by Drew Koppelmann

Gaging With Air

For large production runs, the cost of air gaging can easily be
justified because it is very quick and easy to use.

Every type of gage has particular benefits and liabilities. One of the shortcomings of mechanical gages is their limited resolution-that is, their ability to measure very small dimensional changes. Mechanical gages can resolve down to about 0.0001", but the engineering of mechanical motion transfer isn't reliable much beyond that.

When measurements finer than 0.0001" are required, many automatically turn to electronic gages for the solution. Electronic gages have many beneficial attributes, but they are not necessarily the best tool for every high-resolution gaging task. Air gaging represents a valuable but frequently overlooked measurement option.

Air gages use a stream of compressed air to measure the distance between precision orifices (jets) and the workpiece. A meter measures the back-pressure in the system-the larger the gap, the lower the back-pressure. The jets are installed in tooling or fixtures that are built for specific part-measuring applications.

Air gaging has several benefits compared to mechanical gaging, the first being high resolution. Air gages can easily measure increments of 50 microinches, and, if care is taken, resolution of 5 microinches is possible. Thus, resolution obtained with air gages is two orders of magnitude finer than can commonly be obtained with mechanical gages.

When measuring extremely thin-walled parts or soft materials, contact pressure from a mechanical gage or an electronic gage head can distort or compress the material, producing erroneous results. Air gaging, on the other hand, is a noncontact form of gaging. Pressure from the jet of air is minimal, so soft materials do not become distorted. As a result, gaging results are more reliable. Air gaging is also kind to delicate, highly polished or lapped surfaces.

Because dust, chips and cutting fluids on the surface of a workpiece can interfere with measurements, good gaging practice requires that parts be cleaned before measuring with contact-type gages. In air gaging, the jet of air automatically cleans the part, saving time for the operator. Furthermore, the air flow constantly cleans out the gaging "mechanism" so, unlike mechanical and electronic gages, air gages are resistant to contamination. And the absence of moving parts makes air gages even more durable and reliable than other types of gages.

The most common air-gaging application involves measuring bores, using an air "plug." The plug, with two or more jets, is machined to a specific outside diameter. The operator simply inserts the plug into a bore, and the meter registers the bore diameter as a function of back-pressure. By turning the plug through 180 degrees, the user can see if the part is out-of-round. The same principle applies to outside diameter dimensions. In this application, an air "ring" with one or more jets is placed around a rod or other outside-diameter feature, and the meter measures the gap between them.

Air jets can also be installed in a fixture built to measure a specific type of workpiece. Because the jets are small (typically just 0.050" in diameter) and no mechanical linkages are required, jets can be spaced closely in a tool or fixture. As a result, many closely spaced features can be measured simultaneously. Engine parts are common applications for air-fixture gages.

A new development in air gaging is the "air fork"-a hand-held gage with jets installed on the ends of a two-pronged fork-like device. This device measures the outside diameters of features that are not accessible to air rings.

Air gaging is not limited to measuring feature dimensions. It can also gage a variety of geometric and relational characteristics, including the ovality, taper and straightness of bores, countersink angles, distance between hole centers, and parallelism of features.

Air gaging has its drawbacks. For example, the measurement range is short, with 0.003" being close to the maximum. By comparison, most mechanical indicator gages can measure across a range of at least 0.01" and ranges of several inches are easily accommodated. Air gaging is also not well-suited for use on roughly machined parts because it cannot register the tops of a surface's microscopic peaks. In addition, it requires clean, dry compressed air, which can be somewhat costly.

On the other hand, for large production runs, the cost of air gaging can easily be justified because, in addition to its high levels of accuracy, it is very quick and easy to use. Also, when one production run ends, the gage can be reapplied to the next run by simply adding new tooling.

About the author

As applications manager, gaging products, at Federal Products Co. in Providence, Rhode Island, Drew Koppelmann provides dimensional gaging applications assistance to companies in a wide range of industries. He can be reached by fax at (401) 784-3246.