Measurement: Air Gaging for Orthopedic Devices
Typically, as parts move through the manufacturing process from raw material to final product, dimensional tolerance, surface finish and geometric characteristics become more critical. Tolerances become tighter and gaging processes must increase their performance to match these requirements.
For initial processes, calipers, micrometers, bench stands with indicators, thickness gages and other simple hand tools are usually sufficient. But moving further along in the manufacturing cycle, two important things need to be considered. First, once tolerances tighten, a whole new class of gaging is required. Gages must be more robust, have the proper design characteristics to fixture the parts properly, and have the proper resolution and accuracy to measure the required tolerances.
But the second thing to consider is that as tolerances get tighter, surface finish and form characteristics begin to take up a larger portion of the overall tolerance band. It is not uncommon for a form or surface finish condition to use 25% or more of the total dimensional tolerance available. Thus, form and surface errors can combine with dimensional errors and lead operators to make wrong decisions as to part quality.
Why Air Gaging?One of the tight tolerance dimensional characteristics measured at the end of the process is on the tapers used to match the parts together. Most hip and knee implants use tapers to provide good alignment and to “lock” the components into position. In manufacturing these devices, control of both 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 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 dimension, 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 fact, air gaging can be used in some cases to inspect and estimate certain form features on a part.
But air gaging also is so precise and provides such high resolution that it can be influenced by the surface finish of the part. Thus, 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 inch-and surface roughness is less than 50 microinches Ra. When these precision conditions exist, as with precision medical tapers, air can be the best solution for the application. But even when there are surface finish issues, special steps can be taken to account for them.
Taper DesignFor years the taper design of choice for orthopedic devices was the simple round taper. It functioned well, but new designs, such as complex taper forms employing oblong tapers and special surface finishes have improved product performance. However, these new 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 millimeters in diameter and 12 millimeters 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 valuable 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 coordinate measuring machines (CMMs) with touch probes. But the air jet 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. Neither CMMs nor optical gaging have the speed to measure 100% of 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 inside diameter (lD) or outside diameter (OD). Taper angle, on the other hand, can be controlled by at least three different methods:
Included angle or angle per side
Taper per inch or per foot
Controlling two diameters at specified datum locations.
All can be measured with air.
Types of Air Taper GagesMedical 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% 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 can 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. 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, the reverse is the case and the gage will read positive taper.
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. 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.
The third type of air taper gage is a cross between the aforementioned styles. 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 DesignJust as there are many manufacturers of taper components, there are 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 inch or smaller; sensors can be placed within 0.10 inch of each other and be combined to produce any number of dimensional and geometric results right on the shop floor.
Accounting for FormUnderstanding the form of mating tapers as part of process development is not only important to the function of the locking tapers, but also is important in determining the proper selection and use of dimensional gages. For example, when a manufacturing process inherently produces a two lobe condition, a two jet air probe is most apt to pick up these size and form variations.
Once known, form deviation can be correlated and understood as a component of actual size variation within the part. If the form analysis had indicated a three lobe or odd number of lobes in its results, then air tooling employing three jets would have been designed for the application. By understanding the form of the part prior to gaging, the best application of the air tooling can be employed.
Accounting for SurfaceMeasurements of the stem include the roughness of the polished area found on the proximal portion of the prosthetic femoral stem. This area is subject to close visual inspection for asperities by the orthopedic surgeon. To verify surface and waviness quality of the taper, a contour gage is used to trace the taper in the same measurement.
This causes complications for the application of air gaging. As noted earlier, a good surface is necessary to use air gaging. In normal use the air jet’s curtain of air covers an area on the surface of the part. This air curtain is restricted by the surface to create the back pressure which is required to do the measuring. On a smooth surface, the difference between the average surface and the peaks, which in the case of an ID part means minimal clearance, is pretty much insignificant. But if the surface is very rough, the area, or the point at which the back pressure is built up, can be significantly different. This can register on the air gage display and affect the indicated diameter.
As long as the surface finish is less than 50 microinches, this offset can normally be ignored. However, even if it is not, if the effect is discovered during process development, the error can be compensated for and the effective diameter shown to the machine operator. Q
Quality OnlineFor more information on air gaging, visit www.qualitymag.com to read the following:
“Air Gage Does Double Duty”
“Air Gaging Gets Better with Age”
”Why Air Gaging Still Matters”
Tech TipsAir gages effectively measure all common types of dimension, and are particularly suited to checking dimensional relationships.
Differential air gaging combines the necessary high resolution and accuracy with the speed, ease of use and ruggedness required on the shop floor.
Understanding the form of mating tapers as part of process development is not only important to the function of the locking tapers, but also is important in determining the proper selection and use of dimensional gages.