MEASURING TAPER IN ORTHOPEDIC DEVICES

Tapers play an important role in orthopedic devices. Most hip and knee implants use tapers to provide good alignment and because they 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 in the '40s and '50s. Introduced decades before electronic gaging was widely accepted, it provided the first high performance–50µ in or better–gaging on the shop floor.

Despite its age, air gaging today is still as important and actually growing in popularity. 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.

Air gages effectively measure all common types of dimensions and are particularly suited to checking dimensional relationships. As an inspection tool, air gaging can measure many jobs faster, more conveniently, and more accurately than other gaging methods.

In the measurement of high precision hole conditions, for example, air gaging is unsurpassed for speed and accuracy. And in checking dimensional characteristics, air offers sufficient magnification and reliability to measure tolerances well beyond the scope of mechanical gages.

Air is also easy. 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.

HOW IT WORKS
Air gaging uses the principle of back pressure to determine the size of a measured part. According to the laws of physics, flow and pressure are directly proportional to clearance and react inversely to each other. Thus, the relationship between air pressure and the distance of a restriction (workpiece) to the air escape (jets) can be plotted on a graph, as shown in Figure 1, line (a).

As the distance between jets and work surface increases, the pressure decreases and the ratio becomes linear as represented by the straight section (b). This straight portion of the curve can be accurately calibrated, and represents the scale of the air gage.

Air gaging is not the solution for all shop measurements. It is typically used for applications where the tolerance is fairly tight–usually less than +/-0.001 in –and surface roughness is less than 50µ in Ra. When these precision conditions exist, as with precision medical tapers, air can be the best solution for the application.

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

But all these tapers offer unique challenges to gage designers. The problem is not just measuring taper or diameter, but measuring taper and diameter 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.

The reason air gaging is so important for orthopedic tapers is the air jet–the small orifice that emits air to begin the physical process of creating a gage. No other gaging sensors are as small or can be placed in such close proximity when measuring multiple diameters or geometric forms. Small electronic sensors or eddy current type sensors may approach the size of an air jet, but nothing can match their economy or ability to work in a wet and oily shop environment right at the point of manufacture.

It is certainly possible to measure diameter and taper with either a combination of electronic sensors or CMMs with touch probes. But what is unique about the air jet is that it can be built in a precision tool that can be used to measure the part at the point of manufacture. This can be done in one fast measurement with very little operator involvement.

CMMs and optical gaging may provide an alternative, but neither has the speed to measure 100 percent of the parts on the shop floor, right next to the manufacturing process, while providing immediate feedback on the performance of the process.

Two conditions most important in controlling taper are taper size and angle. Size is controlled by tolerance, and is, therefore, identical to a cylindrical lD or OD. Taper angle, on the other hand, can be controlled by at least three different methods: 1) included angle or angle per side; 2) taper per inch or per foot; 3) controlling two diameters at specified datum locations.

TYPES OF AIR TAPER GAGES
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. This is usually done with differential air gaging, which combines the necessary high resolution and accuracy with the speed, ease of use, and ruggedness required on the shop floor.

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 rate of taper is too great, there will be excessive clearance between the two surfaces at the small end of the taper. If the rate of taper is too small, there will be excessive clearance at the large end.

Either situation can reduce the rigidity of the connection, which over time could cause the lock to fail by becoming loose and/or rotating. 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 (see Figure 2).

If the difference in diameter at the large end of the taper is greater than the difference in diameter at the small end, the upper jets will see more back pressure than the lower jets. This will reflect negative taper, or a larger taper angle.

If the diameter difference at the small end is greater than the difference at the large end, the gage will read positive taper, or a smaller taper angle.

But because a differential air meter displays diametrical differences only, it will not display the part's diameter at either location. So while this type of air tooling provides a good indication of taper wear, and allows us to predict a loss of rigidity in the connection, it does not tell us anything about the taper components' positioning accuracy.

For that, we need a "clearance" style air tool in which an air taper ring cavity is sized to accept the entire taper part. And 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 (in addition to the change in clearance, as with the jam-fit type). 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 (see Figure 3).

There is a third type of air taper gage which 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. When measuring a female taper part, if the taper diameter is too large, the gage will go farther into the part. If the diameter is too small, it will not drop into the part as far as expected.

SELECTING THE RIGHT GAGE
As there are many manufacturers of taper components, there are just as many methods for specifying taper requirements. The reference face may be different between manufacturers, and tolerances can be specified in different ways.

Depending on the way the assembly goes together, tolerances may be tighter on the taper than on the diameters, or vice versa. Or, there may be a combination of taper tolerances on only one diameter. Thus, the specification on the print is the best guide in choosing which air taper gage design to use.

With air gaging, the tooling is made specifically for each different taper application. So it is critical to properly understand the requirements.

Even so, the flexibility and benefits are hard to beat. Think about it: air gaging uses a sensor that is 0.050 in or smaller; sensors can be placed within 0.10 in of each other and be combined to produce any number of dimensional and geometric results right on the shop floor.

Some things do get better with age, and air gaging is one of them.

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George Schuetz is the director of precision gages at Mahr Federal Inc., 1144 Eddy Street, Providence, RI 02905, 401-784-3100, george.schuetz@mahr.com, www.mahr.com.