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Chapter 7
A. Common Gages and
Measurement Instruments
Part II.A
INTRODUCTION
Dimensional metrology is concerned with the measurement or gauging of a variety
of workpiece characteristics including: length, diameter, thickness, angle,
taper, roughness, concentricity, and profile. Different sensing technologies may
be employed to measure or gauge those characteristics, depending on the requirements
for accuracy and other considerations. There are basically five different
technologies that may be used individually or in combination to perform these
inspection functions:
1. Mechanical. Small displacements are amplified by a mechanical
system.
2. Electronic. Utilize an electric or electronic phenomenon such as
electrical resistance.
3. Air or pneumatic. Small variations in the dimension are measured
with respect to a reference dimension and are shown by a variation
in air pressure or the velocity of airflow.
4. Light waves. Utilizing the phenomenon of the interference of light
waves to provide a standard. Such a standard is the wavelength of a
monochromatic light, expressed in terms of the meter.
5. Electron beam. Stabilized lasers are used as working standards for
dimensional measurements, providing a precise and stable frequency
for the standard.
In general, the mechanical and electronic types of measuring and gauging instruments
have sensing devices or probes that come in contact with the workpiece
and are referred to as contact instruments. Air instruments, while employing contacting
elements, rely on air pressure difference to effect measurement. Thus, they
are basically noncontact instruments. Although different technologies are involved
in the light-wave and electron-beam instruments, they both utilize a variety
of optical systems. Thus, they are often grouped together as optical noncontact
instruments.
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