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102 Part II: Metrology
Part II.E.4
to specialized staging, computer manipulation of lighting, image processing, and
the extremely high magnifications available, some vision systems' accuracy
and repeatability can fall within ±0.00005" under certain conditions.
Inspection microscopes use traditional optics to magnify a desired detail. Many
inspection microscopes today are coupled with a video system and offer manual
optical inspection as well as automated video inspection. These microscopes generally
work best when inspecting lightweight and/or flat parts.
Laser inspection instruments, the latest development in optical inspection, offer
the greatest accuracy of any optical inspection instrument. Accuracy and repeatability
within ±0.0000010" under certain conditions can be expected. This type
of instrument utilizes a reflected laser beam to accurately determine distances
by using time-delay calculations. Extremely specialized laser inspection instruments
require fixturing to locate the laser device as well as substantial setup time
to align the instrument. This measurement method is best suited for specialized
production inspection or calibration applications.
Optical Tooling
Telescopes and accessories to establish precisely straight, parallel, perpendicular,
or angled lines are called optical tooling. Two of many applications are shown
in Figure 11.3a and b. One is to check the straightness and truth of ways of a
machine tool bed at various places along the length. The other is to establish reference
lanes for measurements on major aircraft or missile components. Such methods
are especially necessary for large structures. Accuracy of one part in 200,000
is regularly realized; this means that a point at a distance of 2.5 m (100 in.) can be
located within 13 mm (.0005 in.). Common optical tooling procedures are autocollimation,
autoreflection, planizing, leveling, and plumbing.
Autocollimation is done with a telescope having an internal light that projects
a beam through the crosshairs to a target mirror as indicated in Figure 11.3a. If
the mirror face is truly perpendicular to the line of sight, the crosshair image will
be reflected back on itself. The amount the reflected image deviates from the
actual reticle image is an indication of the tilt in the target. A target may have a
cross-line pattern for alignment with the line of sight. An autocollimated image is
not clear for distances over 15.2 m (50 ft.); in these cases a somewhat less accurate
method must be used. This is autoreflection, with an optical flat containing a crossline
pattern mounted on the end of the illuminated telescope and focused to twice
the distance of the target mirror. If the mirror is perpendicular to the line of
sight, the pattern of the flat is reflected in coincidence with the crosshairs in the
telescope.
Planizing comprises fixing planes at 90° with other planes or with a line of sight.
This may be done from accurately placed rails on which transits are mounted in a
tooling dock as indicated in Figure 11.3b. A transit is a telescope mounted to swing
in a plane perpendicular to a horizontal axis. Square lines also may be established
with an optical square or planizing prism mounted on or in front of a telescope,
as depicted in Figure 11.3c. Angles may be precisely set by autocollimating on the
precisely located faces of an optical polygon as in Figure 11.3d.
Leveling establishes a horizontal line of sight or plane. This may be done with
a telescope fitted with a precision spirit level to fix a horizontal line of sight. A