Measuring Complex Implant Geometries in One Setup

Jürgen Klemm, plant manager at Stuckenbrock Medizintechnik, shows some of the high-precision specialty screws the company produces.The complex geometries of prosthetics, implants, and specialty medical screws can be measured with typical measurement equipment with only limited success. However,  3D CNC multi-sensor coordinate measuring machine, with three different sensors, can check all features in one setup.

Prosthetics and implants are becoming more and more complex and delicate. That is why Stuckenbrock Medizintechnik, Tuttlingen, Germany, maintains a high level of technology. In order to produce and inspect very tight tolerances, the equipment must meet very high specifications. Plant manager Jürgen Klemm explains: “For our specialty screws – with regard to thread diameters – we have reached precision levels of hundredths of a millimeter. 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. 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.”

It helps to know that their machine tools work precisely. However, the workpieces must still be checked in order to detect any changes early on, and to take countermeasures in case of any drop in quality. “We have a high level of inspection effort, mainly in the area of in-process measurement,” Klemm says. He is talking about the 3D CNC multi-sensor coordinate measuring machine, the VideoCheck IP400 from Werth Inc.

The plant manager explains that the complexity of their geometries can be measured with typical equipment with only 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 to do this than by using multi-sensor technology.”

A Single Machine
Just five years ago, Stuckenbrock invested in the multi-sensor coordinate measuring machine, which has a measurement range of 400mm x 200mm x 200mm (X, Y, and Z). It is characterized by a constant-stress guide system, an additional fourth axis, and a wide range of sensor options.

Klemm reveals the considerations that preceded this purchase: “Typical tactile measuring machines are too coarse for our requirements. We can barely measure our delicate parts with them, because the probe spheres are ususally too large to approach the critical features. 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 multi-sensor 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.”

The modular system at Stuckenbrock has three sensors: the fiber probe – with a probe sphere as small as 20µm; a high-resolution optical sensor with zoom optics; and a laser that is integrated in the zoom optics.

According to Klemm, its use, and the use of WinWerth software, is uncomplicated and easy to learn. “I completed a training class at Werth Messtechnik, together with my employee Salvatore Bennardo,” Klemm says. “Afterward, we were able to train our machine operators sufficiently in the use of the measuring machine. This was no problem, and neither is the daily use of the VideoCheck.”

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% 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 – since keeping production moving is important. Additionally, 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 – from under 100 parts to up to 5,000 parts.

Measuring Tools, Workpieces
An example of one of the 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 20 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 meet the precise requirements, so that the implant fits perfectly and can be installed later. Without the multi-sensor measuring machine, such measurement tasks would not be able to be implemented.

It is also useful in many other ways. For example, 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. Klemm explains: “We work with profile tools, among other things, that have surface contours that are responsible for the precision of the manufactured part. In order 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 multi-sensor machine.”

In the meantime, the VideoCheck machine 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 of the hardware,” Klemm states. “The Werth Messtechnik technicians were always by our side as fair partners.”

Werth Inc.
Old Saybrook, CT
werthinc.com