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Jeff Bibee

Metrology

Multisensor CMM Measures Complex Implant Geometries

Multisensor system enables complex measurements in one setup.

Published: Monday, January 4, 2010 - 16:20

T

he complex geometries of prosthetics, implants, and specialty medical screws can be measured using typical measurement equipment with only limited success. Prosthetics and implants are becoming more and more complex and delicate. That’s why Stuckenbrock Medizintechnik , in Tuttlingen, Germany, maintains a high level of technology. To be able to produce and inspect very tight tolerances, equipment must meet very high specifications.

“For our specialty screws—with regard to thread diameters—we have reached precision levels of hundredths of millimeters,” explains plant manager Jürgen Klemm. “For threads that have very thin flanks, this is a formidable specification. A normal tibial pin, in contrast, is a relatively large part.”

Stuckenbrock earns its money not with high volume, but with highly intensive parts. The high quality of their complex, delicate parts is also responsible for their success, and reliable production processes are indispensable. Klemm and his employees place great value on high-quality products, starting with their machine purchases. “Our machining equipment consists of about 20 machines–grinders, long and short lathes, and machining centers with up to 12 axes—all from manufacturers with good reputations,” says Klemm. It helps to know that their machine tools are precise and come from reputable companies, but just the same, the workpieces must still be checked to detect any changes early on, and to take action in case of any drop in quality.

Even the best equipment is only “just good enough for us,” says Klemm.

Investment in only a single machine

Five years ago, Stuckenbrock invested in Werth Messtechnik’s 3-D computer numeric controlled multisensor CMM, the VideoCheck IP400, which has a measurement range of X = 400 mm, Y = 200 mm, and Z = 200 mm. It is characterized by a constant-stress guide system, an additional fourth axis, and a wide range of sensor options.

The complexity of Stuckenbrock’s geometries can be measured with typical equipment only with limited success. It would be a rare case if a single sensor would be sufficient for checking all the quality features. “Today, we need more extensive measurement capabilities, so there is really no other way than using multisensor technology,” Klemm explains. “Even with this machine, some needs still demand sophisticated fixtures in order to meet each requirement in a single setup.”

Klemm reveals that several considerations came into play for the purchase of the VideoCheck. “Typical tactile measuring machines are too coarse for our requirements. We can barely measure our delicate parts with them, because the probe spheres are mostly too large to approach the critical features,” says Klemm. Optical measuring machines are one alternative, but they do not cover all applications. Additional sensors are needed. The question was raised as to whether to invest in various measuring machines, or in one multisensor measuring machine. “In order to avoid the disadvantages of multiple setups, we decided on the Werth VideoCheck IP400 solution. The ultra fine, optical-tactile Werth Fiber Probe also convinced us. As far as I know, there is no alternative to its functionality,” Klemm notes.

The modular system at Stuckenbrock has three sensors: the fiber probe, with a probe sphere as small as 20 µm, a high-resolution optical sensor with zoom optics, and a laser that is integrated in the zoom optics. All in all, the machine saves time and space.

The multisensor systems and its WinWerth software are uncomplicated and easy to learn, says Klemm. “I completed a training class at Werth Messtechnik, together with my employee Salvatore Bennardo. Afterward, we were able to train our machine operators sufficiently in the use of the measuring machine,” says Klemm. “This was no problem, and neither is the daily use of the VideoCheck.” Klemm found the use of two screens to be very practical. One is used for the measuring program, and the other displays the video image with edge detection. This eliminates switching back and forth in the software interface.

Salvatore Bennardo is responsible for the measuring machine on the day shift. He writes the measurement programs and takes all the measurements. The machine operators themselves are responsible for measuring during the other shifts. These are primarily in-process measurements for various machines, but also final inspections and 100-percent inspections for particularly challenging parts.

The measurement area has been set up so that the clamps and most fixtures can remain on the machine, enabling more rapid changeover of parts as seen in figure 1. “Sometimes the time can get tight,” Klemm admits. “Measurements are almost always rushed. After all, the machines need to be producing again without long interruptions.”

Figure 1: The measurement area was designed such that clamps and most fixtures can remain on the machine, which allows faster changeover of parts.

 

All measurement programs are available via a network connection to the machine. They can be called up by part number. The product portfolio is now up to about 8,000 to 9,000 different articles, mostly with small production lot sizes, from under 100 parts up to 5,000 parts.

Measuring tools and workpieces

One of Stuckenbrock’s products is a hip implant used for classical fractures to the neck of the femur. It consists of various elements and can be placed in the body without stress. The implant is screwed onto the thigh bone (femur) from the outside.

The load-bearing screw that is screwed into the femur bone must meet nearly twenty quality criteria. For the screw thread, the inner diameter, outer diameter, flank angle, radii, and pitch are checked. The positions of the hex socket, external hex, and central counter bores to each other must exactly meet the requirements, so that the implant fits perfectly and can be installed later. Without a multisensor measuring machine, such measurement tasks would be hard to implement.

It is also useful in other ways. For one thing, the measurement records document the process sequence and the precision of the products, which can also be useful in case of discrepancies with customers. Tools are measured more and more often as well. “We work with profile tools, among other things, that have surface contours that are responsible for the precision of the manufactured part,” Klemm explains. “To get good results right from the start, we have moved to inspecting the tolerances that we require, which was not possible before we had the multisensor machine.”

In the meantime, the VideoCheck has been reconfigured and adapted several times to meet all the requirements. “Completely capturing all features of an implant in one setup is not an easy task, and requires some adaptations on the hardware side as well,” says Klemm. “The Werth Messtechnik technicians were always by our side.”

Long tradition

The roots of Stuckenbrock Medizintechnik go back to the year 1884, the year the predecessor firm, Karl Vögele, was founded. It specialized in the fabrication and sales of surgical instruments.

Production of scalpels is still an important part of the company. Stuckenbrock has become a shareholder corporation of the KLS Martin Group, which also includes Karl Leibfinger Medizintechnik in Mühlheim, the Martin brothers, and other companies. The various medical technology manufacturers jointly use the name KLS Martin in the marketplace.

KLS Martin is a complete provider, selling everything that is needed in the operating room. Its production spectrum has been divided among the member corporations.

Of the 180 employees at Stuckenbrock, about 40 work in a separate production area that specializes in prosthetics and implants. Certification to DIN/EN/ISO 9001, EN 46001 and Appendix II of the Directive 93/42 EWG in 1995 are help ensure the quality of Stuckenbrock’s products.

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