca, IL).
Useful for 2-D measurements, the integrated measuring system with display is designed for convenience, to operate fast, and with no fixturing. It adjusts automatically for location and orientation of parts. With optional software, the machine is programmed with actual parts, prints, or off-line using CAD. "It is especially good for measuring parts where contact might affect the result," says Sergie Shirokov, project manager for Keyence. "Parts that are very thin or have precise etchings, where you cannot get a stylus into. These include soft parts that are difficult to fixture." Measurement accuracies are between 2 and 5 µm. He notes typical medical parts inspected with IM series machines include needles, sutures, stents, or syringes.
The perspective from Micropulse (Columbia City, IN) is a bit different from that of a general job shop. "We decided long ago to focus our expertise on medical manufacturing alone, which is quite challenging all to itself," says Brian Emerick, president and CEO. While acknowledging that cost pressures are becoming more important in an industry once solely dedicated to quality, he predicts growth across the board of 5–10% annually. It is especially promising to be a contract manufacturer. "Some contract manufacturers are better than the medical OEMs at making things," he says. "OEMs are outsourcing 60–70% of their products and we are seeing a new trend with some OEMs that have no manufacturing capability at all."
Variety, small lots, and ever-tighter tolerances characterize his business. "Twenty percent of what we do on a monthly basis is a new part," he says. "Some months, 35% of what we are processing is brand new." That includes materials new to the machining company, such as processing PEEK plastics. "Our established processes for machining, inspecting, and handling of implantable PEEK materials are tightly controlled," says Jeff Hicks of Micropulse. "Manufacturing equipment and trained personnel have been dedicated to processing implantable PEEK to ensure that the product is consistently produced and handled according to specifications. In addition, the product is transported throughout the facility in visually unique containers. This provides notification to all employees that special handling is required in order to prevent contamination of the product."
Dedicated quality technicians will typically run exhaustive first-article inspections for part approvals. They also check first runs of every new lot to ensure process stability after machine setup. Two quality labs are set in strategic locations in their 100,000 ft² (9290 m²) facility, with one CMM and optical comparators set-up on the shop floor itself. Their CMMs are also equipped only with touch probes and scanning analogue probes.
Metrology at Micropulse is not for dedicated technicians alone. "The machinists and quality technicians both have access to the same inspection equipment. Machinists are able to use the appropriate inspection tools to verify product as it is being manufactured," explained Hicks. "Inspection activities for prototype devices, which are manufactured in a dedicated cell, are primarily performed by prototype personnel."
Of course, the goal is to not burden operators with quality checks at locations distant from their machine. Micropulse uses dedicated expediters to track lots as they move through operations and physically move the material from machine to machine and into metrology stations. This keeps machines humming and operators dedicated to what they know best.
"Implantable devices are becoming more common and there are more manufacturers of them," says Jamie Murray, Senior Applications Engineer at OGP. "Because of the critical nature of implantable parts, manufacturers are doing more measurements to validate processes, even as we see smaller lots and ever tighter tolerances." Today, nanometer–level measurement repeatability is a common requirement with some manufacturers. That level of precision was unheard of 10 years ago. OGP offers their SmartScope Specialist 300 for the medical field. It features an array of sensor options including telecentric zoom optics, touch probe, TeleStar telecentric interferometric TTL laser, microprobes, rotary tables, and continuous-contact scanning probe. Accuracy is down to 1.5 + 5L/1000 µm, to the ISO 10360 standard.
Another device useful on the shop floor that OGP recently introduced is its SNAP digital measuring system. Shop-hardened, it has no moving parts and quickly measures in 2-D to an accuracy of 5 + L/150 µm, to the ISO 10360 standard. "We felt the ergonomics of how to use measuring equipment is becoming more important, which is certainly true in the medical field," says Murray, even though the SNAP is a general-purpose machine. "SNAP features AutoCorrelate, recognizing the part it measures regardless of the orientation. This reduces [or eliminates] the need for special jigs or fixtures to hold parts," says Murray.
Long-term use of data is another trend in medical. "We are well past the point where we look at measurement data, say the part is good, and then discard the metrology data. All of the data, good or bad, is being fed back into some kind of process-control loop," said Nate Rose, Applications Engineering Manager at OGP. This often includes comparing the data to the part’s CAD model to determine how well the part complies with the design intent.
OGP offers more than equipment to help their medical device manufacturing customers, especially smaller operators for whom medical manufacturing is relatively new. For example, OGP offers tools and services to help companies meet regulatory requirements for installation qualification (IQ) and operational qualification (OQ), two of three phases required to make measurement systems compliant with FDA regulation. "We also offer workbooks to assist companies in preparing for the process qualification [PQ] phase, although they must do the actual qualification on their own," explains Rose. "We are seeing an increase in the services that we do. I think that is because there are more manufacturers getting involved in the medical device supply chain. A number of aerospace component manufacturers have added medical because, really, there is much similarity between the two."
Establishing a foundation of accuracy is as important as in-process control. "The most challenging areas within the medical market are makers of implants, such as hip and knee replacements," agrees Clive Warren of Renishaw (Hoffman Estates, IL). "These require complex machining processes to exacting standards." Complying with medical manufacturing quality standards means proving CNC machines are capable of cutting to stated tolerances. "Ballbar analysis is a proven method for determining machine tool capability, and is the most practical, convenient and comprehensive tool for assessing the performance and accuracy of CNC machines," according to Warren. The QC-20W measures to ±0.1 µm. "The new QC-20 wireless ballbar means testing is much faster and safer, since you do not have to route the wire out of the machine or override the door interlock," he says.
Currently, ballbar tests defined in ASME and ISO standards are limited to the three primary working planes and do not address the challenges presented by four and five-axis machines. However, new test routines have been proposed that include a machine’s rotary axes and are currently being considered for inclusion in a new ISO 10971-6 standard, and there are also references to four and five-axis tests in Appendix J of ASME B5.54 2005, according to Warren. To determine the angular accuracy and repeatability of rotary axes, Renishaw recently launched the XR20-W, a wireless rotary-axis calibrator. "It uses an ultra-high accuracy encoder, originally designed for the new generation of active probe heads such as the new Renishaw PH20," said Warren. Designed to work with their XL-80 or ML-10 laser interferometer systems, the XR20-W measures to better than ±1 arc-second. ME