NOTHING LESS IS ACCEPTABLE

Medical devices may not be the first thing that comes to mind when someone mentions precision manufacturing, but products like artificial hip and knee joints, the "rods" and "straps" used to mend shattered bones, even hypodermic needles and the staplers used to close wounds in open heart surgery are all produced with processes every bit as sophisticated as those used in the aerospace and electronics industries.

Even the materials – titanium, magnesium, cobalt, stainless steel and other equally exotic alloys – are more commonly thought of as aerospace components than replacement parts for the human body.

Obviously, medical components must be as close to functional perfection as modern technology can make them, but that is not the only critical parameter. In an increasingly litigious society, any imperfection, even an unmeasurably small mark left by a gauge contact, can be the basis for a liability lawsuit.

MARK-FREE TITANIUM HIP JOINTS
More than a quarter million hip replacement surgeries are performed annually in the U.S. and every one of them requires an ultra-precision ball and socket set sized to the needs of an individual patient.

The ball portion, commonly made of titanium, is attached to a long stem that is inserted into the femur, or upper leg bone. The socket portion is attached to the pelvic bone. The ball and socket must mate perfectly and both need to be superfinished to provide a nearly friction-free range of motion. The grinding and polishing operations are done on specialized machine tools and are monitored and controlled by equally specialized in-process gauges.

Marposs Corporation (Auburn Hills, MI), has supplied a number of systems to artificial joint manufacturers designed to meet particular application requirements. In the case of titanium hip balls, for example, the need to inspect the part without leaving any marks presented an unusual challenge.

Titanium is a highly reactive metal that attaches readily to carbon-based materials like diamond, which is commonly used for precision gauge contacts. Naturally, a build-up of workpiece material on contact of a gauging system measuring workpieces with tolerances of 0.0002 in to 0.00025 in is unacceptable. Even more unacceptable is the mark left on the workpiece when the titanium attaches itself to the diamond.

Marposs evaluated a number of alternative contact materials, including ruby and sapphire, which are less reactive, but quite soft. The ultimate solution, however, was found in modern ceramics that are quite hard and essentially impervious to chemical bonding by titanium. Engineers helped the user fine-tune the grinding process so that the last pass of the wheel is made after the last measurement is taken. That way, even an unmeasurable mark on the ball is eliminated while still maintaining complete control of the process.

In an interesting bit of serendipity, a technology called conical compensation was applied to the process that finishes the tapered stem of the hip joint. This method uses a pair of gauges, one locating on a reference surface and the other measuring the tapered surface, to provide an accurate measurement of taper regardless of the axial location of the part.

Conical compensation was originally developed almost 30 years ago to control the manufacture of tapered breech blocks for 155 mm cannons built for the U.S. Army. Of course, then it was an analog process. Now it?s a built-in function provided by the control software.

TAKING THE 'OUCH' OUT OF HYPODERMIC NEEDLES
The company has also adapted a standard through-feed grinder gauge to inspect hypodermic needles as they emerge from a centerless grinding operation. Here the challenge is to inspect huge quantities of very small needles that come out of the grinder "like a stream of water".

The answer is an optical sensor offered as an option on a standard through-feed gauging system. The optical approach also solves the problem of reliably touching the fast-moving, lightweight needles coming out of the grinder.

TINY HOLES IN SURGICAL STAPLES
Today, surgeons close incisions in delicate tissues with surgical staples rather than sutures. None are more critical or delicate than those used during open heart surgery, which depend on holes as small as 0.001 in diameter.

Of course, drilling a 0.001 in hole requires a drill 0.001 in diameter and a tool setting and monitoring system capable of handling something that small and delicate. To make this challenge even more interesting, the monitoring must be done while the drill is rotating in the spindle at 30,000 to 40,000 rpm.

The solution is a programmable laser system for in-machine non-contact tool monitoring with a built-in microprocessor that allows the threshold of the laser to be tuned to compensate for the presence of coolant mist as the drill is measured. Since the mist droplets can be as big as the drill itself in this application, this ability is critical to success.

Before the laser system was installed, the user had to inspect 100 percent of the parts being manufactured with a magnifying glass to see if the hole was present. This manual inspection had to be done because the user never knew if the micro-size drill had broken during the machining process of a batch of 160 pieces. The manufacturer processed the staples 40 pieces per side with a 4-sided indexing fixture, meaning a tool failure could occur in batches of 40-80-120 or up to 160 pieces of scrap in one setting.

Now the drill is inspected using the laser system after each 40-part run (one rotation of the 4th axis as processed per side of the fixture). If the drill is broken, a redundant tool is measured, the necessary offsets are implemented, and the 40 parts are re-processed, virtually eliminating scrap. Moreover, by detecting any wobble as the tool rotates, it is possible to calculate wear and predict imminent breakage, further reducing downtime and scrap production.

In a similar application, laser tool inspection monitors the condition of the cutting edges of tools used to machine magnesium components. Dull tools can create enough heat to ignite the highly-flammable magnesium, so continuous monitoring is both a quality and a safety issue in the plant.

MONITORING BONE REPAIR DEVICES IN THREE DIMENSIONS
Not everything used in medical procedures is small and delicate. The rods and braces used to repair shattered bones, for example, can actually be quite large. They are also oddly shaped to match the contours of the bones to which they are attached.

Checking these 3D contours and inspecting all of the holes for the screws and bolts used to attach them presents a different set of unique monitoring and control challenges involved in accurately measuring part shape in three dimensions on the machine immediately following the machining process. The solution involves the use of Mida spindle probes that feed data to 3D Shape Inspector (3DSI), which is specialized software specifically created to map three-dimensional surfaces.

The manufacturer also uses 3DSI in conjunction with QuickSPC software to monitor and control mold making operations in a branch of their business dedicated to "made-to-order body parts", along with the firm?s aerospace division. This real-time three dimensional monitoring with the longer-term data storage and analysis allowed this manufacturer to meet stringent quality requirements without investing in a CMM to inspect the finished molds.

FIRST PART, GOOD PART
In addition to coping with exotic materials and exacting quality standards, the parts created by medical products manufacturers have extremely high value, which places a premium on scrap reduction. In all of these applications, gauging systems, laser tool setters, spindle-mounted probes and specialized software solutions offer a "first part, good part" capability that keeps scrap, and cost, to a minimum.

The challenges of medical manufacturing are unique – and so are many of the measurement and control solutions developed to meet them. But in an industry dedicated to making products as close to perfect as technology allows, nothing less is ever acceptable.

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Marposs Corporation, 3300 Cross Creek Parkway, Auburn Hills, MI 48326-2758, 248-370-0404, Fax: 248-370-0621, www.marposs.com.