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80 Part II: Metrology
task at hand. While 2-D X-ray imaging is sufficient in many cases, 3-D X-ray images
can reveal complex inner structures in an intuitive format so that anomalies can
be viewed in their 3-D positions. (Rademaker 2005)
3. PNEUMATIC GAGING
Part II.B.3
Identify and describe basic components of
pneumatic gaging, including air columns,
probes, rings, etc. (Comprehension)
Body of Knowledge II.B.3
A pneumatic gage is an instrument for measuring, comparing, or checking dimensions
by sensing the flow of air through the space between the gage head and
workpiece surface. The gage head is applied to each workpiece in the same way,
and the clearance between the two varies with the size of the piece. The amount
the airflow is restricted depends on the clearance. There are four basic types of
air gage sensors, shown in Figure 8.3a, b, c, and d. All have a controlled constantpressure
air supply.
The back-pressure gage (a) responds to the increase in pressure when the airflow
is reduced. It can magnify from 1000:1 to over 5000:1, depending on range,
but is somewhat slow because of the reaction of air to changing pressure.
The differential gage (b) is more sensitive. Air passes through this gage in one
line to the gage head and in a parallel line to the atmosphere though a setting
valve. The pressure between the two lines is measured.
There is no time lag in the flow gage (c), where the rate of airflow raises an indicator
in a tapered tube. The dimension is read from the position of the indicating
float. This gage is simple, does not have a mechanism to wear, is free from hysteresis,
and can amplify to over 500,000:1 without accessories.
The venturi gage (d) measures the drop in pressure of the air flowing through
a venturi tube. It combines the elements of the back-pressure and flow gages and
is fast, but sacrifices simplicity.
A few of the many kinds of gage heads and applications are also shown in
Figure 8.3 (e through i). Practically all inside and outside linear and geometric
dimensions can be checked by air gauging.
Air match gauging, depicted in Figure 8.3i, measures the clearance between two
mating parts. This provides a means of controlling an operation to machine one
part to a specified fit with the other. A multidimension gage has a set of cartridge or
contact gage heads (Figure 8.3h) to check several dimensions on a part at the same
time. The basic gage sensor can be used for a large variety of jobs, but a different
gage head and setting master are needed for almost every job and size.
A major advantage of a pneumatic gage is that the gage head does not have
to tightly fit the part. A clearance of up to 0.08 mm (.003 in.) between the gage
head and workpiece is permissible, and even more in some cases. Thus, there is