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
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
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
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
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