CLEAR THE AIR

Tapers play an important role in orthopedic devices. Most hip and knee implants use tapers because they provide good alignment and can be “locked” into position. In manufacturing these devices, the control of taper and size determines how well the implants perform over their lifetime. Increasingly, air gaging has become the inspection tool of choice for controlling these critical parameters.

“Air gaging was the breakthrough gaging technology of the ’40s and ’50s,” says George Schuetz, director of Precision Gages for Mahr Federal (Providence, RI). “Introduced decades before electronic gaging, it provided the first high performance ? 50µ” or better ? gaging on the shop floor. Despite its age, air gaging today is actually growing in popularity, especially in high precision areas like orthopedic devices. It is fast, easy to use, provides high precision results even under the toughest shop conditions, and can last for years measuring literally millions of parts.”

For years the taper design of choice for orthopedic devices was the simple round taper, explains Schuetz. ?It functioned well, but with demands for better performance, new designs have been developed. Innovations such as special surface finishes and complex taper forms employing oblong tapers have improved orthopedic device performance.?

But all these tapers offer unique challenges to gage designers. “The problem is not just measuring taper or diameter,” notes Schuetz, “but measuring these dimensions on parts that may be only 12 mm in diameter and 12 mm in length. Even on parts that are longer, more points frequently need to be measured and more data collected.”

Air gaging uses orifices that are 0.050 in or smaller. They can be placed within 0.10 in of each other and combined to produce any number of dimensional and geometric results. “Small electronic sensors or eddy current type sensors may approach the size of an air jet,” adds Schuetz, “but nothing can match air’s economy or ability to work in a wet and oily shop environment right at the point of manufacture.”

Air gaging uses the principle of back pressure to determine the size of a measured part, Schuetz explained. It is typically used for applications like precision medical tapers, where tolerance is fairly tight ? usually less than ±0.001 in ? and surface roughness is less than 50µ” Ra.

Medical implants take a beating and the taper fit between the female and male components is critical. The two pieces have to lock together and sit at the correct height. During manufacture it is common to inspect 100 percent of parts to ensure the accuracy of both components, and air gaging offers three ways to do this.

The most common type of air gage taper tooling has two pairs of jets on opposing air circuits, and is designed for a “jam fit” between the part and the tool. If the taper angle is incorrect, there will be excessive clearance between the two surfaces at one or the other end of the taper. Either situation can reduce the rigidity of the connection, which over time can cause the lock to fail. If the taper angle is correct but the size is incorrect, then the overall length of the orthopedic assembly will be incorrect and provide unexpected results after implant.

Jam-fit tooling does not measure part diameters, per se. Rather, it displays the diametrical difference at two points on the workpiece, as compared to the same two points on the master. To learn more about the taper components’ positioning accuracy, you need a “clearance” style air tool in which an air taper ring cavity is sized to accept the entire taper part. Depending on where the part’s reference surface is, the part can be referenced on the end of the taper or on a flange against the top surface of the part. This makes it possible to measure diameters at known heights. An additional set of jets may be added, to inspect for bell-mouth and barrel-shape, two more conditions that reduce the contact area between the male and female components.

The third type of air taper gage is a cross between the styles mentioned above. This is called a “simultaneous fit” taper gage. It is basically a jam-fit air tool with an indicator that references on the face of the datum surface. This indicates how far the air tool goes into the part being measured. So while the air gage provides a reading of the taper angle, the indicator provides an indication of the size of the diameters.

“But the best part about air gaging is that it’s easy,” says Schuetz. “The pressurized air from the gage tends to clean the part as it’s being measured, and production workers do not require special training to use air gages. To check a hole, for instance, it is not necessary to develop skill in “rocking the gage.” To find the true diameter, merely insert the air plug in the hole and read the meter. It is as simple as that.”

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Mahr Federal Inc., 1144 Eddy Street, Providence RI 02905, 800-333-4243 or 401-784-3100, Fax: 401-784-3246, www.mahr.com .